Schematic for relay tester

[mbenelli1014]

I am fairly new to electronics design. For a school project I have selected to build a relay tester...more specifically a circuit to read the resistance across a standard 12VDC relay when voltage is applied to the coil. I would like the circuit to have three LEDS...one to light up if the coil resistance is "low," below let's say 50 ohms. The second LED to light up if the resistance falls betwen 50, and up to 120 ohms. Anything over 120 would light up the third LED to show a "high" resistance coil. Is this easily possible to do. I have some thoughts, but I am afraid I will overlook something. Thanks for any input.

 

[MikeMl]

Force a constant current, read the resultant voltage. Its called an Ohmmeter.

 

[KeepItSimpleStupid]

Look for "comparator", "window comparator", "voltage reference", "LM339 datasheet" and "LM334 datasheet". You also may need to look at hysteresis in comparitor circuits. You'll have to pay attention to voltage swings.

Try to pass a specific current through the coil. It does not have to activate the relay and compare the volatge drop to two thresholds.