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.
  • 1
    \$\begingroup\$ This question just warmed my heart. I thought nobody else would ask such a thing. It's not a silly question at all. It takes some real work to get a robust Mueller-C with relays. R and C elements get down to sensible sizes and dissipations for 750ohm coils and up. Much below that and things get out of hand. \$\endgroup\$ Jul 31, 2022 at 3:09

2 Answers 2


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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    \$\begingroup\$ Relays aren't really resistive loads, at least not during turn-on. At the moment you apply the input voltage, they are open circuit, since the current across the coil starts from zero. A usable model for a relay coil is an inductor with series resistance. Per my measurements of a stock of several models of relays, the L/R electrical time constants for "small" PCB-mount relays vary between a fraction of a millisecond to several ms. \$\endgroup\$ Jul 29, 2022 at 5:51
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    \$\begingroup\$ @Kubahasn'tforgottenMonica - this design assumes that your output relay is snubbed. You can often (badly) approximate an appropriately-tuned overdamped RL||RC as a resistance. See e.g. tinyurl.com/25l5s79j . (Though this has other problems...) \$\endgroup\$
    – TLW
    Jul 31, 2022 at 1:50

If you want to use the C-element in arbitration / asynchronous logic, then a so-called MrGo circuit - a Seitz arbiter with an anti-metastable element - is a much more robust solution. All async arbitration relay logic needs anti-metastable elements, otherwise it's glitchy exactly as you describe.

Below is a tested 2-input Seitz arbiter design for armature relays. Diode snubbing is not recommended, since it slows everything down. R-C snubbing is the way to go. On low-voltage relays, the external resistor may be necessary. On most relays, though, the internal coil resistance provides all the attenuation needed, and only an external capacitor across the coil is needed.

There are three copies of this circuit on a card with 36 edge connector terminals - thus the STOP contact outputs are paralleled, as there wouldn't be enough pins otherwise :)

I've ran this circuit for about 200E6 cycles at 50Hz using 3-decade-old NOS Japanese armature relays whose contact life was rated only for 1E6 operations. It ran without mis-arbitrations throughout the entire test. Eventually I just got tired of the buzzing and turned the test bench off. For all I know, these things would last a billion cycles.

The particular relays I'm using are 3V, 0.5W coil models, last made two decades ago. These are not amenable to "capacitor based" solutions. I've tried: the necessary capacitors run into millifarads. The coil resistance is 18 ohms... It will work fine on higher coil voltage (and coil resistance!) relays of course - just adjust the snubber values and LED dropper resistors.

The schematic of a Seitz Arbiter / MrGo

The design of the arbiter is a reimplementation from CMOS after Marly Rocken & al., after Seitz's arbiter. The Asynchronous Research Center at Portland State University, founded by Marly Roncken and Ivan Sutherland (yes, that I.S.), is chock full of materials that can inspire asynchronous relay designs - lots of excellent published work there.

The first request to come in "wins", and gets granted. While either request is active, if another request comes in, it gets deferred until the original request is removed.

When two requests arrive at once, one gets granted, but it's indeterminate which one. In practical terms with relays, the initial behavior favors one side when both requests are driven from the same source. Eventually, as the relays get hot, both sides of the circuit start to balance out in terms of speed, and the grant selection becomes chaotic/random.

I'm using this one in a larger design of an asynchronous relay computer, where it is used to condition all asynchronous external inputs, such as clock stopping, front panel toggles and test and debug (per Marly Roncken, Swetha Mettala Gilla & al.).

It is a robust and dependable circuit. An NMOS-like reimplementation that uses fast SPST NO reed relays arbitrates coincidental requests in about 300us worst case :)

  • \$\begingroup\$ an excellent response, 7 years later. A pity it uses a billion relays... \$\endgroup\$
    – TLW
    Jul 31, 2022 at 2:03
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    \$\begingroup\$ 6 DPST relays or 8 SPST reed relays + some neon bulbs and a 100V supply :) \$\endgroup\$ Jul 31, 2022 at 2:58
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    \$\begingroup\$ I'm a purist as well, and I don't use diodes at all, other than as LEDs, just to be practical, since incandescent bulbs or neon bulbs as indicators are too much hassle. For ROM and RAM, the switching matrix is implemented with reed sense relays and neon bulb switches - one neon bulb per bit. The address space is 4096 12-bit words, half of it RAM, so when I look for neon bulbs, it's 1k-10k at a time... Also the thing needs a 100V supply for the 5mm neon bulbs, in addition to LV supplies for the relays. But no diodes! Tip: 1kohm coil reed relays run just fine from 100V with a neon in series. \$\endgroup\$ Jul 31, 2022 at 3:05
  • \$\begingroup\$ I try to keep my hobby projects LV if possible. Negative differential resistance elements as memory sounds interesting though! What's the switching time like? \$\endgroup\$
    – TLW
    Jul 31, 2022 at 6:27

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