I am trying to revive some old reed switches that I found at house that we bought. They were part of some proprietary expensive security system (which doesn’t work anymore). They didn’t seemed to work originally, so I took one apart and found out that the two sides are connected via a resistor, which puzzles me - anyone knows why it’s there? When I disconnect the resistor, the switch starts to work as expected with my ESPHome config.
This is what it looked like (with the disconnected resistor):
If this is (was) a part of protective device then having a resistor is normal. These protective devices work in a way that normal condition is usually certain resistance. Then depends on device - it could be in your case that having resistor is normal, when reed contact is closed it represents another state - door closed, open… but based on image i think that only option is that resistor was connected in parallel with reed, since there are only 2 wires and they are (still) connected. Usually open wires represent error in wiring, so i think it’s what you have now (if reed is not actuated).
But, check reed relay if it’s ok, too… (with a magnet and buzzer on instrument)
I have seen where there is a resistor used to always determine things are connected as a certain voltage is available through the circuit as a normal state.
If someone cuts the wires it triggers the alarm as the voltage goes to 0.
If the reed switch moves it provides a connected circuit and the resistor is bypassed and thus produced a different voltage.
Pretty simple solution to allow detection of a tampered or malfunctioning circuit.
Ha, interesting, thank you a lot for the explanation, makes sense.
Do you think if there is a way how I could make it work with ESPHome+nodemcuv2 so it would work with the resistance change so I wouldn’t have to disconnect the resistor everywhere? GPIO Binary Sensor — ESPHome that I am using now doesn’t seem to have any option that would deal with this scenario .
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