relay logic

Tony Duell ard at
Tue Jan 3 18:20:04 CST 2006

> I'm quite well aware of how to make basic gates, FF, counters etc out of
> relays but I'm curious about anyone elses experiences.  I can contemplate 
> all kinds of weird problems like mechanical shock from so many relays 

I've never heard of that being a problem. None of the books I have on 
telephone exchanges (which used thousands of relays) mention it.

> clicking at once, or the relay equivalent of contact bounce, but I don't

Countact bounce shouldn't be a problem either (well, maybe if you're 
interconnecting very different relays, trying to control a high-speed 
reed relay from a much slower type thing...). The response time of the 
driven relay won't be fast enough to respond to contact bounce.

Be warned if you look at some of the telephone exchange circuits that 
they used (at least in the UK) some odd relays. Not only 
make-before-break changeover contacts, but also slow-energise and 
slow-releasy relays (copper 'slugs' in the coil was one way to get this 
IIRC). You may have problems finding that.

Incidentally, I made a relay flup-flop without realising it a few months 
back. I had a pair of contactors (high-power relays, basically) that were 
mechnaically interlocked so they couldn't both close together and 
electrically interlocked so that the 2 coils couldn't be energised at the 
same time. The latter was done by feeding each coil through a 
normally-closed contact on the other contactor.

Anyway, if I turned on the supply to one contactor (let's call it 'A'), 
it pulled in. Turning on the supply to the other one ('B') did nothing 
(since the current path to B's coil was broken by the NC contact on A). 
But now if I turned off A's coild supply, A dropped out and B pulled in. 
I could then turn on the supply to A again and nothing happened until I 
momentarily opened the supply to B.

Of course this is exactly what was supposed to happen and I fairly 
quickly relaised it was logically equivalent to a pair of cross-coupled 
NOR gates -- the classic Eccles-Jordan circuit.


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