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555 timer Relay driver with TTL input

licketylips

New Member
Hello,
I want to drive a 12v relay off a 555 timer after being triggered by a 12v PIR sensor. The sensor has a 3.3V TTL output which goes HI when the sensor is triggered. Once the sensor turns off the TTL output returns to 0V.
-I want the Relay to turn on for 3 minutes (can be adjustable) after being triggered.

I think I need a monostable circuit & I know the trigger needs a low input, but I'm unable to figure out how to put this together.
Is anyone able to fill in the gaps or provide a known working circuit?

20200523_093742.jpg

Thnaks in advance,
M.W.
 

gophert

Well-Known Member
Most Helpful Member
The blue and yellow highlited parts are optional. Those allow the PIR to retrigger the 555 and keep the relay active 3 more minutes for each PIR excitation. Without them, the relay will turn off after 3 minutes. And, once off, it can be retriggered for the next PIR excitation.

4D593BAB-FE90-4089-87D0-3550DB0EC536.png
 

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