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PIR motion sensor interface for a microcontroller
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e100:
I have a typical PIR sensor as you would find in a home security system. It has an isolated reed relay that opens when motion is detected (or when power is disconnected). I believe in alarm terminology this is called a "normally closed" circuit, which confusingly means that it uses a "normally open" relay (closes when energised).
These are used with long cables (non-twisted conductors), have to be resistant to interference, and be super reliable.
I went looking for examples of alarm panel interface circuit diagrams but could only find installation wiring diagrams, however there are various documents describing the use of "end of line" resistors to detect wiring faults, and these are typically 5.6k which given the typical 12v supply means that loop currents are in the sub 2mA range.
Does this point to interfaces using opto isolators or would they be using a direct connection to the microcontroller ADC/digital input with protection resistor & clamping diodes?
viperidae:
They would be using two comparators to detect open circuits and short circuits. That's why they have a resistor in there that is significantly higher than the wire resistance.
You could also just an opamp as a buffer to feed an ADC.
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