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Alarm Power Cut-Off Circuit

Hi everyone,

I've got a burglar alarm that works off a 12V DC transformer. The only trouble with the alarm system is when the alarm siren 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 power feed (from the alarm unit) that will shut the alarm off after 20 minutes but will re-set when the voltage feed from the unit to the siren is finally lost. 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?

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

Thank you for your time.

Regards, Mark Hodgkinson

Reply to
mark.hodgkinson2
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On 2006-06-30, snipped-for-privacy@googlemail.com wrote:

yes, splice this into wires running to the siren, it'll do what you ask. _o-------------> to siren from alarm 12V DC _-~ >---------+---+--------+---------------o~ : o--- | | | _:_ | | | .-------|\\ | relay coil resistance ===== | . . . .|. . . . | | \\ | more than 60 ohms 220uF | | . VCC(8) . | .--| \\| | | . . | | ~~~ | +-RES(4) OUT(3)---+-| | . . | +->|--+------TR(2) CV(5)-- | | | . . | | / . GND(1) . | | \\5.6M . . . .|. . . . | | / | | | | | | >----+-----+------------+-------------+ the diodes are 1N4000 (or similar eg 1N4004) With the components shown you should get about 20 minutes delay before it cuts off. (you can test this with the siren disconnected) pick a relay with contacts rated for the current the siren draws, an automotive horn relay should be suitable (I think they have coil resistance about 120 ohms) but you can probably get by with a smaller relay,

use an bipolar 555 like NE555 or LM555 not a CMOS part like TLC555 or LM7555 as they don't have the strength to drive the relay

Bye.

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Bye.
   Jasen
Reply to
jasen

Certainly, the 555 should do fine.

By the way, depending on your local laws you may have to have a timed shut-of in your alarm to protect the neighbours. Here in Germany an alarm may sound only for, IIRC, 2 min, unless re-triggered.

Reply to
Dr Engelbert Buxbaum

This will do what you want:

formatting link 

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell 

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20 minutes = 1200 seconds


     T = 1.1 RtCt = 1.1 * 1E6R * 220E-6F = 242 seconds
Reply to
John Fields

Yep, my mistake. Took some head-scratching to sort. I had calculated the value, entered it in my simulator, double-checked that the circuit worked, and noted the time was something like 1150 s. But I forgot to change the *schematic* from an earlier value of 1M. In the 'Label-Value' box for Rt's Properties in CircuitMaker I'd entered '4.7meg', as the syntax unfortunately requires, and unchecked 'Visible', so it slipped through.

Well-spotted!

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Terry, West Sussex, UK
Reply to
Terry Pinnell

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