Reduction by mirroring

A project log for Clockwork germanium

A retro version of Yet Another (Discrete) Clock, with vintage parts

Yann Guidon / YGDESYann Guidon / YGDES 03/02/2020 at 02:280 Comments

The previous logs Pulser-sequencer and Towards a smaller shift register try to reduce the complexity (and parts count !) as much as possible but the last circuit was still pretty daunting and looks overkill. I couldn't figure out what was feeling wrong until... I tried to reverse the circuit, and that would save a whole layer !

So the new version both does the sequencing AND pulls the pass transistor bases. In the real/final version, the only transistor per bit will be a NPN, and it saves a bunch of auxiliary parts.

I added another transistor (+1D1R) at the start to format the input pulse length and the curves show

  1. very short pulse (it works nicely)
  2. long pulse (it works well too)

So far the cost is 1T 2D 1C 2R and the pass transistor would have the pulse-shortening circuit...

So the circuit is completed with the 2K2 pull-down replaced by the pass transistor and its RC cell. Here is the circuit :

My inventory shows I have a quantity (tbd) of 5K6 and about 100× 2K4. The pulse width ratio is given by the ratio of the capacitor values. Here we have 1µF / 220nf so it's approx. 1/4 to 1/5. This provides a niiice clean edge ! If I find a large quantity of cheap non-polarized capacitors at about 1µF, I'll put 2 or 3 of them in parallel on the sequencer side, and only one on the pass transistor.

The power consumption is still very low when idle, and it can work for the slow parts : seconds, minutes, hours.

Note that for the sake of evaluation, the "pass transistor" here is a NPN with 1K to the positive rail but the final circuit will have the polarity reversed and the pass transistor (a PNP OC70) will not be tied to ground but to the other latches. I expect that the sequencer-side OC139 will work well.

The cost so far : 1P 1N 2D 2C 5R (11 parts, ignore the 1K pull-up)