I would like a circuit to come into effect if my Arduino locks up. Is it possible to get a relay that relies on a constant output from the Arduino?
I would like a circuit to come into effect if my Arduino locks up
Do you mean like the processor's built-in watchdog?
I'm not entirely sure what you mean by that? However I am not referring to only software lockups, but also failures in the hardware itself (for example if I just came along and smashed the Arduino with a hammer I would want this other circuit to come into effect).
Yeh, that would stop the watch-dog
if I just came along and smashed the Arduino with a hammer
There are several "supervisor" chips that expect a pulse from the processor and will act if it doesn't appear for a while.
A simple 555 timer can be wired to do the same thing.
Do you know the names of any?
MAX791 MAX6316/17/18/19/20/21 aTPS3600
Should get you started.
Google something like "microprocessor supervisor"
Also look at circuits for a 555 timer in "monostable" mode (I haven't used a 555 for decades but I think it will hold its output low/high as long as it gets trigger pulses).
Another option is to use an ATtiny85 with your own 5-line program (well maybe 10 lines :)) to do the same thing.
Just out of interest, how do you propose to handle the eventuality of your failure detector system failing. Laugh if you want but these sort of problems are typical of what Emergency Shutdown Systems designers have to handle. The simplest way of doing it is by having multiple processors running the application in parallel then voting on a 2-out-of-3 basis. As far as your application is concerned, whatever it may be, a simple watch-dog output is all you need - that's what it's designed for.