I am looking for a simple alarm circuit, trip a wire and alarm sounds, but with a timer that stops the alarm after 5 minutes or whatever can be set.
I have seen dozens of them online but none with a timer that allows the alarm to stop automatically. Anyone have such a circuit laying around?
A fully controlled alarm with an adjustable preset time given by the user can do with a microcontroller. It can turn ON the alarm and OFF the alarm automatically by the given time.
But this is getting complicated for you.
What about a timer done in a simple timer IC like 555. There are a large number of circuits on the NET regarding 555 timers. Did you search?
If the wires too long you'll notice two interesting effects:
One: the capacitance in the wire will cause it to trigger on power on.
Two: it will trigger randomly as the noise picked up by the cable will cause false triggering.
To stop this:
One: connect pin 4 to +V via a 100k resistor and to 0V via a 100nF capacitor.
Two: add a 100nF capacitor across the trigger.
The 100nF capacitor on the trigger will filter out any noise and the 100k and 100nF on pin 4 (reset) will stop it from being triggerd for a short perioud after turn-on.