PIR on time question?

The PIR sensor is a great motion sensor but the 3 second on time after first sensing the movement is a long time before it can start sensing again. Is there any way I could reduce that on time some?


  • Chris SavageChris Savage Parallax Engineering Posts: 14,406

    No, there isn't really a way. The nature of the sensor kind of makes it this way. Unlike an IR or ultrasonic sensor, the PIR doesn't just see things as there or not there. The PIR sensor has to settle (balance) on the current PIR field it sees. Once it does this, motion upsets that balance and it takes a finite time for it to stabilize again.
  • Hi Chris, I thought I would ask here I ordered two PIR sensors Rev. B and haven't received them yet. Can you tell me if the output is open collector or push-pull or what exactly it is?

    Yes I did read the documentation. Also, might you know if there is any penalty in sensor performance running it at 3.3 volts instead of 5 volts (and conditioning the 5V output to work with a Propeller)?

    Thanks in advance, -bryan
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406

    The OUT line is connected to a PNP transistor and a 1M pull-down resistor. The transistor is also connected to the supply rail, effectively driving the output to the supply rail when triggered. As for 3.3V versus 5V on performance. I have not characterized the Rev. B in that aspect but I wouldn't expect too much difference as long as you're within the rated input voltage.
  • Thank you sir!
  • About speed of response, there is a distinction between the raw pyroelectric element that is at the heart of the sensor module and the electronics that processes the signal to an on/off decision at the output. The raw sensor has response over a range from about 0.1Hz up to around 10Hz, with a peak around 1Hz. Subsequent electronics filter out noise and stretch the output pulse. Here is an example of a dual element pyroelectric sensor. And an ST app-note. The circuits in the app note are fairly typical and the OP may be able with tradeoffs to tap or hack into the PIR module at an earlier stage to pick off a faster signal.

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