EOL resistor trick


Active Member
I read in a DIY book about 20 years ago that alarms were wired with normally open switches in parallel to resistors that would complete the circuit. Then, multiple switches would be wired in series within a zone loop (door, windows, etc.). Each switch in a single zone would have a different resistor value. Once a a sensor was triggered and corresponding switch was closed, the resistance across the complete loop would go down by a specific amount. Therefore, it was possible to know exactly what sensor was triggered within the loop, based in the specific change in resistance.

For an M1 required total loop resistance of 2200 ohms you could use this combination of resistors:

1000 ohms
470 ohms
330 ohms
220 ohms
2020 total ohms

These 2020 ohms are within 10% of the require value. If you want to get closer just buy several 20% tolerance resistors of each value and combine them as required to reach the 2200 ohms.

The smallest resistor here is just 10% of the total 2200 ohms required EOL. The big question here is: Would the Elk detect this 10% sudden change in resistance if the sensor at the 220 ohms resistor is triggered?

If yes, would it be possible to inquiry the change in voltage (or current, depending how it is wired) to determine the exact sensor tripped? It looks like a nice feature for the HomeSeer plug-in. You could monitor your house at a granularity 4 times better than possible with the physical available inputs.
Sounds overly complicated. I believe in KISS. There is no shortage of inputs in M1, why not just have each sensor on its own zone. Makes it real easy to control or know which one tripped. You can still use EOL for supervision of a cut or shorted wire. Nice thing is you can configure each zone for EOL,N/C or N/O.
The problem you may run into is the M1 has a wide tolerance for resistance changes on the input loop. The M1 will not know which switch is violated.

There is a new feature on the next software revision, Version 4.3.8 in which you can use two 2200 ohm resistors on a normally closed supervisory zone to get an alarm and tamper. This is mainly used on Motion detectors with a tamper switch.

Good idea though. You are almost asking for a multiplexed zone input. Another thread was asking for a multiplexed fire input zone.