ard at p850ug1.demon.co.uk
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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