What is a goniometer?

Goniometer - The name means 'angle measurer'. In radio context, it is a set of coils arranged to determine the bearing (direction) of a transmitter. The scan at right is from the 1938 Admiralty Handbook of Wireless Telegraphy, a two-volume set which has spent more than half of its existence in my collection.

The name Goniometer was chosen for the blog because although its relevance to modern electronics is marginal, the word stands out eyegrabbingly in a page full of search results; this gets more traffic to the blog without resorting to the less ethical SEO methods.


A goniometer is useless without a set of source antennas, and at left I have another page from the Handbook, showing a set of Bellini-Tosi direction-finding antennas. The triangular loops at the top are the antennas, and the three two-turn coils at the bottom are simplified representation of the goniometer coils.

The operation of the system is straightforward. The sense coil ('S' in Fig.18) is rotated until a null (little or no output) is heard in the receiver. The sense coils are connected to a pointer, which indicates the direction of the source transmitter.

003 Fitting Part 1

Hacksawing, filing, finishing

Metal, and other materials from which we make the hardware of our projects, will need to be cut to shape to form the basic components of cabinets, panels, PCBs and other parts. Even when ready-made cases and enclosures are purchased, some cutting operations will be necessary to make the item fit our purpose.

Hacksaws

Hacksaws are the basic cutting tool for hard materials. Two common types are the 12-inch (300mm) hand hacksaw, and the 6-inch (150mm) junior hacksaw. Junior hacksaws are an ideal first purchase for a home electronics constructor. The frame and a pack of blades can be bought very cheaply, and will last years. Full-size 12" hacksaws will cost more, and there is a choice of blades available for different materials and sizes of stock. Coarse blades (18 teeth per inch (TPI)) are suitable for soft material such as wood, plastic and aluminium, while finer-pitched blades (24 to 32 TPI) work better on hard stock like steel and will cut a much thinner material without the blade's teeth jamming over the work.

Blade fitting and tuning

Hacksaw blades are usually fitted with the teeth angled forward. There are two exceptions to this; when they are fitted to power hacksaws, where the blade is pulled back toward the case of the tool to perform the cut, and when a blind-hole saw is made. A description of how to make and use a blind-hole saw can be found at the end of this section.

To fit a blade to a junior hacksaw, simply apply one end of the blade in one of the mounting slots, pull the frame in to shorten it under tension and slide the far end of the blade into position. Take care not to slip, the blade will be very sharp when new.

Full-size hand hacksaws vary in the way they attach blades, so read the instructions supplied. In general, there will be a way of relaxing one of the blade-holders so that the blade's holes can be fitted into them. Full-size hacksaws need to have their blades tensioned. Again, follow the maker's instruction, but as a rule, if it is a screw-tension saw then two and a half turns from the point where the slack is taken-up is about right.

Sawing

The work must be firmly clamped. Use a vice, or clamp the workpiece to a stout table with a G-clamp. Use packing pieces made of wood if the clamp or vice threatens to mark the face of the work. Make sure that the line of the cut is as close to the clamp or vice jaws as possible. Cutting too far from a solid mount will let the work vibrate. This creates noise, slows the cutting and the operation will be more difficult as the saw jumps around.

Remember that the saw has a 'kerf', a well-defined thickness of cut. Allow for this when you apply the blade to the work. Make the first stroke a light one, to get the blade started in the cut. Subsequent strokes can be more forceful. Do not apply undue pressure to the blade; for most work the weight of the saw will be enough. Too much force will make the blade wander around, as it squirms under the strain.

Stance is important. Relax the knee joints, put your left foot forward and your right foot back, angled out 45deg. Keep your back erect, and your torso facing slightly right of centre. Martial arts student will recognise this stance; it is common in many schools of Tai Chi Chuan. Use the full length of the blade, and keep the blade running parallel at all times. When the cut is nearly complete, the offcut part may need support; use your left hand reaching over the saw to do this. Left-handed workers should reverse all of the above instructions for their comfort.

Blind-hole saw; making and using

Blind-hole saws are used where access to both sides of the work is difficult or impossible. It's easy, and requires only a hacksaw blade of appropriate TPI and some duct tape. Wind the tape around the blade, at the end normally fitted to the front of the saw frame. Put plenty of tape on, it will act as a handle and protect the user's hand from the teeth. To use, simply pass the blade into a hole in the workpiece and pull the blade back while applying pressure downward. Be careful when pushing the blade back in for the next stroke; it's all to easy to bend and break it.

Other Saws

Some craft-knife kits (X-acto) contain a miniature tenon saw blade. This is very useful for fine work, such as cutting the copper cladding on circuit panels.

Files

Files are used to smooth rough-cut surfaces, to remove small amounts of material and to take off 'burrs' created by other operations. There is a wide range of shapes and degrees of coarseness, but the home constructor needs just a couple to start with. A 150mm second-cut file and a 150mm round needle file. Needle files can be bought in sets from bargain stores, market traders and tool shops. In terms of quality, you get what you pay for. The second-cut file will be more expensive, but with care will last a long time. You can usefully add a 250mm bastard file (coarse-cut) and a 6mm diameter rat-tail (round) file to your collection for more aggressive work.

Pictured:

1. 250mm bastard file
2. 200mm half-round file
3. 6mm rat-tail file
4. 150mm second-cut file
5. 100mm three-square file
   

Files, like hacksaws, only cut in the forward direction. However, lightly drawing the file back over the work can help to clear particulate matter from the teeth with some materials. Some metals, like copper and aluminium, are a real problem when being filed. These malleable materials clog the teeth readily, and picking the smears of metal out every so often can be a chore. A partial solution to this is to rub the file over a piece of chalk before work begins. This fills the teeth, and being soft it allows the work to proceed. The chalk lubricates the cut and prevents the soft metal from clogging the file.

Never use a file without a handle firmly attached. If you try to use a file by holding the bare tang, it would take only a moment's inattention for the file to slip and the tang to enter your wrist. Don't do it.

Filing

Take a stance as described in 'Sawing', above. The main point to watch is that the file must be kept parallel to the work, or the surface will be curved instead of flat. If you want to form a curve, then start with the front tipped downward, and push it upward as the stroke proceeds. This is counter-intuitive, and you may feel it better to push the file 'over the top', but try it and see; it works better.

Burrs and sharp edges can be dressed-off with one or two light strokes at 45 degrees; sometimes a wider 45 degree 'chamfer' is more appropriate. Sharp corners can be knocked-off in a similar way.

Care of Files

The cutting teeth of files are glass-hard, and liable to chip off if abused. Keep files separate from each other in storage, and take care not to drop them, they may break. Files do wear out, so be prepared to replace them after some extended use. Worn-out files can be ground into other tools, such as scrapers and other edge-tools.

Other Cutting Tools

You may occasionally find use for a small, round gouge, as used by wood-carvers. You can modify PCB tracks with this.

An Abrafile (a thin, coarse round file mounted in a hacksaw frame) is great for piercing odd-shaped holes in panels, but they are becoming difficult to find.

Hobbyist drills (ie Dremel) are wonderful for fine work, and are ideal for occasional PCB drilling.

A small cabinet scraper will help when preparing wood, and it will be useful for removing burrs from copperclad board. Make one from a piece of old file; just remember to leave the burr on the edge when grinding it, this is the cutting edge.

Finishing

To prepare a panel for painting, it needs to be keyed and cleaned. The best way of providing a key for paint is to use either a disc abrader (DA), or an orbital sander. Small parts can be prepared by hand using Scotchbrite or a similar nylon abrasive pad. Degrease with soap and water, or with a solvent cleaner.

Painting

Spray painting gives the smoothest finish, but other methods can be used. Hand-painting with a brush can be effective if applied with the panel horizontal, and time is taken to brush-out the paint evenly. Curing of spray paints is accelerated if the work is gently warmed with a hair drier first. Choose a colour which suits the purpose; matt black or olive green are trendy, but light aircraft grey is easy on the eye. Leave the paint to cure / dry, and don't be tempted to handle the work until the paint is hard.

Panel Marking

I was brought up to use rub-down lettering, but this method is slow, liable to damage and gives poor results except in the very best hands. Today, it is far easier to use a computer to lay-out and make self-adhesive panel covers. Use your favourite paint program, and with a little planning you can make a one-piece label to cover your front panel. Complex features and symbols are possible. If your home-made equipment needs a calibration chart, print one of those, too, on self-adhesive label, and stick on the case. If you can't find continuous A4 sheets, then choose sheets of labels which have the individual labels butted together, and simply reassemble them on your front panel. Test spray lacquers or brush varnishes to find one which doesn't make your printing run or bleed, and you have a complete system. Holes for control shafts and lamps are cut out with a craft knife.

USB-Powered Direct-Conversion Receiver

I've built and blogged one of these before, but now I've developed the idea a little. The original suffered with a poor antenna and although it worked, it was more than a little deaf. The new USBrx has eighteen components, including the connectors, antenna and panel.

1. Loop antenna wire (32feet, 9.75m) - any insulated wire will do.
2. Loop antenna connectors male (2 required)
   
3. Loop antenna connectors female (2 required)
   
4. Tuning capacitor, 20-200pF, polyvaricon
5. Knob to suit tuning capacitor
   
6. Ferrite toroid, 9mm 4c65, FT37 type, any small HF ferrite
7. SA602 mixer / oscillator
8. Quartz crystal, frequency at band edge
9. Emitter capacitor, 30pF
10. Feedback capacitor, 30pF
11. Bypass capacitor, 3n3 (3300pF)
12. Coupling capacitor, 100nF
13. Supply decoupling capacitor, 10uF
14. USB extension cable
15. Old earbud cable
16. Copperclad panel (FR4), 55x55mm

You can save on some parts by permanently mounting the loop wire, without using the connectors. I use my 32-foot loop on other radios, so I need to be able to disconnect it. If you plunder an old AM radio for the tuning capacitor, you may have a knob with it already.

The antenna is a loop, 32 feet long, and works as a quarter-wave magnetic loop for 40 meters. It was first described by Ben Edginton G0CWT, to whom I'm indebted for the idea. It's basically an opened-out version of the 'original' USBrx antenna, itself a copy of the Poundshop antenna, first used by me in 2005 as part of the Poundshop Radio. These antennas are directional, and very compact. An eight-foot square for forty metres is something which can be occasionally strung across a room (I use lengths of nylon string and small bulldog clips), and taken down when business is done. The loop is resonated with the polyvaricon variable capacitor, which can have one side connected to the panel. I made a 64-foot version in 2007, and with extra windings on the transformer I used it on Topband.

The antenna has a very low impedance, in the region of 2.5 ohms, so a transformer is used to match the loop antenna to the 1500 ohms of the SA602. One turn (a single pass through the toroid's hole) for the antenna and 25 turns of 30SWG enamelled copper wire for the SA602 input.

The output, using the tip and body contacts of the 'earbud' cable, will drive the microphone input of a PC soundcard. You can listen to it with headphones directly from the soundcard output, but you won't like it. Everything from the crystal's frequency to 25kHz in both directions will be coming at you, and it's quite a cacophony. To make sense of it, you need a little help from one of my favourite pieces of software, the SAQrx. You can download this from here. This is intended for use with LF antennas for monitoring the famous Alexanderson Alternator transmissions, which use the callsign SAQ, but it has a host of other uses, including rudimentary digital signal processing. Because the SAQrx software can filter out any narrow band from near DC to over 20kHz, you can use it to choose a spot frequency to listen to. You can choose narrow, medium and wide bandwidths, and the gain can be adjusted to suit. The program works well under Wine in a Linux machine, as shown in the photograph. Comment if you need any help with this one!

002 Hand Tools

Some tools are absolutely necessary for building radio equipment. The following short list should be shopped for, before attempting any practical work. These are the bare minimum required to assemble the projects detailed in this book, and may be all you ever need...

Soldering iron
Pliers
Cutters
Screwdrivers
Knife
Spanners
Trim-tool
Multimeter

We'll discuss each tool in turn, and suggest what to buy to get the best value and utility.

Soldering Irons

We use soldering irons to melt solder, which is an alloy designed to melt at relatively low temperatures. The solder is allowed to flow over the joint area where we wish to connect one component to another. The actual art of soldering is described in section 007 of this book.

Soldering irons can be bought very cheaply from discount shops; the quality of these will vary from tolerable to unpleasant, but they will be cheap, and will get you started. The power rating to look for is in the range 20 to 40 watts. Anything more powerful will be too large physically, and if the temperature is unregulated (highly likely in a cheap iron), then it will be difficult too achieve a clean soldered joint. Big irons are useful for heavy work, such as antenna work or soldering pieces of PCB material together. Besides the iron, you will need a damp cloth to wipe the tip of the iron on to remove the accumulated flux residue. Better-quality irons have a piece of special sponge material set into a small tray in the stand; keep this damp and wipe your iron's tip regularly.

I use three irons for work at home; a temperature-controlled soldering station, a 100W high-power iron and a small butane-powered portable. The soldering station is good for general work, and mine has a cleaning feature with rotating sponges inside the housing, which retains the moisture the sponges need to work. The 100W iron is useful for soldering pieces of copper-clad together to make cases or complex modules. Larger screening cans are best soldered with a big iron. The gas iron has a very small tip, and I use it for surface-mount devices. It has an integral flint wheel in the cap.

Pliers

You will need at least one pair of pliers. A pair of snipe-nosed pliers: get miniature 'electronic' types, as what you are working with will be small, and the leads may not be gripped by larger types. A pair of large, engineer's pliers will also prove useful for gripping larger items. One special pair to get if you decide to take your wiring skill to the next stage is the round-nosed plier. These, as their name suggests, have a pair of long, thin conical tips; these are used to curl wire into a ring shape for making 'mechanical' joints. I modify mine to be as versatile as possible by filing one of the tips smaller than the other, to make a wider variety of joint sizes.

Cutters

Cutters are available in a bewildering array of shapes and sizes. The ones to buy first are electronic side-cutters. These are for cutting thin copper leads on components, but can also cope with light-gauge copper connecting wire. Never use them on steel wire; they'll lose their edge or break. For very fine work, a pair of 'flush-cut' cutters can be added to the kit. If you anticipate cutting heavy cable, such as hard-drawn antenna wire, then a stronger pair will be needed. A farmer's fencing tool is great for this, as it includes a range of functions in the head, including a hammer and a nail-extractor.

Screwdrivers

Buy just two screwdrivers initially, and add to the range as you feel you need them. The ones you need are a small Pozidrive, and a 3mm flat-blade. Philips screwdrivers are different to Pozidrive; the angle is steeper and they only have four large flutes. Pozidrive tips have an additional four smaller flutes. They look similar, but you feel the difference when you try turning a tight screw. Extra screwdrivers can be bought as needed, a 6mm flat-blade and a larger Pozidrive are sensible additions. If you decide to buy a screwdiver kit which includes a handle and a range of interchangeable tips, then you have lots of options, including hex bits and Torx bits. Be careful about quality; poor alloy choice means that the tips will wear quickly and chew-up your screw heads.

Knives

Use whatever knife style you feel comfortable with. Here at G1INF, the favourite is a Stanley blade in a home-made handle. The choice is between fine blades with good access for detailed work, and heavy blades with fat handles for meatier cutting.

Spanners

The most likely spanner sizes you will need are 5.5mm (M3), 7mm (M4), 8mm (M5), 10mm (M6) and it would be wise to get a selection of larger sizes, or a small adjustable spanner, for working with control spindle nuts. Small kits of combination spanners (ring one end, open the other) are to be found in hardware stores, bargain shops and market stalls. These are ideal for light home use, but be sure you get that essential 5.5mm spanner. Tip - A 5BA spanner works on M3.

Trim-tool

Unless you have worked with radio or electronics before, you are unlikely to have seen a trim-tool. They are essential for setting-up trimmer capacitors. It is tempting for a beginner to use a small screwdriver to adjust a trimmer, but you'll soon learn by your mistake. Screwdrivers have a considerable metal mass, and this adds to the capacitance of the trimmer (and the circuit you're trying to tune). The process will be very frustrating, and although possible if you have the time to cut-and-try the over- or under-adjustment, it's far easier and quicker to use the real thing. Cheaply available from electronic suppliers, trim-tools are a tough plastic rod with a tiny piece of hard metal in each end. They do not appreciably change the capacitance, and you can adjust the tuning in 'real-time'. One word of warning - Trim-tools are not tough enough to be used as a screwdriver, so don't be tempted.

Multimeter

A vital first piece of test equipment is a small, cheap, digital multimeter. At the time of writing (2009), these could be bought for around £5 in bargain stores. There are cheaper models, but they may not have the ranges you require. Cheap analogue (moving-needle) meters are rarely worth buying; they are usually too insensitive. Digital meters have a high-impedance buffer amplifier before the ADC, and are very sensitive. A better-quality analogue meter has its uses. You can see trends in the measured value, and for some measurements it simply feels more comfortable to watch a needle swinging.

Minimum specification for a multimeter:

DC Volts ranges 0.5V to 500V
DC Current ranges 100mA to 10A
Resistance ranges 100 ohms to 2M ohms
Sensitivity - 20,000 ohms per volt (20K ohm / V)

Direct current (DC) voltage and DC current are the main parameters measured when testing a circuit. It is useful to check resistance in a number of situations. If you are unsure of a resistor's value, then measuring it with a meter will confirm it. Circuits can be checked (with the power off) with a resistance meter, and items like fuses and connectors can be analysed for faults.

The sensitivity is where many very cheap meters will fall by the wayside. The lower the sensitivity, the more current is taken from the circuit under test. If you apply a low-sensitivity meter to a high-resistance circuit, you will get a false reading, as much of the circuit's voltage will be drained by the meter. [graphic of meter loading]

One aspect of cheaper multimeters is the quality of the test leads. They will break long before the meter gives trouble, so note that both the pictured meters are fitted with home-made test leads. These are easy to make. Red 'bullet' crimp terminals are compatible with standard 4mm test connectors, and are cheap and easy to buy. A crimp tool for applying them should cost less than £3-.

Other Tools

Tools are seen by some as an object of desire in themselves. People collect tools, not because they need them, but because they look interesting, or they think they may one day have a use for them. If you want to amass a toolkit of your dreams, then go ahead, It's part of human nature. However, if you are fastidious about utility, then the tools above may be all you ever need. There are some extra tools worth considering, and they are described next.

Desoldering Pump

If you plan to make printed circuit boards, then a desoldering pump will be needed if you need to repair them. When a device is removed from a PCB, the hole invariably remains filled with solder, making it impossible to fit a replacement device. A desoldering pump is used with a soldering iron to remove the solder, leaving the hole open. They work like a back-to-front bicycle pump; you push the piston all the way into the cylinder until it latches, heat the joint to melt the solder and then apply the tip of the pump to the joint. Pushing the release button causes the piston to fly back under spring pressure, sucking the molten solder up the tip. The solder is collected in the cylinder, and will need to be emptied from time to time. Unscrew the tip carrier, and shake out the solder droplets. Any which remains stuck to the face of the piston can be released with a small flat-bladed screwdriver. Always wash your hands after this operation, as the finely-divided solder powder is easily ingested, and is toxic. There may be local regulations for the disposal of heavy metals, so either take advice from your local authority about disposal, or consider recycling the solder.

Wire Strippers

I have deliberately left wire stripper out of the list of essentials, because they are unnecessary for low-volume non-commercial wiring. I only have one wire stripper in my radio toolbox, and that is a special one for a particular fine, single-core wire I sometimes use when prototyping digital equipment. I keep wire strippers in my 'commercial' wirer's toolbox, because my clients expect me to use them, and I do. But at home, I have other ways.

There are at least three ways to take the insulation from the end of a wire without using wire strippers. The first, and least recommended, is to place the wire between your teeth and pull. The chances of chipping a tooth are high, and you run the risk of ingesting slivers of wire. Don't do it! Another way is to slice around the insulation with a knife, and pull off the resulting tube of insulation. This works well, but care must be taken not to cut the wire, or your fingers.

The most frequently-used method is to use your side-cutters. Grip the wire with your cutters, with the flat side of the cutters toward the end of the wire, and pull. It takes practice to remove the insulation cleanly, but once the knack is acquired, you won't look back. This method works well with multi-stranded wire, which is the commonest type. It is flexible, due to its rope-like structure, and the strands deform under the pressure of the cutters, so that more of the cut is applied to the insulation. When stripping single-core wire, make a rotating cut with the cutters before applying the pull.

Don't use this method if you are working in industry, as it is not approved by any employer I've met. You will inevitably mark or even cut through strands of wire when you're learning, but practice makes perfect.

In my experience, a cheap pair of strippers will cause more damage than the use of side-cutters, especially in inexperienced hands. Try the side-cutter method, and by all means invest in a pair of strippers if you feel the need. The type popular with installation electricians, which have a pair of jaws with a dozen or so tiny teeth, are ideal for everyday uses. For high-quality work, a pair of separation-jaw strippers will give first-class results. These are approved for use on aircraft and military equipment and are expensive.

As I mentioned in the first paragraph of this section, I keep a special stripper for miniature solid wire. It is specially designed to cut the insulation, and leave the core conductor intact. These, like the separation strippers described above, are not cheap. The one in the photograph has lasted the author for over twenty-five years.

Test Equipment

You will be able to do a lot of testing with just your multimeter. Its capabilities can be extended by making add-on modules, some of which are described in the projects section of this book. They will allow you to measure capacitance and radio-frequency power. There are other projects there which don't involve the multimeter; these include the noise generator, audio amplifier and gate dip oscillator. Other items of test equipment which will be useful to a radio experimenter are outlined in section 017, but as they are expensive and complex, they will not be discussed at length. The two main equipments in this category are the oscilloscope and the spectrum analyser; get an oscilloscope if you can, but check article 017 'Radio Frequency Testing' before you buy.

Care of Tools

I possess tools which date back to my apprenticeship, over thirty years ago. They've stayed with me because I've looked after them, and they've looked after me. Storage of tools is important, to keep them from damage (and from damaging people); an old adage - 'A place for every tool, and every tool in its place'. Keep your tools in a dedicated box, or set aside one or two drawers of your desk for them. Keep them clean; wipe them down with a dry (or slightly oily) cloth, and lubricate moving parts when needed. Pliers and cutters need their hinge joints oiled at least twice a year, more frequently if you use them full-time.

A useful way of storing test leads is to coil them individually, and pop them into small ziplock bags. Squeeze the air out before the seal shuts, and they snug down into a drawer or box very neatly without sprawling everywhere and tangling. If you have wall-space, hang them on hooks for easy access.

Instruments need special care. Check the battery occasionally. Some instruments use a tiny current, and the battery can degrade and leak before it is exhausted. Protect meters and other sensitive equipment from mechanical shock; moving-coils meters are particularly delicate, and a little attention to handling and storage will keep them safe. Protect them when storing, tarnsporting or even in use by means of polythene foam cushioning.

Other Workshop Facilities

A good stout bench, desk or table is needed to work on. I know amatuer constructors who cheerfully work on a bureau, wooden tray on the lap and even on the kitchen work surface. There have been designs published in the old paper magazines for special electronic hobbyist's work-stations.

A vice can be very handy; make sure it's well secured to the bench, and that the rear (fixed) jaw is overhanging the edge of the bench. Cupboards for storing equipment and components, small boxes for keeping things tidy, drawer-cabinets for small items. Take time to think how best to arrange your working space, and ask the rest of the household for their opinion!

Location

Think carefully before setting up shop in a shed or other outside structure. Is it dry? Does it freeze? Some devices (LCDs are an example) can be damaged by very low temperatures. Inside the home is best, but an outside shack can be a rewarding retreat if you can keep the temperature up and the damp down in winter. My mentor, Eric G3HTP (SK), kept his station under the stairs; he was quite comfortable, and far enough from the sitting-room not to disturb the family. I work in a lean-to extension at the back of our house, but I have used a metal shed, which made an excellent ground for my antennas.

24MHz HF Theremin

I've been wanting to blog this for a few months, but life got in the way of preparing enough material for release. I've updated the static site at g1inf4u with enough information for an experimenter to try their luck with one. The full link is here, and do please comment if you'd like more information.

The theremin is a 'mature' project; it's been around for a couple of years. The original still works, and I find it a great asset when I'm teaching the art of the Boatswain's Call. The two instruments are very close in character, and the way they are played. They share the distinction of being the only instruments whose pitch is controlled without the player's fingers touching it; only the distance from the tone-determining antenna or hole is relevant.

Theremins are relatively rare. I've just had to add the word to my netbook's dictionary, and few people recognise the word, either. The sound is unmistakeable, however, and while similar to a bowed saw, the range of frequencies is quite large. My theremin can make any tone from a low ten-hertz growl to a ten-kilohertz shrill.

I've seen pocket-sized theremins cropping up here and there, but none offer level control. I've used an ORP12 photocell with a white LED, and this gives a useful dynamic range. It would need a window amplifier to cut off the signal entirely, but then the circuit begins to get complex. Currently, it's simple and elegant.

001 Health and Safety

When I was sixteen, I began my apprenticeship as an avionics / instrument technician. The first thing we did was to attend a lecture on industrial safety, complete with scary movies of industrial accidents and a long list of 'thou shalt not' behaviour modes. This was before we'd even seen the workshop which would be our daytime 'home' for the next forty weeks.

It would be wrong of me to treat my readers any differently; safety first and always. This brief post will outline the essentials of radio experimenter's safety knowledge, but be aware that the author is not a H&S professional, and the reader is advised to take local expert advice before beginning work.

The potential hazards include:

Risk of burning. Soldering irons have a very hot functional end; the temperature is usually set to around 360 degrees C, but may be higher where an unregulated iron is used. Molten solder is at the same, high temperature, and because it is a liquid (and a heavy one), it can flow, drop or spray and cause burns.

Risk of cutting / abrasion / amputation. Some tools used have functional sharp edges; knives, hacksaws, chisels, etc. Always cut away from your body / hand / other parts. Take great care when cutting thin sheet materials; the edges will easily cut you.

Risk of puncture wounds. Some tools have functional sharp points, and care must be taken to avoid being stabbed by your own spike, needle, needle-nose pliers etc. Tightening a cable tie with pliers is particularly hazardous; the tie may break, and the pliers will move fast. Take care with cut wire ends. Larger wire sizes make sharp, stiff spikes.

Risk of poisoning. Some of the chemicals and materials used in electronics are dangerous. Do not open electronic components. Many contain susbstances hazardous to health, such as beryllia, polychlorinated biphenyls and lead alloys. Solder may contain lead in high proportion. Always wash your hands after handling solder.

Risk of respiratory irritation. Be very careful when cutting fibreglass panels; the dust is a respiratory irritant. Wear a dust mask, and if possible operate a vacuum cleaner by the work when cutting. Solder contains a resinous 'flux', which will vapourise when heated. Make sure you have adequate ventilation, and use a small fan if possible to draw the fumes away from you.

Risk of electrocution. Do not use mains power for your projects. The risks far outweigh the convenience; you can change a battery, but you only get one life. Make sure you electric-powered tools (including soldering iron and bench lamp) are in good condition, with no frayed or damaged wires. If in doubt, get a qualified electrician to inspect them.

Risk of radio-frequency burns. The projects, as published, do not produce enough power to cause significant danger of RF burns, but you should be aware that even modest RF energy can produce very painful and potentially dangerous burns on the skin.

This list is by no means exhaustive, and other safety information will be included in future articles as necessary.

I repeat once more - I'm not an expert - take advice if you are at all unsure. Work safe!

Basic Skills - Introduction

In support of the projects, I will be releasing some skill-related posts. These articles give the reader an introduction to each skill, and will also give links out to any resources the author finds during his research. The content is based on the author's own experience and learning, but there's a lot more to know.

The set begins as it should, with words of caution. Electronics assembly and experimentation is potentially hazardous, and the reader must make him / herself aware of these hazards. This first 'Health and Safety' post is by no means definitive. Safety information will be given occasionally as the topical need arises in future posts.

Take note that the author is not a Health and Safety professional, and whilst the advice given in this blog is given in good faith, the reader assumes all responsibility for his / her own safety. If in any doubt, take professional advice before undertaking any potentially hazardous operation.

List of forthcoming posts:
001 Health and Safety. Safety first, and always.
002 Hand tools. Outline of basic radio-construction hand tools. Other tools will be introduced and discussed topically, later in the series.
003 Basic fitting skills 01 - Hacksawing, filing, finishing. How to cut metal.
004 Basic fitting skills 02 - Drilling. How to make holes.
005 Basic fitting skills 03 - Tapping and threadcutting. How to make threads.
006 Electronic skills 01 - Cable preparation. How to strip cables and wires.
007 Electronic skills 02 - Soldered joints. How to make a joint, and solder it.
008 Electronic skills 03 - Wirewrapping. How to make joints without solder.
009 Electronic skills 04 - PCB making. How to make your own printed circuit boards.
010 Electronic skills 05 - PCB assembly. How to fit devices to a PCB.
011 Electronic skills 06 - Elementary panel wiring. How to wire-up electronic equipment.
012 Electronic skills 07 - Advanced panel wiring. Advanced techniques, including lacing.
013 Electronic skills 08 - Prototyping with stripboard. How to make projects using stripboard.
014 Electronic skills 09 - RF prototyping with copperclad. How to 'skywire' circuitry.
015 Electronic skills 10 - Basic circuit testing. How to make sure your project works.
016 Electronic skills 11 - Audio frequency testing. How to check for sound quality.
017 Electronic skills 12 - Radio frequency testing. How to check that new radio.
018 Light fabrication 01 - Sheet work. How to cut and fold cabinets.
019 Light fabrication 02 - Soldering. How to join sheet materials with solder.

There is no timeline here because these 'skills' posts will be interspersed with the regular Project posts. When a significant number are published, I'll add a sidebar navigation to allow easy access to both the skills and the projects.

Simple AF Amplifier

The Simple Audio Frequency (AF) Amplifier. An easy and useful introduction to home-build electronics, this project will take a low-level audio-frequency signal and boost it to 200 times its original strength. It has a level control to adjust the output for comfort, and will drive a pair of iPod / MP3 player headphones, or a small loudspeaker. It uses a 9V battery (PP3 / MN1604), and requires no setting-up or test equipment. To make it, you will need the following parts:

1. LM386 audio amplifier device
2. 10k logarithmic pot with switch
3. 1uF 16V electrolytic capacitor
4. 10uF 16V electrolytic capacitor
5. 100uF 16V electrolytic capacitor
6. Red 5mm LED
7. 1K-ohm resistor (1/8 watt)
8. 3.5mm stereo jack socket
9. 2-off 4mm banana sockets
10. Thin insulated wire (150mm)
11. PP3 battery clip
12. PP3 9V battery
13. 100x55mm single-sided copper-clad board

An experienced constructor would find most (or all) of this in their junk-box, but a newcomer to the art will need to buy these things new. I plan to market a kit of parts for this project, along with all those which will follow in the weeks and months ahead. Two big-name suppliers of components are Farnell ( http://uk.farnell.com/ ) and RS Components ( http://rswww.com ). Farnell may be the better bet for the non-corporate buyer, and they carry the stereo socket ( p/n 1280747 ), whilst RS do not. You will have to buy the sockets in multiples of five, as that is their packet quantity. No worries - you''l find uses for the other four in other projects and experiments. Another source of components is from old or broken equipment; see my article - http://www.lulu.com/content/e-book/component-harvesting/5327360 for more information.

You will of course need tools, and solder. Soon, I'll be adding teach-in articles (including videos) on tools and techniques; but for now, unless you already have the skills and tools, ask someone who knows how before attempting to make this project. Stick around! I'll tell you how it's done. I've spent over thirty years in the electronics industry, in several varied jobs, and I've picked-up a lot of knowledge and skills down the years. I intend to pass it all on to you.

The panel drilling is very straightforward. There are nine holes, and you'll need 3mm, 5mm, 6mm, 8mm and 10mm drill bits. Drill all holes initially with the 3mm bit; this makes it much easier to start the larger drills, and this first drilling size is called a 'pilot hole'. Make sure you clamp the panel when drilling. If the drill bit snatches the panel, it'll whip round and catch your hand. A G-clamp with a piece of wood to spread the load makes a good makeshift clamp. Clean the raised burr from around the hole edges with a larger-sized drill bit (hand-held), and then buff-up the copper surface with scouring pad ready for soldering.

Mount the sockets and pot first, locating them in their holes and carefully tightening their nuts. With care, a pair of snipe-nosed pliers may be used to tighten the nut of the stereo socket. Bend pins 3 and 4 of the LM386 up, and then outward, level with the top surface of the plastic case. These will be soldered to the panel as 'ground' connections. Turn the LM386 over. With the writing-side down, you have the remaining six pins sticking upright like a dead insect. Connection to these pins is now easy, and they are clear of the panel. Bend the 10uF capacitor's leads around as in the layout picture, and solder the capacitor to pins 1 and 8 of the LM386. These pins are at the 'notch' end of the device. Now add the remaining components. The red LED is pushed through the panel, and it's cathode lead is soldered directly to the panel. This is different to the circuit diagram; the diagram shows the resistor in between the cathode of the LED and the panel. It will work either way round; this is a good example of the flexibility which can be employed in electronics. Things aren't always this easy or straightforward, but I haven't the space now. I'll expand on this and much more, later.

Connect a PP3, a pair of headphones, and switch on. A buzzing will be heard in the headphones if the input socket is touched. What can you amplify? Try a magnetic (moving-coil) microphone, or an electric guitar. Search the web for 'crystal set' radio designs; the output can be amplified by this project. If you want to use a computer-type microphone (electret), it will need a power supply. This can be done by connecting a 47k resistor between the input socket and the switched side of the power switch. This will put around two volts on the microphone.

This simple circuit is simpler than normal for an LM386-based amplifier. I've left out three components; and these may be added if the amplifier is unstable or noisy. They will not normally be necessary, but you should be aware of them. A 100uF capacitor can be added between pin 6 of the LM386 and the panel, with the + of the cap to pin 6. This 'decouples' the power supply, stopping spurious signals getting into the device via the power supply. It also helps to stabilise the voltage, acting as a reservoir. Two other devices normally used in circuits of this nature are a 10-ohm resistor and a 100nF capacitor in series, connected from pin 5 (the output) and the panel. This 'Zobel network' helps to cancel-out the reactance of the speaker's coil, but if headphones are used the inductive reactance is too small to be of consequence. If you hear popping or howling when using a speaker, add the Zobel components.

Next week, I'll offer up some skills. There's a lot to learn, and the basics need covering first. When I was trained as a wirer, many years ago, we were instructed to wire-up panels fitted with tagstips and terminals, and the joints were made such that they held together mechanically. The units we made had to work before they were soldered. The wires had to be laid straight, and the stripping accurate. Any deviation from the rigorous standards, and the instructor pushed a screwdriver through the wire, and politely asked us to 'do it again!'. I'm not that overbearing; but I will teach you all I know about wiring, fitting and electronics assembly. Valves, Transistors, ICs, panel wiring, PCBs, wirewrapping, soldering, stripboard, the lot.

Theory will be added into the mix as required. I will not be teaching Maxwell's Laws of Electromagnetism, but I will tell you why you don't need them.

Cheap and Easy 4mm Connectors

An interim post; I felt it was worth sharing. I have used red bullet crimp connectors for some years as a cheap alternative to 4mm 'banana' plugs, which are used for test and measurement connections. Only the red ones work; the yellow and blue crimps are too big. Earlier today, I found it was easy to use the female crimps to make workmanlike sockets for use in test equipment.

All you need is a red female bullet crimp, and a pair of M8 nuts. It's simple. Use a nut to cut a shallow thread in the body of the crimp, and then use a pair of nuts to mount the crimp in the front panel of your new home-brewed meter, power supply, generator, whatever. I found it easier if the wire was already crimped into the terminal; the flattened, crimped section can be held with pliers while the nut is wound up the barrel of the crimp terminal.

For a really smart finish, you need a powerful soldering iron. A 100-Watt soldering iron has enough power to solder an M8 nut to a piece of FR4 copperclad. Drill an 8mm hole in the panel, lightly assemble a nut to the rear of the panel with a short bolt in the hole, and heat the nut. When it's hot enough, flow a little solder around the nut. Just add enough to give a 3mm 'fillet' joint. Wait until the nut has cooled before winding the crimp barrel into it; if you are tempted to do this in a hot nut, the crimp will shrink with the heat and drop out!

These crimp terminals are sold in motor factors, DIY stores and good, old-fashioned hardware shops. You'll find the M8 nuts in the same places. The connectors made in this way are fully compatible with 4mm banana plugs and sockets, and the tooling is cheap, too. Buy the simple pressed-steel cheap one you see in the photos; you can pay a lot more, but these work well enough for our purposes. Besides, you get a wire stripper, screw cutter and wire cutter in the tool as well.

Back to Basics

I've decided to start over. New ground rules for myself, and heaps of benefits for readers.

(1) I will post updates here regularly, every Thursday. If anything can't wait, it'll get posted, but the Thursday post is the routine.

(2) The amateur construction ebook will have its projects posted as news items here, with links to PDFs as they are completed. The project PDFs are free for anyone to download, anytime, anywhere. All I ask is that you download, and not link to them. 'Bandwidth thieves' will be dealt with.

That's it! Just a little more commitment and organisation this end. Enjoy.


So, starting over means back to square one? Yes, and it's real basic. The first project is a fundamentally useful one, and it has just ten components, including all connectors and the FR4 panel it's built on. I can't claim originality for this one; it's straight out of the data-sheet for the LM386 audio amplifier. Anyone with basic electronic tools can build this, and the parts will cost you around £3-50 total. If I were to present this as a kit, it'll cost you £7-50 including post and packing to UK mainland, battery not included. That's a thought I'm going to mull over.

The LM386 amplifier is available in a variety of sub-types; the only ones worth noting are the N-1 and N-4. The N-1 is the cheapest, and will deliver around 300mW. The maximum voltage is 12V, making it ideal for use with a 9V PP3 battery. For higher voltages and more power, the LM386N-4 can use up to 18V and supply 1000mW (a whole watt) of audio power, great for small speakers. Its Farnell code is 1184987, and they sell it for 57p.
Next post here will include links to this first project, and give an overview of the next. I'll also include links to things I've discovered while researching 'out there', a practice harking back to the original purpose of blogging; a list of links and commentaries on websites.

Diversion into sales

I'm letting the new radio percolate for a few days, while I sort out some sale stock for my eBay account. I've built up a significant amount of surplus over the years, and I'm disposing of some of it to make room, and funds, for the kits project. I'll be releasing the stuff onto the lists over the weekend, and I'll update here with links and information on the items available.

Another thread of the business is the information products. I'll be unable to commit any serious time to this for the next few weeks, as the hardware sales will take precedence. The only new information likely to be published will be for the kits!

Early Development Prototype - CW84

I'm calling it the CW84 because it's a refinement of an 80-metre cw transceiver I made about four years ago, which I called the CW80. Not only is it four years younger, but the true wavelength of a 3.550MHz signal is 84 metres. It's a direct-conversion receiver, very straightforward in design, with an equally simple transmitter tacked onto it. A 700Hz offset is added to the transmitted signal by adjusting the control voltage fed to the tuning varactor, which is a forward-biassed LED.

Today, I achieved some satisfactory results with an initial development prototype. It's cobbled together on stripboard, and the picture shows the diagram of the circuit as it stands. There is as yet no RF input to the SA602 mixer / oscillator, and I haven't tried to recover any audio from the thing. It does, however, tune it's oscillator sweetly between 3.550 and 3.583MHz, which is exactly the 30kHz I wanted; this little band covers the 3.555 and 3.564 slow Morse code (QRS) 'sandpits', and the 3.560 low-power (QRP) centre of activity.

The front end may be a simple bandpass filter and a 1k-ohm pot attenuator, or I may include a FET preamp as I did in the CW80. I'll try to avoid complexity wherever possible. The whole idea is one of simplicity; very little setup, few wound components and even fewer trimmer capacitors. Today, I eliminated a padder from the oscillator tuning, and that's the kind of change which I welcome. There will be no AF gain pot. The original CW80 doesn't use one, simply because the RF attenuator does all the gain-setting required. It also stops a novice operator from setting the RF gain too high, backing off the AF gain and wondering why the mixer is getting swamped with powerful signals. The SA602 is a lovely device, if you respect the fact that Gilbert cell mixers are easily overloaded, but have good conversion gain and hence work well at low input levels. Another simplification is in the AF output stage. The LM386 is noted for its low ancillary component count, but I've taken the liberty of doing without the bypass on pin 7 and the Zobel network on the output at pin 5. If they prove necessary, I'll add them; but only if.

Tomorrow (hopefully), I'll add a front end and a pair of 'phones. I may even try listening to my gate-dip oscillator later tonight. Whatever happens, and whenever I get around to doing it, I'll post it right here. Remember; as always, although I assert copyright on my work, please feel free to try these things for yourself. I intend to market kits for these projects, and support them with this blog and other web-based materials, but use your junk-box stock or the shopping-lists which I publish with the designs.

Witlessness and woe

If I've learned anything this evening, it's how to waste three hours work on one simple detail. The new radio was laid out three nights ago, and it looked elegant, compact and concise in its prototype stripboard form. The resonators, 1K pots and 100p COG capacitors arrived today, so it all looked good for a first build. Thursday evenings are free of Sea Cadet obligations, and nothing was planned by the family. I settled down in my untidy position at my desk in the extension, and carefully loaded the stripboard. My youngest son came by to wish me goodnight at around 2100, and I was just hooking-up the tuning pot. The tuning LED glowed and dimmed, the volts were where they were meant to be, but no output from the SA602. I'd put a 100p cap on pin 7 (the oscillator emitter), to take the signal to a keyed buffer for the transmitter. I was patiently listening with my old Realistic DX-392, looking for signs of life. I searched the board for open joints, solder whiskers, anything. I changed the ceramic resonator for another type, I removed the bandsetting padder, I changed the LED coupling capacitor, nothing.

Then it hit me, like a huge, soggy mattress. I am so used to using chips upside-down on a piece of copperclad, that I'd laid the board out with the two chips back to front. The entire layout is scrap, and I face a fresh piece of quadrille paper to start afresh. Ho-hum.

It's wonderful to watch your ideas take shape, and weird to watch the shape change as the ideas mature into workable products. The latest radio is no exception. I went through a panic stage four days ago, and the direct-conversion receiver / cw rig turned into a Pixie II, and back again. Do a search for the Pixie II, it's a fascinating exercise in minimalism. The PA transistor doubles as the receive mixer; keying the PA emitter shorts the receive path and puts the transistor into full drive. On key-up, recovered audio is passed to an LM386, and into a pair of 32-ohm headphones. Usual power source is a 9V PP3, and the housing can be anything from an Altoids tin to al fresco.

I did say it turned back again, and it remains a variant of the Rev. George Dobbs G3RJV's famous 'Sudden' receiver. I'm using Micrometals iron toroids in the receive preselector, and a 3.58MHz ceramic resonator as the frequency controlling element. I need tighter control than George's VFO original, because it's also transmitting. I've settled (for the time being) on a 30kHz band, from 3.55 to 3.58. This includes the QRS 'sandpit' at 3.555 and the QRP centre of activity at 3.560. The radio has a 700Hz transmit offset, given by arranging for a pair of resistors to be switched in and out of the frequency control voltage 'totem-pole', which feeds the LED varactor. Not only am I using a LED as a varactor, but I find that forward bias gives more linearity. This method is not original, but has been used with success in the radio home-brewing world for some years. The other feature of the forward bias is that the LED can be brought out to the front panel, adding interest as the radio is tuned. The LED dims and brightens, glowing well at the lowest frequency.

I'm currently waiting for parts, and I have two evenings ahead of me where I'm committed elsewhere, so it'll be just ideas until the latter part of the next weekend. With a following wind, the prototype may be running inside a week, and taking reports from Southern England. We'll see.

Morse Code Advocacy

This Saturday gone was hard work, but great fun. I delivered a talk and teach-in to forty-eight 10 - 12 year-old Junior Sea Cadets, along with their twelve adult and senior Cadet team leaders at an Adventure Camp. I presented a potted history, a practical demonstration of current amateur radio usage and then let them loose with keys and oscillators. The kids loved it. They were given a copy of the code, on an A5 sheet, and they sent me their names. I divided them into three-member sub-teams and gave one group the task of composing a distress message, while the other three were shown more about amateur radio. When ready, the distressed group sent their message, and the 'helpers' decoded it and responded...

There was a wide spectrum of interest; some Cadets wanted to know if they could talk to aliens, others wanted to know where they could get keys and beepers. I think a couple of them may be hooked, and we might hear them in future when they have their Foundation Licences.

I had to return home late Saturday evening for domestic reasons, but I've had reports that there were flashing-light Morse signals passed from tent-to-tent after lights-out. Seeing how Morse can capture the imagination of these youngsters, I'm wondering if more could be done to raise the profile of what remains one of the most efficient ways of moving information around our planet. I wrote a short pamphlet about it, and I'm considering writing an extended monograph for just this purpose.

As I often say in this blog, more follows...

A New Station - M6BMO

My youngest son, Billy, recently qualified as a Foundation licensee. He studied with the Worthing and District Amateur Radio Club, to which we are both very grateful for their help and support. Now Bill is an M6, he needs a radio, and there lies a problem. I can't afford a 'black box' rig for him, and as a Foundation ham he can't use my homebrews. He can, however, use a radio made from a commercially available kit. Some of Tim Walford G3PCJ's kits ( www.users.globalnet.co.uk/~walfor ) are suitable, and maybe I'll buy Bill one for Christmas. Until then, I'll simply have to become a vendor myself!

What I have in mind is a simple kit based on Billy's immediate desires, and which may prove useful to others in the same boat (impoverished, Foundation licence). He wants to use 20m, and cw. This fills my heart with gladness; 20m is my favourite band, the rig is technically unchallenging and can be made in a variety of ways. First thoughts are with either a Pixie II clone or a Sudden / VXO / Pebblecrusher mix. What ever happens, I hope to have a prototype going quickly, and a kit advertised soon after. Then, Bill can build a kit bought with his pocket-money (I'm hoping it'll be that cheap), and get going.

Somewhere new to rest my case

New website!
Yahoo! Geocities is due to close its ports later this year, so I'm migrating the best bits of http://uk.geocities.com/egnaro937@btinternet.com/index.html to a Google Site - http://sites.google.com/site/g1inf4u/home . I've already added a page about the Poundshop DC receiver, along with basic info on the 24MHz theremin and the closed-circuit RF trainer projects.

The new site will take some little time to build, but I thought I'd give a heads-up on the move.

Closed-Circuit Radio

I've wanted to do this for some years, and I've finally taken the plunge. All the non-radiating radio network trainers I've come across have used some form of multi-port telephone system, where the students and tutor are connected together and may talk amongst themselves. They have PTT switches, headsets and boxes full of electronics, but they lack the one thing real radio networks have - a radio spectrum to explore.

I've prototyped a closed-circuit radio network. It's a very basic DSB (dual-sideband) system, using only three active devices in each 'station'. Little effort has been put into emissions control, because there are no emissions. The network is contained in a loose web of 75R TV coax, and the signal levels are tiny.

I've used my favourite integrated circuit, the Philips SA602 mixer-oscillator. This eight-pin device contains all you need to make a basic DSB 'modem', able to modulate and demodulate voice onto a RF carrier which is produced by its internal oscillator. Frequency control is by a ceramic resonator, whose resonant frequency is changeable by adding capacitance between it and ground. This 'pulling' of the resonator's frequency is analogous to a blues harp player lowering the note of their harmonica by increasing the volume of the mouth on some of the 'draw' notes.

Controls are simple; PTT, tuning and AF gain. Power for the student stations comes along the coax, and is segregated from the signals by a simple inductor / capacitor diplexer (much like a HiFi speaker crossover filter). I plan to add a couple of feature to the tutor station, namely a variable noise level for the system, and an interference simulator (piped in from an MP3 player?) to add realism.

The really big thing here is the necessity for each student to learn how to tune their radio, netting-in on the signals they hear, and maintain the setting. It is quite possible (and may even be useful) for several nets to exist simultaneously, given that several tens of kilohertz of bandwidth are available. Multiple tutors could train the students; interfering, breaking-in and calling.

So why bother? The system outlined here would cost little more (if any more) than an AF system, has much more realism, and would be FUN to use. People who require controlled experience of HF communications without actually using our precious ionosphere before they are qualified would benefit greatly.

Once I've hardened the design, I intend to publish it, and perhaps provide kits for interested parties. The photo shows a matrix-board prototype which offers some idea of how a PCB layout would look. It measures 37 x 70mm, and it should be noted that the microphone socket is missing from the bottom left. I have successful prototypes constructed on stripboard and ugly-style over a copper-clad groundplane. I plan to publish layouts for stripboard, and downloadable Gerber files for those who wish them.

No promises on time-frame here. That's one of the pleasures of doing electronics as a hobby, and doing other stuff for a living!

Forty-Metre SSB Superhet Transceiver

Yet another HF superhet rig

I like doing unusual radios, so when I decided to produce yet another HF superhet, I felt compelled to give it a twist.

The current project uses two SA602 mixer/oscillators, and a DPDT relay to swap the signal path around to take the rig from receive to transmit. I used the fortuitous layout of the SA602's inputs and outputs, along with the equally serendipitous pinout of the miniature relay to form a very compact and efficient SSB 'modem'.

So far, I've managed to get it running as a receiver, and added an AGC circuit to soften the blow when a strong signal is encountered. The AGC is pretty standard; just a charge pump and capacitor driving an NPN transistor, but the control devices are unusual. I've put a pair of red LEDs in a stack from the +12V line down to the AGC transistor, and coupled them into the RF input to the first mixer. As well as smoothly attenuating the signal, I get a pair of signal-strength LEDs as a bonus. Two birds, one stone (well, two bits of silicon).

I've housed it in a pair of 2oz tobacco tins, mounting the circuitry in the lids, which I've soldered together. This makes it compact and lightweight; ideal for backpacking. The photo shows an early incarnation, before I boxed it. The two mixers, the relay and the three-crystal filter are at the top. The fourth crystal is for the BFO / carrier, and the 12MHz ceramic resonator at left is for the 4.915MHz / 7.1MHz conversion. This choice of IF meant I could terminate the filter directly to the 1500R ports of the SA602s, and still maintain great passband shape even with just three crystals.

A simple direct conversion receiver

Back to my roots - simplicity
I tried a little experiment. I hooked-up my Poundshop receiver (Google it...) to my netbook, and fired-up the wonderful SAQrx Panoramic Receiver software. I found I could use SAQrx as a digital signal processing AF subsystem, and tune-around inside the Poundshop's gigantic audio bandwidth. I was satisfied with the results, but the instability of the Poundshops's 7088 chip meant it drifted too quickly for comfort when the passband was narrowed. I needed crystal control, and a fixed reference.

Why use batteries?
Most of my projects run from 12V lead-acid gel-cells, or 9V alkalines if low-powered. The current project has two features; a single active device with a 4.5V - 8V supply range, and a close association with a small computer. Why not use the power from a USB port? It's 5V, and capable of supplying 100mA (more if care is taken). So, I found a redundant USB cable, an old earbud cable (the kids tread on the earpieces!) and scribbled out a plan on the Schematic Editor.

My favourite chip
It had to be a SA602 mixer-oscillator. These have poor big-signal handling, but are very easy to use. I attached a 7030kHz crystal, a smattering of capacitors and a loop antenna (one metre length of four-core cable), and hooked-up the computer power and signal cables. It worked as soon as I'd turned the SA602 over; a blunder which deserves it's own paragraph...

A creature of habit
Nearly all my projects are produced in 'ugly' style, where chips are turned-over, and used legs-up. So, when I came to use a 'familiar' chip in a piece of stripboard, I reversed all the connections. Happily, the SA602 survived this abuse; I simply desoldered it and mounted it in the solder-side of the board. Success!

A little deaf, but it works
It's a starting point. I may try to improve the antenna, and / or add another SA602 as a RF amp stage. Because the antenna and mixer inputs are balanced, this is an ideal choice. I get an extra 15 - 18dB of gain, controllable by applying a bias voltage to the oscillator emitter pin (6). This works well. I have a home-brew guitar amplifier which uses this scheme as a tremolo effect, with pin 6 fed from a sine-wave generator via a 'level' pot.