A simple tester can be used for routine checks for resistance on production lines of relays, coils, or similar components where frequent changes in resistance to be tested are not required. The tester is built around a single quad op amp chip, the LM324. R, which is chosen to be around 80 times the resistance to be checked, and the 5 V supply form the current source. The first op amp buffers the voltage generated across the resistance tinder test, Rx. The second op amp amplifies this voltage.
The third and fourth op amps compare the amplified voltage with high and low limits. The high and low limits are set on multitum presets with high and low limit resistors connected in place of Rx. LED 1 (red) lights when the resistance is high. LED 2 (green) shows that the resistance is within limits. LED 3 (red) indicates that the resistance is low.