Quadruple Low Voltage Electronic Tester

What's this thing?

A versatile quadruple low voltage tester, contributing to a greener world because with the help of this little gadget a lot of broken electronic devices can get a second or third life, and will not be sent to the garbage dump !

You can test wiring connections, leds, listen to audio, use a 1kHz test signal and apply a test voltage of 9-12V DC.

The tester was designed and constructed in 2016. in 2025 after intensive use at the repaircafe I updated the design with the following:

A plug to use a separate wall wart power supply 9-12V DC because the 9V battery did not last as long as I expected. The audio amplifier uses relative much current.

A separate ON/OFF switch for the audio amplifier and the signal generator for the same reason. (to extend the battery lifetime).

A small loudspeaker on the front to replace the earbuds, deleting the 100 Ohm resistor in the output. This loudspeaker was salvaged from an old headphone.

A little L-shaped protector directly next to the ON/OFF switch because the tester was sometimes switched ON by accident when transported in my toolbag, draining the battery.

An additional connector on the side for the connection of the 9V DC or the 12V DC for testing.

Safety warning:

Be careful whatever you test, the device can only be used to test apparatus with safe low voltage inside.

Never use it to connect to dangerous voltages !

See the following steps and the circuit diagrams for all components used for this tester.

Where is the best place to use this quadruple tester? At the Repair Café !

A Repair Café is a meeting in which people repair household electrical and mechanical devices, computers, bicycles, clothing, etc. They are organised by and for local residents. Repair Cafés are held at a fixed location where tools are available and where they can fix their broken goods with the help of volunteers. Its objectives are to reduce waste, to maintain skills around repairs and to strengthen social cohesion.

Parts list for the amplifier:

1 Velleman kit type K4001

1 potentiometer 10 kOhm

1 knob for the potentiometer

1 resistor 10 kOhm 1/8 Watt

1 small loudspeaker (salvaged from an old headphone)

The audio amplifier is a K4001 kit from Velleman, a 7 Watt mono amplifier.

Follow this link and you will find all neccessary details. (assembly instructions, parts list etc).

As shown in the circuit diagram, I added a potentiometer (10kOhm) and a series resistor of 10 kOhm on the input to adjust the audio level and limit the influences of the amplifier on the test object.

I also added a 47nF capacitor from the input to GND to minimize the disturbance from the signal generator.

I also mounted a small speaker on the front of the box, salvaged from an old headphone.

Finally I removed a few mm from the top of the black metal heatsink, else the amplifier would not fit into the plastic box.

Parts list for this signal generator:

1 small part prototype PCB

2 resistors 47 kOhm1/8 Watt

1 resistor 100 kOhm 1/8 Watt

2 capacitor 10 nF

1 capacitor 10 uF

1 IC type NE555

This generator generates a small block type signal with a frequency of appr. 1 kHz, you can use it to send into a test object and follow the signal via the amplifier part of the tester.

You can find a lot of NE555 applications on the instructables website, just search for 555 to find a zillion examples..

Parts list for this wiring tester and LED tester:

2 resistors 1 Mohm 1/8 Watt

1 resistor 100 kOhm 1/8 Watt

3 resistors 1 kOhm 1/8 Watt

1 resistor 10 kOhm

1 green LED 5 mm

1 red LED 5 mm

1 buzzer 9V DC

1 capacitor 10 nF

2 ON/OFF switches

1 connetion plug for an external power supply 9-12V DC

1 battery 9V DC

1 transistor BC547 (NPN)

1 transistor BF472 (PNP)

1 small part prototype PCB

The wiring tester is a very sensitive and high impedance detector and will light the green LED (via the BF472 transistor) when the input is connected to GND.

It has a second little amplifier (the BC547 transistor) to activate the buzzer.

The tester has a additional 1 kOhm resistor, connected to the VCC 9V DC to test LED's. A LED can be connected between this resistor (connected to the white input plug) and GND (connected to the black input plug).

An additional connection is available for an external power supply 9-12V DC.

When the external power supply is plugged in the internal battery will be disconnected.

Parts list:

1 plastic box

6 female plugs for 6 mm banana plugs (yellow, red, green, white, blue, black)

1 plug for an external power supply 9-12V DC.

. colored wiring.

1 L-shaped plastic little cover to protect the ON/OFF switch from acidentally switching ON.

Build the three separate circuits (amplifier, wiring tester and signal generator) into a small plastic box.

Mount a small loudspeaker (from an old headphone) on the front of the box.

Connect the following plugs for 6 mm banana plugs:

YELLOW: connection of the power supply for testing. (9V DC or 12V DC)

RED: audio IN

WHITE: LED test

BLUE: Wiring test

GREEN: Audio signal out

BLACK: Ground

To switch the quadruple tester ON and OFF mount the ON/OFF switch on one side including a small L-shaped plastic cover together with the red ON/OFF status LED. On the front the knob for the audio level potentiometer is placed, together with the buzzer, the green wiring detection led and the small loudspeaker including cover for the audio amplifier.

The audio amplifier and the signal generatror can be switched ON/OFF seperately. When you only use the wiring test circuit the load current for the generator and the amplifier will not drain the battery.

What can you test?

Via the blue connector: WIRING.

You can test all types of electrical connections.

Basically it is a high impedance resistance tester with a buzzer and a green LED to detect failing copper connections on printed circuit boards and failing wiring.

Because it beeps very loud you can keep your eyes on the test location.

Via the white connector: LED.

Nearly all types of LED can be tested without worrying about serial resistors.

It is a 9 or 12 Volt DC connection with a serial resisor of 1000 Ohm.

Via the red connector: SOUNDS.

Sounds! Here you can listen to electric signals inside the device you are testing.

Via the green connector: AUDIO.

Here you have an 1 kHz audio test signal (block type) for example to test a circuit with amplifiers.

Via the yellow connector: POWER SUPPLY.

Connect the internal power supply (9V DC or 12V DC) to test the object you want to repair.

The black connector:

This is the ground connection.