Simple circuit selects LEDs?

I was puzzling over the classic bistable circuit and wondering if it was possible to expand it to any number of channels.

TriStable 640

And this is what I came up with.

BTW: To make it more interesting (claims I), it is modified to light one of three LEDs by pushing the appropriate push button.

I have no idea if it works as I have no time to make it, model it, or indeed ponder it much πŸ™

Is there a glaring error?

Could some kind soul with Spice have a go?

The idea is that whichever channel is selected steals gate drive from the other mosfets via the diode NOR gates.

It relies on the mosfets being pretty well ‘off’ with 0.6V on their gates, and properly ‘on’ with 3V (the drop of a lighting led) on their gates.

By way of a confession, I did try to think up a circuit with npn transistors, but gave up because I kept ending up with diodes in the emitters (wasting LED power) or using Schottky diodes for the diode logic NOR gates which seemed marginal to say the least – any ideas?



  1. Blast from the past !! I used to build multi-astables using leggy OC 71 & OC 72 germanium PNPs on my solderless S-Decs. Three stages made happy flip-flap-flops, four lock-stepped, five had a runt pulse, six lock-stepped etc etc. I didn’t get to a big enough ring to have ‘chase’ sequence…

    • Hi Nik
      I had to look up flap-flap-flop πŸ™‚ A ternary logic latch? How cunning.
      About a million years ago I also used OC71s in a little white-line-following robot that I never got working – not understanding that a triple Darlington is pretty much on all the time with germanium transistors, due to leakage. Oh well.

  2. Hi Steve.
    I sort of gave up.
    Rather than make a pcb, I made the something similar with a bunch of toggle switches.
    So, job done, but the boring way :-/
    Inspired by your analogous way, I did toy with using a single PIC – but am too rusty to write code quick enough !


  3. My own instinct is to just do it with flip-flops, but if one is compelled to do it with discrete parts, then two stages of transistors might be a way to do it. Not sure if you can get away without a lot of diodes or resistors, though.

  4. Hi Steve – are you saying I should have another think because there is a simpler way of doing it out there?

    Hmmm, I shall have a think.
    To try it out, I started to lay out a 5 way version over the weekend.
    It has n(n-1) diodes – and it is quite hard to fit 20 diodes onto a small PCB, it turns out.
    Also, I am failing to find a 6x6mm tactile switch model in Eagle PCB – anyone know which library it is in?

  5. I’ve seen something analogous in a brushless motor. It used a somewhat complicated array involving gate drive mosfets in addition to the mosfets that energized the motor coils. It’s proprietary, so I can’t share any of it, but it was simpler (in # of components) than what you’ve sketched out.

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