2 Wire Resistance and Continuity Tester

This circuit is similar to continuity tester using transistor but this version provides a simple estimation of the wire resistance. It can be use also as an ohmmeter to measure resistance of resistors from 0 ohms to 500 kilo ohms. The circuit has three LED indicators and the operation is summarized below.
1. All LED are ON; meaning resistance is about 0 ohm to 200 ohms.
2. Only is LED1 OFF; resistance is around 300 ohms to 10 kilo ohms.
3. Only LED 3 is ON; resistance is around 10 to 500 kilo ohms.

resistance tester schematic

Part List:
R1 - 100 ohms 1/4W
R2 - 33 kilo ohms 1/4W
R3 - 120 ohms 1/4W
R4 - 39 ohms 1/4W
R5 - 56 ohms 1/4W
LED1, LED2, LED3 - red LED (see LED info for pinout and markings)
Q1, Q2 - 2N4403 or similar PNP transistor
Probe - just use solid type wire or conductor
Bat - 3V battery

2N4403 pinout
Note: The tester is approximation only and might defer in actual results. When we say that the LED is OFF, it does not necessarily mean it’s completely off since in actual it is still emitting light.  But this light is very low enough that we can consider it as OFF.


  1. can i use BC558 transistor for Q1 and Q2?

  2. the type of transistor is not critical thus you can replace it with BC558 or any similar tansistor