The 120 Ohm Detector

Thread Starter


Joined Aug 2, 2006
I'm trying to make a 120 (+ or - 10%) ohm detector, in which a green LED lights up when there is a 120 ohm resistor placed at the input, and a red LED for anything else (Open circuit, short circuit, ect....). I'm having problems getting around the possibility of getting a short circuit as an input and I was hoping for some advice.

Unfortunately I can't seem to draw what I have came up with so far but imagine a power source going to a large resistor (10M ohms), the input is placed in parallel with this resistor, and then these resistors are in series with a relay. This relay controls contacters that turn on and off the green lights.

This way when a 120 ohm resistor is placed at the input, the 10Mohm resistance is bypassed and will allow the relay to be powered. BUT this also happens when a short appears at the input, this is where I get stuck. I want to use only DC power.


Joined Apr 20, 2004

The circuit attached to this is a window detector. It can be made with a dual comparator instead of the two LM311s. Everything runs off 12 volts. The three 10K resistors can be made smaller, but not below 2.4K just to keep current down.