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alah Posted - Nov 08 2016 : 2:34:07 PM
Hi, its been some time. I need help with my circuit for an alarm system. It will have a few sensors and an END OF LINE resistor to indicate if the wiring has been broken. Thanks.
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Aaron Cake Posted - Nov 10 2016 : 5:24:38 PM
So you're looking for something different than just an NC alarm loop that will go off if the wire is broken or the sensor is tripped to open. The simple wire loop that goes off if the loop is broken is found here:

You could build a comparator circuit for each loop with sensors being NO switches in parallel with the resistor (obviously resistor at alarm side, sensor way out on door/window/whatever). Soon as sensor contact closes, resistor is shorted and comparator swings.

But the way I'd do it is with an Arduino of your choice, which would depend on how many loops you want. An ATTINY45/85 can any combination of inputs and outputs totaling 5 by default. Set up a voltage divider feeding to each input with your NO sensor shorting the "top" resistor in the divider when it trips (that resistor located in sensor). In software just use AnalogRead in a loop to constantly check values. Some voltage = line normal. No voltage = line cut. High voltage = sensor tripped. Then decide what to do based on the condition. Sound an alarm by enabling an output, or light a fault light with another output.

You can use the bare chip or one of the boards (Uno, etc.). The boards are easier to work with and break out serial connections and such so you can connect to a panel, LCD, etc. The bare chips are more for integrating onto a board in a product.

Beats setting up a bunch of comparators or a single comparator with a multiplexor and a clock.

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