Here is the problem I face. I have a commercial PIR and wondering if it can be seamlessly connected with my pi zero w and how.

PIR specs: https://www.thecrowgroup.com/cr/uploads/2019/03/Datasheet-Swan-Quad-Crow.pdf

Here is some more info: https://www.thecrowgroup.com/cr/uploads/2019/03/Manual-Swan-Quad-Crow.pdf

Any help would be appreciated.


  • Pi can accept any digital input right out of the box. The product you're using seems like something that can give out digital output. So, try to connect it to your Pi. – Irfanuddin Aug 31 '19 at 19:51
  • I'm basically stuck on this: Alarm Output N.C 28VDC 0.1 A with 27Ohm series protection resistor – vincent Aug 31 '19 at 19:52
  • You can use a transistor to trigger a pin in Raspberry Pi and write a Python or even scratch code to do something on trigger. – Irfanuddin Aug 31 '19 at 19:55
  • Thanks for the quick response. Not entirely sure how to do that, I mean what exactly to use and how to connect them. I was planning to power the PIR externally and connect the two cables of PIR's relay directly to PI's GPIO, but I have a feeling this is not a good idea. – vincent Aug 31 '19 at 19:58
  • The GPIO cannot handle 28V DC. You'd fry your Raspberry Pi if you do so. You need to trigger you Pi with 3.3 -5V DC. Do your research on how to do that. There are many simple solutions for doing it. – Irfanuddin Aug 31 '19 at 20:00

Well, the PIR spec says the following about output:

Alarm Outputs Form A N.C 28Vdc 0.1 A (with 10 Ohm series protection resistor)

I think this means that the output is in the form of a (electromagnetic) relay, of "Form A", "Normally Closed". The normally closed switch has a 10 Ohm in series current protecting resistor (so that no excessive current can pass the closed relay switch).

The usual wiring is the following: Extract from the relay switch, two long wires (can be as long as 100 meters say). At the end of the two wires, one end to Rpi 3V3 (just for example), another end a 10k to Rpi ground.

Then Rpi continuous monitors that the voltage at the resistor end is High (3v3) because the relay switch is closed. If the PIR detects a cat coming in, it will change the relay switch open, now Rpi will find resistor end no current flowing, therefore voltage becomes zero (actually depends of the wiring polarity).

The relay switches on/off a DC circuit (in this case, 3V3 > 10K > ground). This DC circuit has no AC circuit noise problem if the wiring is over 100 meters long (assuming connect wire is heavy duty/thick, so voltage drop is small).

In other words, 28V DC 0.1A is the relay spec. To clarify things, for example, a cheapie 5V relay module, the relay spec is similarly 200V 6/10A, DC30V 10A (for switching high power motors, lamps). In PIR case, the relay is not passing current to turn on a fan or motor, so only 28V 0.1A is enough to make a signal switch.

And of course you don't need to use Rpi. Instead you can use a status LED/buzzer which is on when no cat comes, and off otherwise.

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