Need fundamental help understanding this circuit

Thread Starter


Joined Jan 16, 2016
Hi. In my home alarm panel, the door sensor circuits are each landed on a pair of screw terminals. Across the terminals I read 13.5vdc with nothing attached. Each door sensor circuit is a loop through a normally closed contact, with a 2200 ohm resistor on one end. Most of the pairs of terminals for the door sensors read ~5vdc when the door is closed, and ~12.5vdc when the door is opened.
Except for one of the sets of terminals. It reads around 13 volts with the door closed and very slightly higher, like 13.4v with the door open. I confirmed the resistor value, even jumpered a new resistor in its place. Why wouldn't 2200 ohms draw down the voltage in this one circuit the same as it does in the other ones?


Joined Nov 23, 2012
One resistor in a circuit doesn't change the voltage, it only limits the current. You need to find the other part of the sensor that divides the voltage (whether a resistor. Photo-cell, a switch with a second resistor, ...). Maybe the other half of that circuit is damaged or improperly installed.


Joined Feb 21, 2014
Can you connect the resistor directly across the panel terminals? That would prove correct panel operation.
The resistor(as you may know) allows the system to sense if the contact is open and if the wires are shorted by some fault or fiend.
As you have measured volts across open contact is about 12v and about 5v closed on good sensors. That sounds about right.
Since you have replaced the resistor maybe the internal relay is open/faulty