Re: Timed Power Cut-Off Circuit

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.


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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
.