Re: 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.

I hope you can come up with something for me.

John Fields wrote:
On 30 Jun 2006 01:34:42 -0700, "Hodgey"
<mark.hodgkinson2@xxxxxxxxxxxxxx> wrote:

Hi everyone,

I've got a burglar alarm that works off a 12V transformer. The only
trouble is when the alarm is activated via the system it doesn't stop
until the system code is entered into the keypad.

I'm wanting a circuit I can cut into the 12V siren feed (from the alarm
unit) that will shut the alarm off after 20 minutes but will re-set
when the voltage feed from the system is finally lost.


---
What DC supply voltage do you have available in the alarm box and
what does the signal to the siren look like?
---

It has been
along time since I looked at electronic circuits so any advice would be
greatly appreciated.

I have worked with 555 timers before. Could a monostable 555 timer be
used for this application?

---
20 minutes is a long time for a 555. I'd go with something based on
a 4060.
---

I'm looking for a not too overly complex solution that I can easily put
together myself using breadboard.

---
I'm thinking a couple of chips and a relay, maybe...


--
John Fields
Professional Circuit Designer

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