1. Resistor
   • R1 (68K , 1/2W)
   • R2 (330K , 1/2W)
   • R3 (1K, 1/2W)
   • R4 (680 ohms, 1/2W)
   • R5 (22 ohms, 1/2W)
2. Transistor
   • T1 (BC107)
   • T2 (AC128 or AC188)
3. LED
   • LED1
4. Miscellaneous
   • Battery, Dry cell 6V
   • (L1, L2) Testing probes or crocodile clips
   • S1 ( Slide switch, single pole)
   • PCB or Breadboard
   • Flexible Wire

This is a very simple and useful circuit which can be used for testing some electronic components. Like multimeter one can use this circuit for :- continuity testing, resistor testing, capacitor testing and diode testing.

In this circuit LED glows only when probes L1 and L2 are short-circuited or has a path for the current to flow. This fact will be used to test components.

Continuity Testing : Connect the two probes to the points between which the continuity has to be tested. If the LED glows! the points are electrically continuous otherwise not.

Resistor Testing : This circuit can test upto 10K resistors. If LED glows, resistor is good otherwise faulty.

Capacitor Testing : When a electrolytic capacitors is put in between L1 and L2 the LED should show glow momentarily. If the LED glows continuous, the capacitor is defective.

Diode Testing : A good diode will produce a glow in one direction only. If a diode produced glow in both directions, or in none of the direction, diode is faulty.

Transformer coil Testing : A good transformer winding should glow LED, but in case of "open circuited" faulty coil, the LED won't glow.