Timed Power Cut-Off Circuit

John, Thanks for your prompt reply which is appreciated.

To fill you in on the problem I've got here's the story:

To provide tamper protection the actual external siren is controlled by a relay in the external alarm box (i.e. if the wire to the box is cut the relay drops and a battery in the box powers the siren). Trouble is, this circuit no longer works because I'm not getting any power from the main alarm control box associated terminals to keep the relay up so the siren sounds continuously.

Because my system is over 10 years old, I don't want to get a new system on the grounds of the above system fault and the fact that it is virtually impossible to get into the alarm box without great difficulty, I've decided the tamper feature isn't critical and I want to power the siren direct from a separate set of terminals I've found that give around 13.66V across them when the alarm is activated.

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Sorry to have confused you. I think I'm telling you more than is necessary so I will try to be percise.

The siren did run continuously because of some problem in the control box. To get over this problem I have done some re-wiring and have directly connected the siren to a set of terminals which give a +12V on alarm activation. Previously the siren was connected via a tamper relay which activated the alarm when the relay was down (i.e. power was lost). This relay was powered via a different feed from the control box. It is this feed that is faulty.

Anyhow, the circuit I'm wanting is simply a timer circuit which can be cut into the 12V siren feed. The circuit needs to provide a 12V output to sound the siren for approx 20 minutes after a 12V input supply is provided and then cut-off (i.e. provide a 0V output). The timer should be re-set when the 12V input feed is lost (i.e. after the code is entered into the keypad).

The basic problem I've got is that my alarm doesn't stop until the code is entered but if no one is home it may run for days if not weeks! There is a possible fine of up to =A32000 ($3700) if an alarm runs more than 20 minutes.

Regards, Mark