I have an old alarm system in my house that I would like to integrate with a modern system. The old system has hardwired circuits where multiple reed sensors in series monitor a zone with as many windows (A). The modern system has wireless contact sensors that use a reed switch to detect open/closed state (of a door or window). I thought if I could bypass the reed switch in the wireless sensor with the hardwired circuit, the wireless sensor would monitor the hardwired zone (B).
Some have tried this and results are inconsistent. One person states that the wireless sensor must be within 10 ft of the reed sensor in the house's circuit.
The circuit in my house is at least 100 feet long, has around 20 reed switches and I measured ~30 Ohm on it. The setup (B) does not work. I measured resistance on the reed switch in my wireless sensor (it is a Ring "alarm contact sensor") when it's closed and that's about 1 Ohm. The wireless sensor won’t detect open/close state changes of the house’s circuit, although upon wiring it up as in (B) it detects the initial state correctly.
Could I solve this situation by using the houses circuit as a low voltage control circuit for a relay that would open/close a circuit for the wireless sensor like in (C)?
Answer
The problem was caused by induced voltage on the circuit. This can be seen by measuring voltage on the closed circuit. It is not much but it varies wildly and it is enough to confuse the wireless sensor.
The solution for me was to employ a pull-up resistor so the interference would not matter anymore. I implemented this with an Arduino and it is documented at this Instructable.
For your information, there is a DIY-alarm company that is offering a device to integrate existing hardwired alarm circuits with their solution.
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