555 timer Relay driver with TTL input

Thread Starter


Joined May 22, 2020

I wish 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?
Thanks in advance,


Joined Aug 1, 2013
The 3.3 V output is high enough to turn on an NPN transistor acting as an inverting switch. The transistor collector goes to the 555 trigger input with a pullup resistor.

Sensor output > base current limiting resistor > NPN transistor > 555

A 3 minute period for a 555 is pretty long, even for a CMOS LMC555. Consider a counting circuit such as something based on a CD4060.



Joined Jan 29, 2010
hi LL,
With regard to the Timing period of the 555 Mono, check page #3 of this PDF, ref Trigger Requirements.
If the Trigger is held Low for periods longer than the R/C timing period, the 555 Output remain High while the Trigger is Low.

Some PIR modules have adjustable Time On periods, so set the PIR On period to suit the project when using a 555 Mono, driven by a transistor Trigger.
If that is not possible use the AC coupling capacitor method shown in the PDF.