Simple Component and Continuity Tester

This simple project may be used for testing components, as well as checking circuit board tracks, wires and connections for continuity (conduction). It tries to pass a small current through the item being tested and the LED will light brightly, dimly or not at all according to the resistance of the item:

• LED bright means the resistance is low, less than about 1kΩ
• LED dim means the resistance is medium, a few kΩ
• LED off means the resistance is high, more than about 10kΩ

When not in use the 9V PP3 battery should be unclipped or the crocodile clips attached to a piece of card or plastic to prevent them touching. You could add a push-switch in the red wire from the battery clip and this may be the best option if you mount the simple tester in a box.

If you think this project is too simple to be useful, please see the table of components which it can be used to test and think again!

Stripboard Layout

Testing components

Connect a crocodile clip on each side of the component. They can be connected either way round unless stated otherwise in the table below.

Test results for components in good condition

Resistor LED bright for low resistance, less than 1kΩ. LED dim for medium resistance, a few kΩ. LED off for high resistance, more than 10kΩ.

Variable Resistor Across the two ends of the track the LED brightness will depend on the resistance value (see above). Between one end of the track and the wiper the LED brightness should vary as you adjust the variable resistor. For high resistances (>10kΩ) the LED will only light at one end.

Diode LED bright with red lead to anode and black lead to cathode. LED off with black lead to anode and red lead to cathode. a = anode, k = cathode (end with stripe)

Zener Diode LED bright with red lead to anode and black lead to cathode (stripe). LED dim with black lead to anode and red lead to cathode (stripe) if the zener diode voltage is less than about 7V. LED off with black lead to anode and red lead to cathode (stripe) if the zener diode voltage is greater than about 7V.

LED LED bright with red lead to anode and black lead to cathode (short lead) - the LED being tested will also light. LED off with black lead to anode and red lead to cathode.

Transistor B = base, C = collector, E = emitter. See a catalogue or website to identify the leads. For each pair of transistor leads connect the tester leads first one way, then the other way. For NPN transistor in good condition: CE pair: LED off both ways. BC & BE pairs: LED bright with red lead on B, LED off the other way. For PNP transistor in good condition: CE pair: LED off both ways. BC & BE pairs: LED bright with black lead on B, LED off the other way. You can use the tester to identify the B lead (it always conducts one way) and to distinguish NPN and PNP transistors (by the tester lead colour when B conducts). However, the tester cannot distinguish the C and E leads.

Capacitor less than 1µF LED off (a broken connection will give the same result).

Capacitor 1µF and greater (polarised) Connect the red lead to positive (+) and the black lead to negative (-): the LED will flash briefly when first connected. Reverse the connections: the LED will give another brief flash. With low values like 1µF the flash will be almost too brief to see, but larger values will give longer flashes. Some capacitors leak a little charge when connected the wrong way, making the LED light dimly continuously.

LDR LED bright with LDR in bright light. LED dim with LDR in normal room light. LED off with LDR in darkness.

Thermistor LED dim when the thermistor is warm. LED off when the thermistor is cold. These are typical results, the exact results depend on the thermistor's resistance.

Lamp LED bright. The lamp itself will NOT light because the test current is too small.

Switch LED bright with switch contacts closed (on). LED off with switch contacts open (off).

Loudspeaker, Inductor, Relay coil Wire, Motor and Fuse. LED bright.

Testing connections, stripboard, PCB tracks and wires

Connect a crocodile clip on each side of the suspected fault:

• LED bright means there is a connection.
• LED off means there is no connection.

If you are testing a stripboard or PCB which has components soldered in place, beware of possible connections via the components and allow for this when interpreting the results.

Stripboard circuits can suffer from two common problems: solder bridging between adjacent tracks making a connection where there should be none, and tracks broken with a track cutter which have an almost invisible thread of copper conducting across the break.

If a PCB has etched poorly the tracks may be very thin in places or there may be traces of copper bridging between adjacent tracks.

Wires and connections may be checked for continuity (conduction).

Circuit diagram