Fluctuating digital inputs

[Dave]

Robert, An alarm system hey, When I worked for Westinghouse alarm company about 40 years ago we would use from 15 to 20 Ma. for our contact wetting current in loops. When using switches, you can't rely on anything less to break the oxidization of NO contacts. The system I designed for my home uses a cunstant current source for the NC loop as well as current source's the all NO switches inseries with the LED's powered by 12 volts. I have never had a problem with it. Using constant current allows for some loop resistance variation over time as switches and wiring connections age. No false alarms...... Also you DON'T have to worry about shielding the wiring due to electrical noise.

[Demon]

So you just have a resistor and LED on every line amd that's it?

That could help diagnostics in case the keypad/display circuit acts up.

Robert

[Dave]

Robert, I don't know what you mean by a resistor and led on each line. I am talking about all of the sensor loops, and individual sensors coming into the control board from some distance away. Having an additional led in series with each of the curcuits would also be an ideal way to visually diagnose the system externals though. What I was talking about was using something like a constant current diode or what I used is a LM317 in a TO-92 package and a resistor configured as a current regurator of 20 Ma. in each of the source circuits for the individual sensor circuits and the loop circuits. They were all powered by the 12 volt power supply.