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I have a simple security/alarm system I built for my carport after a thief took advantage of me. It simply has a strobe light and an extremely loud siren, using photoelectric sensors. When the sensors are tripped, the strobe and siren sound/light up. What I want to do is turn the system off after about 5 minutes once tripped, then be able to engage itself again if need be. Any suggestions on a simple timer? This is a low voltage dc system.

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  • \$\begingroup\$ 555 conected as monostable circuit - it needs about 1mF capacity to reach 5min time constant. Or 555 as astable with smaller time constant (about 0.85 sec) and inceased duration to 5min with counter IC (8bit). Reset at overflow. \$\endgroup\$ – Michal Podmanický May 5 at 22:33
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A monostable multivibration with a time period of 5 min in the astable region should do the trick.

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Follow these instructions :

  1. Change variable resistor( number 1 ) to 1 Mega-Ohm It gives you 5 minuts ON-State then on delay and then it goes to OFF/State (in this state it will be ready for another trigger).

  2. Your trigger is number 2 : You can use a transistor instead of push button key. Connect emitter and collector of your transistor instead of push-button's pins and you have to connect your command signal line to the base of this added transistor (i didn't about your output signal's voltage and i didn't design this part for you bur it's so easy and i think you can do it by yourself).

  3. Connect a suitable relay instead of marked LED & resistor ( remember, you have to connect a common diode to your relay's coil in parallel but in inverse bias, it prevents fly-back surge voltage damage to you transistor and you must install it)

If something in this post wasn't clear for you, talk about it, i will explain more

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