2009-01-02
Discrete Ring Counters
For a long time I've wanted to build a clock. It seems to be a bit of a rite-of-passage for electronics geeks. Sure, I've programmed microcontrollers to implement clocks, done the charlieplexed display thing, but I've always wanted to build a clock that is somewhat unique. Ring counters seem like a good way to implement a simple clock, although I'll admit it is a lot of work to build the individual stages using discrete flip-flops. The idea stuck me to use the Silicon Controlled Switch to implement the latching cells for each stage of the ring counter, no doubt because the four-layer devices have been something I've been playing with of late.
The SCS was once available commercially in four-leaded TO-92 packages. Now days these are rather rare, but like the UJT they are easily synthesised using a complementary pair. It turned out that what I eventually used was more like a SCR than a true dual-gate SCS, but both gates of the SCS could be arranged in a similar and perhaps more elegant circuit...
Spice was helpful in modelling, but the practical circuit taught me a lot more about making it count robustly. In particular the gate biasing voltage should be fairly low, near mid-rail works well which is achieved simply enough with an emitter follower biased with equal resistors. The entire circuit can be built with just 10k and 100 Ohm resistors (actually their values aren't critical), the current limiting resistor (R8) is picked to match the supply voltage and desired LED current. C3 is different to the other coupling caps just to make the simulation happy, in the practical circuit this is not required. Similarly with R14 which is not strictly required, but is a good idea even in practice.
Each stage is identical and you can cascade as many as you want, in theory. It can count correctly up to many tens of kHz (with the values shown) which is plenty for a clock. Here is a video of the prototype 4-stage circuit. I am clocking it with my function generator.
Note that this is very much like the old-fashioned idea of using specially built Neon bulbs to implement ring counters. I dimly recall seeing an old magazine article about building a "computer" (automated adding machine really) using Neon ring counters and a decadic dialling mechanism from a telephone to generate the clocking pulses. It used 9's complement to allow subtraction, very clever really, logic completely without Silicon (well it used Silicon diodes, but it could have used vacuum diodes or perhaps Selenium).
Now, on to designing a clock using this basic building block...
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Attachments
| title | type | size |
|---|---|---|
| LT Spice Model of Discrete Ring Counter | application/octet-stream | 6.269 kbytes |