# Short Circuit Detection in wire

I am looking for a way to detect a short in a piece of wire. I want for an LED to light up when the wire is shorted. i started off by using an op-amp comparator but this didnt work.

the specs of the task "

*The requirement is to design a circuit, to drive an electromagnet solenoid, over a length of connecting cable. The cable could be damaged (short-circuit or open-circuit), so the circuit must detect these conditions.

If the solenoid is open-circuit, a red LED must be illuminated indicating open-circuit. If it is short-circuit, then another red LED must be illuminated. If neither, then it’s assumed OK, and a third (green) LED must be illuminated.

The circuit needs a push-button to turn the solenoid ‘on’ (but only if it is OK) and another button to turn it ‘off’ again. When the solenoid is energised, a fourth (power on) LED must be lit.

If the solenoid goes open or short circuit while powered (such as by someone wiggling the cable), the drive to the solenoid must be cut off and the appropriate LED illuminated. And if it then comes good again (such as an intermittent cable), the ‘on’ button must be pressed again to re-apply power.

The solenoid characteristics are: 28V DC operating voltage (operating range 18 – 32V), coil resistance is 76-84Ω. The solenoid will not operate below 5V, definitely operates above 18V, ie somewhere between 5 and 18V it pulls in. When once pulled in, it drops out between 12V and 2V (so if the drive circuit supplies less than 2V the electromagnet will definitely let go).

Assume that a power supply 28V or thereabouts is available to operate the circuit, and that it’s quite permissible to put low-level power to the solenoid to detect if it is OK or not. The task is to sketch out a circuit which will do this (with approximate component values, and type numbers of any diodes, transistors, IC’s etc used), with a brief explanation of how it works."*

I have attached my first attempt as a sketch.

Any help is appreciated.

• Identify EMC, ESD requirements and do you want low V detect too? or fault in the driver? Commented Mar 12, 2018 at 15:29
• It is not so clear which wire you're actually monitoring, I'm assuming it is the red one. When open (not shorted, also: shorted to what?) the opamp's inputs are undefined: that's never a good idea. Commented Mar 12, 2018 at 15:32
• Hi, Tony this is going to sound bad but i am not sure. this is part of an application. I am an electronics student and i didn't think id get this far. I'm just going on what the spec sheet say. as for the driver, only the wire needs to be detected because its not supposed to turn on unless the wire is ok Commented Mar 12, 2018 at 15:33
• "i started off by using an op-amp comparator but this didnt work" - what about this didn't work? Also, I recommend you 1) redraw your hand-sketched circuit in the built-in schematic tool, and 2) label the solenoid and cable. It's not clear in this diagram how the red cable (which Bimpelrekkie thinks is the one you're monitoring) connects to the solenoid (which I assume is the part in the middle right of the circuit and in no way connected to the cable) Commented Mar 12, 2018 at 15:39
• Selvek, thanks i will clarify. sorry for the ambiguity. my intention was to have the output of the NOR gate be responsible for turning on the solenoid by ANDing it with the power on switch Commented Mar 12, 2018 at 15:43