Relay computers

From: Tom Jennings <>
Date: Fri Sep 24 14:57:07 2004

On Fri, 2004-09-24 at 07:39, Ade Vickers wrote:

> If each relay needs, say, 25mA _at_ 6v to operate, then the peak current
> draw
> of our R80 (as I shall call it) could be around 600A (I think). And
> that's
> before we've added memory, i/o, etc.

Last things first :-) the place to look for this is telco. They did
really for real. Most of them are 48V; it lowers current plus higher
voltages help with the inductive time constant thing (take a look at how
teletype loops are done; HV, a series resistor shortens the *effective*
time constant (eg. the time to reach 60mA in the tty case).

> (either way), so I don't see how you could clock the R80 at anything
> faster
> than around 40Hz; and you'd probably want to drop to 20Hz to be on the
> safe
> side.

First things second :-) doing a z80 might be silly, but I bet there's
architectures more amenable to relays.

Plus, if speed were you goal (probably not :-) asynchronous logic would
likely be better, at least within functional units. Setup inputs to a
net, when the output appears, there you go.

> The same relays quote a typical lifetime of 10e7 operations; so at
> 20Hz, your R80 should last a little under 139 continuous hours of
> operation
> before relays started failing...


Abd yes, wire-OR (or whatever, depending on conventions) is totally

I made a digital clock using relay T-type flipflops to do divide a
1-second timebase (pendulum! not made yet, substituted a 555 for
testing) 2, 4, 8, 16, 32nds of a minute, ANDed to get 60 in the usual
fashion, which drove a stepper 1-of-6, 1-of-10, and 1-of-12 decoders to
drive edge-lit display.

(Don't spoil the fun by pointing out it can be all done with the

You can actually *hear* ripple carry, which is cool.

It uses "sensitive" relays, a few mA at 36V I think. I never made the
pendulum timebase, nor finished the wiring for the display, as I
installed all the flops onto boards (redwood!) in a cardcage, realized
it needs to be re-laid out such that you can see all the relays at once.
I will eventually get around to it.
Received on Fri Sep 24 2004 - 14:57:07 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:37:31 BST