I wanted to have a simple contact sensor to monitor my furnace burner. I first looked at the Sage Doorbell sensor but it had no heartbeat or battery capability. I know there are other contact sensors available with external contact ability but I had the Aqara.
Its small ~1.5+ X 7/8 inches
Uses a CR1632 battery
My first concern was the possibility of EMC or static coming in on the external wires. However looking at the board the Reed Relay has a capacitor to ground then a 500K resistor into the processor. I figured the input is not bulletproof but should provide enough safety margin to not be an issue.
The wire I used is a #28 stranded with silicone insulation. this wire is extremely flexible and the insulation doesn't melt back when soldering.
The steps: (any questions please let me know).