The 120 Ohm Detector

Discussion in 'The Projects Forum' started by ECAM, Aug 2, 2006.

  1. ECAM

    Thread Starter New Member

    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.
  2. mrmeval

    AAC Fanatic!

    Jun 30, 2006
  3. beenthere

    Retired Moderator

    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.