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555 timer Relay driver with TTL input

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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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The problem with the 555 timer is that it is not edge triggered and it is retriggerable. You would like a one-shot that is rising edge triggered. Retriggerible means as long as  a moose stands in front of the PIR the 555 would output a pulse; having inverted the output with a transistor. Thus the relay would stay energized. That is okay  if you want to  make a movie of a moose but not good for one  photo.

The 74121  is a nonretriggerable and is edge triggerable, that would be a better fit.

I will look at your circuit to see if it be used.  I am thinking perhaps one could invert the PIR output with a transistor and discharge a capacitor to momentarily  pull trigger pin 2 low.

What is the resistance of your relay if you already have one?  One needs to know how much current it would draw.



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the circuit below works in the LTspice simulator and as a prototype:


Traces from the simulator:


The green trace is the input from the sensor, the yellow trace is at pin 2, and the red trace is the output at pin 3. Note the positive going pulse when the input drops back to zero. That is why the Zener diode is required to limit it. Here it is  a 15v Zener diode.

The output from prototype the circuit: Here I am using 9 volts as I only have 12 volt Zeners.


Here the blue trace is the trigger signal at pin 2 and the yellow trace is the output at pin 3 across a 150 ohm resistor for a relay load. The diode across the relay coil can be any general purpose diode like a 1N007 for example.  The output is much wider then I would expect for R4 at 1Megohms and C3 at 1 microfarad. Expected 1 sec.  not 1.4   There  are numerous online calculators for calculating these values.  

The values for the resistors are not critical except what you pick for R4:

  • R1: 1000 to 1500 ohms
  • R2; 20k   to  30K   "
  • R3: 30K  to  50K   "

all work in the simulator.



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