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555 timer Relay driver with TTL input

licketylips

May 23, 2020
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May 23, 2020
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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 up to 1 minute (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?
 

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Harald Kapp

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See here for a very basic inverter circuit (NPN resistor–transistor logic inverter) that can at the same time work as level shifter.

Here's a sim:
upload_2020-5-23_11-5-14.png
The green trace shows the output of the PIR sensor: 0 V idle, 3 V active.
The blue trace shows the trigger input to the 555: 12 V idle, 0 V pulse at the rising edge of the PIR's output becoming active.

As for the relay driver, read our resource, please. Do not forget the clamping diode across the relay coil (think of motor M1 in figure 1.2 in this resource as the relay coil).
 
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