Why I Like Transistors by Philip Miller Tate

from BAEC Newsletter No. 128 June 1998

Anybody who read my last contribution to this august journal may have got entirely the correct impression that I collect cheap transistors like some people collect stamps. In fact, my father was a keen stamp collector and I never could fathom out what he got out of it until I discovered that circuit building was nothing like as relaxing as a happy evening spent with a magnifying glass, a data book and 100 mixed small-signal silicon transistors... but there is more to it than that.

Some years ago I realised that the major problem with electronics as a hobby was the time delay between being enthused by a circuit and actually getting to build it. Everyday Electronics falls on your doorstep on the first Friday of the month. You open it and there's some really clever little gadget to build, so you grab your catalogues and start looking up every little resistor and veropin. Three days later the order goes in the post, except for the specialist IC that you need to get from some other supplier. Surprisingly, the latter arrives by return but the main order takes ten days to come (waiting for your cheque to clear). Eventually, it all arrives and you fire up the soldering iron only to discover that the lµF axial electrolytic was out of stock and you can't solder in a credit note for 9p instead. A week after that you have all the parts. It takes two evenings to build the circuit but you can't drill the box until you can get into the shed at the weekend. When you finally complete the job it stubbornly refuses to work. It stays like this until the next issue of the magazine falls on your doormat, complete with the Errata listing the mistakes in last month's circuit. By this time it's all you can do not to throw it in the bin.

By the early '80s I had a job and some disposable income and so I decided to overcome some of these problems with a small stock of 'standard' components, things that appeared time and time again, like 10 of every resistor from 10R to 1M, some electrolytics, polyesters and, of course, some BC108s. I was spurred on by those American electronics books from 'Sams' that filled a small corner of the library when I was a kid with 101 circuits that promised they could be built with "...parts from your spares box...". Gradually I became aware of companies like Bi-Pak, Bull and Greenweld that sold components cheaper than fullspec devices and collected a few transistors together, and even some ICs such as 741s and 555s-everyone seemed to use them. Eventually I reached the point where about 90% of simple circuits could be lashed together in an evening on veroboard - read it, build it, play with it. Some of those things are still around, some got taken apart almost as quickly, but my understanding and knowledge could now advance on a daily, rather than monthly, basis and the hobby now seemed much more fun.

Ten years went by and I started to run out of ideas. I began to kick myself for having given permission to my parents to dispose of my old 1970s copies of Practical Wireless and Practical Electronics (at least they were sold to a fellow enthusiast and didn't go on the bonfire). I could remember all sorts of fun circuits-radios, little amplifiers, those skin-resistance 'lie detector' circuits, games, and so on-but not the actual circuits! I had a desperate need to revisit my childhood (with my son for an excuse) and actually build the circuits that I had merely admired when I was a lad (hey, two 2N2926Gs were a week's pocket money then!). A few old copies had somehow accompanied me on my travels and had turned up one or two nice little circuits. Where could I find more?

One day I recalled that when I had lived in Northampton many years before (in the days when I worked for Plessey), I had spent many happy Saturday mornings going through Northampton library's excellent collection of old electronics magazines. It gradually dawned on me that I now lived quite close to London and the British Library, so I called up their Web site and searched for a stock of old magazines - yes! There were Practical Wireless and Practical Electronics virtually complete, and even some ETI and Elektor (not my favourite mags, though), but sadly no Everyday Electronics prior to their merger with PE (anyone know where we can find them?). If this article interests you enough to follow in my footsteps, you need to visit the British Library Holborn Reading Rooms at 25, Southampton Buildings, Chancery Lane. Telephone 0171 412 7494 / 7496 the day before and ask for whatever volumes of PW or PE you want. They won't bat an eyelid if you were to ask for a whole decade of either, but be realistic in what you can get through in a day! The references are (PW) HOLBORN:(P) RU 10-E(2) and (PE) HOLBORN (P) RK 01-E(2).

I would hope (but am not sure) that other city libraries might also have stocks or be able to obtain them, and I know that some University libraries are also well endowed and do not turn away individuals. Photocopying articles is slow and unreasonably expensive (especially at the BL!) and so I took an A4 notebook, pencils and eraser and sketched interesting circuits by hand. Leave space to add your own construction notes. In two visits totalling 14 hours I transcribed 43 circuits of interest from the two journals throughout the 1970s. I noticed that the journals, though well-bound, were in poor condition and would be incomplete but for photocopied sections. Maybe someone will donate a better set one day! I also noted how rarely they are accessed, and hope that the BL doesn't declare them to be of no further interest and dump them (maybe if others made use of them ... ?). Incidentally, I found that 1960s PW, while interesting, were mainly valves and 'chassis-bashing' and the layout was difficult to read. By 1980, the magazines were well into ICs and (horror of horrors!) computers, and so the 1970s are definitely where my interests lie.

There are some great circuits and ideas in those old journals, which are just as relevant to hobbyists today as they ever were. The all-transistor circuits, returning to my theme, were the most useful. I found a range of radio circuits, most of which I have got to work, but there was one three-transistor TRF receiver circuit which promised to equal the performance of the 'ubiquitous' (and now extinct) ZN414 chip, and that claim seems to be only a slight exaggeration! Maybe our children will return to circuits like that since the demise of the '414 which has ranked supreme for the last two decades. What struck me, though, was the number of circuits that started to appear in the late 70s featuring ICs that I've never heard of and that the world will never see again. With a few obvious exceptions, ICs just don't seem to have the staying power that discrete transistors have demonstrated.

As I pointed out in my last contribution, it takes a bit of shopping around to find real bargain transistors, but they are out there. With Greenweld having just announced (April) their third 'One Million Transistors' clear out from Zetex, maybe this article is a little late, but I don't think so. (If you did take part in that bonanza and have access to the Internet, http://www.zetex.com will provide data for most of their devices.) Don't be fooled by the large range of component numbers listed into thinking that transistor types are very individualised; most circuits can use 'general purpose' silicon NPN transistors and any device of this type with a gain of 200-300 or so and a collector current limit of 100mA will do. A few PNPs of similar specification will be needed, but nothing like so many. Some higher power devices-say 1-2A collector current and dissipation-will prove useful. Curiously, most BF-prefix devices, intended for radio frequency operation, tend to have lower cut-off frequencies than modern BC-prefix devices, so it seems unnecessary to collect RF transistors unless you intend to work at UHF (but I have a few 600MHz devices stashed away). This probably just reflects improved device manufacturing standards. Small power transistors are occasionally available as bargains and are worth snapping a few up, but check the type numbers - PNPs are again much less useful, and consequently cheaper and more widely available, than NPNs.

Power darlingtons are great for things like motor speed controllers driven from op-amps. I find JFETS great fun to play with-all those touch switches and proximity detector circuits-but for devices which in my youth were widely tipped to replace bipolars completely, they are remarkably rare these days (well, not really, but they're all inside ICs). A typical bargain price would be about 20p (or rather, five for a pound) which is a considerable saving on 35p or so for the 2N3819. (Hint: ring Bardwell's on 0114 2552886 and ask for 2N4093s.) You can have a lot of fun experimenting with JFETs (they make for good radio front ends, too). In recent years I've seen several published circuits using expensive MOSFET op-amps to do what my notebook tells me can be done with a JFET and a bipolar or two. To give you an idea of what can be achieved, the March 1998 edition of Everyday Practical Electronics featured a project called 'The Handy Thing', which is basically a gadget for defining positive and negative polarity electric fields using a JFET biased via a bipolar, and it can be used for detecting current flow, a.c. fields, static electricity, measuring resistance, testing batteries, fuses, capacitors, diodes etc.; in fact, it's the nearest thing to Dr Who's sonic screwdriver that I've ever seen published. EPE presented it as a 'surface-mount' device project and costed it at 17. I built it on veroboard (I wasn't about to pay 6 for a one-inch by two-inch PCB) in one evening with discrete devices, and fitted it in the same box, for about 3.50-and 1.50 was for the box!

I hope that some of you will be grinning by now, because you agree with me, but I hope further that even more of you will be beginning to question the current wisdom of what we should be constructing, and paying, to pursue our hobby. Take advantage of the industry's wasteful ways, and encourage others, especially newcomers to the delights of electronics, to do the same. I believe that the new millennium could see a resurgence of the popularity of our hobby after the low profile it has had in recent years. I have a dream...