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security alarm modification

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security alarm modification

Postby jac » Sat May 10, 2014 9:30 pm

hi all, i'm new to this forum...

i bought a cheap alarm system that just has an IR beam emitter and reflector and analog circuit with small speaker that sounds an audible tone or alarm siren when the beam is interrupted (probably used in department stores for when someone enters, a tone is sounded). It has an output of 0.5V - 0.8V for a counter (not included) that i'd like to use in some sort of relay, whereby, i can energize a better external siren & flashing strobe light that i am buying separately. This new siren runs on 12V with a current draw of about 200mA. The alarm system can output the 12V too, and i assume the counter feed can handle the 200ma draw from this external siren/light. My questing is: do you know of a component that can input the 0.5V - 0.8V 'trigger'/counter signal (when the IR beam is interrupted) and allow the 12V to run the external siren/light for about 60s then open and return back to an 'armed state'? (similar to a car alarm scenario in terms of 60s time and reset). Currently, this counter signal only occurs when the beam is interrupted but immediately disappears when the beam is free of obstruction...so, naturally, i'd like to have the external siren/light keep running for 60s to help scare away my intruder, even if he only momentarily interrupted the beam.

Thanks in advance for any advice or assistance.

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Joined: Thu May 08, 2014 5:26 pm

Re: security alarm modification

Postby pebe » Wed May 14, 2014 1:34 pm

0.5V is a bit too low to trigger most things. If the minimum was 0.6v and there was some form of current limiting of the source (through a resistor?), then you could get it to trigger an NPN transistor and that in turn could trigger a 555 timer set to give the 60 sec period you want. Even then, a 555 can only source or sink 200mA so you would maybe need another NPN or FET to drive the siren.
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Re: security alarm modification

Postby jac » Thu May 15, 2014 4:20 pm

thanks for reply...

Posts: 2
Joined: Thu May 08, 2014 5:26 pm

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