I've done some modifications from the circuit I posted the other day in any other business... basically, turned the whole thing upside down so it can have an active high count which I need later. I've never used an SCR so any advice gratefully received; the device in question has a current limit of 200mA; trigger current of 200uA, trigger voltage of 0.8v.
Here's the theory:
At power on, none of the scrs are turned on because the gate drain resistors R1, 4, 7 etc keep the gates off. A high pulse via R32/C21/R11 triggers SCR1 on. At that point, LED 1 is lit and the voltage at the cathode of SCR1 is close to the positive rail. Along comes a positive going trigger pulse. At every SCR which is currently not conducting, it finds that it has a clamp to ground via the LED and limiting resistor (hmm, might need to move the LED above the SCR?) but at SCR1 it (a) takes the cathode above the anode (which is somewhat below the rail due to R31) thus turning it off, and also driving through the coupling cap C3 and turning on SCR2.
The more I think about it the more I think the LED has to be above the SCR...
And as for values - I haven't a clue. It needs to clock at 50H, so C1R1 has to have a time constant somewhat less, and I think R3 should be lowish compared to R2 - R2 is easy, of course, it's just the usual LED current limit, in series with R31. R1 can be quite high, I think. And I'm not sure quite what C2 is doing, save that there was one in the design I modified :)
Assuming a 15v rail (because I have a 15v power supply!) and 5mA for the LEDs gives a total load resistance of ~12/5ma ~= 2.5k - so let's make a guess at 2k load resistor R2 and 470 for the standoff resistor R31. Say 470 again for R3 and - finger in the air - 10k for R1? Sound sensible?
It's been too long since I did analogue...