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Completing a circuit when a circuit is broken

I'm wondering how a circuit is made similar to an alarm. For example, when a point in a circuit is broken, it activates something else (led, etc.$

gmoon5 years ago
The simplest way is to use a relay; a "normally closed" relay would close a circuit when the relay coil is off. The alarm loop keeps the relay coil activated.

But there are plenty of other electronic devices (transistors, etc.) that can invert incoming signals, and trigger other actions...
Are you looking for a specific point that breaks or any point in the circuit?

Basically that is what a UPS or a set of emergency lights does. When it stops detecting power from the main lines of the building it turns on the backup battery. A similar device can be used to monitor a specific point in any circuit. When the expected voltage or resistance across that point changes to an unknown value an LED an be light to indicate the problem.

How you go about setting this kind of thing up and the method you use to achieve it will vary based on the circuit in question. A more complex circuit need allot more to monitor it if you want to watch multiple point. A simple series circuit would be easy to rig up.
cyberguy91 (author)  mpilchfamily5 years ago
I was just thinking a proof of concept. Like say a dc motor that had a switch to turn it on or off and when it was switched off, an led would light up. I didn't really have a specific application in mind; just wanted to see how they work.