I want a design for a circuit that triggers a buzzer and/or flashes a
light after a random interval. It has to be very cheap to build and
should be simple. It should be powered by a small battery. I suspect
it could be built out of a few TTL components.
Here's the spec. A press button starts the 'timer' and after a random
interval the circuit operates a buzzer for, say, 1 second. The circuit
then resets.
The lenght of the random interval should be more than 15 seconds and
less than 3 minutes, approximately - these are not critical. HOwever
I'd like to know the equations for the component values so I can
calculate different parameters for the delay.
It does not matter if the randomness is not truly random e.g. if there
are, say, only 10 or so actual intervals and the circuit simply
selects one of them. There must not be an easily recognisable sequence
however.

the simpler and cheaper the better!

Thanks

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Clarification of Question by playitagainsam-ga on 15 Jan 2004 02:05 PST

to answer this, a circuit schematic would be ideal but if that's
problematical to post, maybe there's a similar circuit on the web
somewhere you could give the link to, then add some notes to describe
how the solution would differ? It's the main principle I'm after.
If it involves a 555 timer chip I can probably figure it out from a
clear description. If it doesn't I might need a bit more guidance.

Thanks!

Hi there Playitagainsam,
I am sure I could give you what you would like, "if" I could quickly
figure out how to send you the schematic.

Regards,