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This is a bit of a silly question, and I realize that. It's hard to imagine why one would use relays for this purpose, especially given relay bounce / etc. Nonetheless, it's an interesting exercise, if nothing else.

Any suggestions for how to go about implementing a Muller c-element in relay logic? Ideally, "pure" relay logic, as in no diodes / etc.

(For those who don't know, it's a latch variant. Output goes high when both inputs are high, low when both inputs are low, and stays the same as it was otherwise)

The best I've come up with thus far is this:

enter image description here

It has a few "amusements" though. Let's see:

  • R1 is required for the LL(L) -> HL(L) transition, as the first relay must switch before the second
  • C1 is required or else the second relay toggles off during the blip when relay one switches during the transition from HH(H) -> LH(H).
  • The output glitches slightly (to ~1/2 a volt, in this case, but the numbers are wildly wrong) during a LL(L) -> HL(L) transition if the circuit was previously in a state with output high due to C1 being connected to the output.
  • I have no idea how to size R1 and C1 beyond empirically "as small as possible".
  • It glitches if the output is too inductive. Rather interestingly, too.
  • It might glitch if the input is too inductive.
  • It has no snubbing across the coils. (Well, aside from C1)
  • It uses 1=high 0=low logic, whereas some things are easier with 1=high 0=floating.
  • It uses two relays, whereas there's only one bit of info being stored.
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After some experimenting and such, I found a 1-DPDT-relay (one single-throw pole and one double-throw pole actually needed) implementation that works much better:

enter image description here

Component values are approximate, and the output resistor approximates an output relay.

It still has some glitching if you're not careful, and the output capacitor needs to be sized depending on the number of outputs its feeding, but it works and is much simpler.

Oh, and it actually works (read: I breadboarded it.)

It does have the downside that the relay can't be diode-snubbed.

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