How to connect a cheap motion sensor to a hapcan button input?
The HC-SR501 has 3 pins (5V, GND and Out) and is very popular. I want to build this in "blind cover plate".
Is it possible to use a optocoupler to switch a hapcan button input?
Yes, but You have to deliver 5V power supply to the sensor. It is safer to use external voltage stabilizer instead of getting power from button module. And also, don't forget that 14 buttons module has no overvoltage protection and wire length should not exceed 25cm. If You are using optocoupler the distance can be larger.
I've tested external modules connection posibilities some time ago. I've connect 7805 stabilizer to Hapcan power bus and to PIR. Then connect optocoupler with 1k resistor from PIR out pin. Opto output connected to button module input and to ground. Works pretty well.