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Sunday 30 March 2014

Room Management System – PIR discovery


When I went shopping for a real life test of the unit, I made another discovery about PIR's. What seems to be so simple and just using a PIR with relay output, became quite difficult. Unfortunately the common PIR's on the market having a Line in, Line out and a common neutral. If it's rated for 240 Volt (Europe) well, 240 volt go in and 240 volt come out which is for our purpose not a great advantage except we want to add a relay ourself. It's possible but would just put up the costs of the unit unnecessary. Since a couple of days I am experimenting with the motion sensor coming with some of the Arduino starter kits, the HC-SR501. Advantage of this sensor is, that it runs of our 5 V power supply and the output can go straight into the shift register input. I also have 3 PIR's running in parallel without blocking diodes and it works just fine.

First we need to make the connections to the last 5 Volt and Ground bus. Then pin 1 from our PIR connects to ground, pin 3 to the 5 Volt bus and the middle one (pin 2) connects to the pin of our S1 which goes to the input pin (PI 1) of the first CD4021B shift register. Since I couldn't find a fitting diode in my electronic box, I am running 3 PIR's parallel without any problems. But for active use as shown in the image it would definitely be good practice to put a diode like a 1N4148 in the PIR's output.



Again a small warning at this stage. Please do not attempt to connect all PIR's you are planning to use to the circuit as shown. The same goes for relays. Connect 1 for demonstration purposes and 2 to 3 PIR's and you are fine. If you like to connect more, please check carefully the power consumption not to blow your Arduino board.

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