Laserliner contactless voltage tester teardown

Here is a look what in inside a broken Laserliner Active Finder Plus contactless voltage tester that is designed to detect

live lines from 24 V AC to 1000 V AC:

Not much see on this side of circuit board. All I can see is place for two 1.5V batteries, beeper and LED.

The other side has all the main components:

4069 CMOS inverter IC based circuit

The CD4069 hex CMOS inverter chip is quite classic circuit to build contactless AC voltage detection circuits. I did not trace all circuit on this one, but at quick look the circuit looks somewhat similar in the implementation to this circuit idea from From 100 IC circuits free book:

This circuit will also detects active mains at 15cm but has the advantage of producing a squeal so you can keep your eyes on the job. The mains must be active and will not work when the light-switch is turned off.

In this specific circuit several 4069 IC ic gates are used as an amplifiers and signal detectors. CMOS devices have large input impedance with input currents on the order of 0.01nA, which is quite ideal for a circuit that tries to pick up signals coupled from wire to circuit though very small capacitance through the air.

Links to other circuit ideas based on 4069 chip:

Button x10 sensitivity


  1. Utthunga says:

    Hi Tomi,

    Nice Experiment! Keep up the good work.

  2. Tomi Engdahl says:

    EEVblog #267 – Voltage Detection Stick Teardown

    Ever wondered what’s inside those mains voltage detection sticks?
    Dave cracks open an IDEAL VoltAware, and giggles at a patent.

  3. Tomi Engdahl says:

    Volt stick circuits and tests

    A look inside a generic Volt Stick clone. I had one of the first Sagab versions with what I think was a metal body, and I recall the circuitry as being similar to this.
    They work by amplifying capacitively coupled current with reference to ambient ground (through you).

    These units are a handy extra test option for checking if an insulated cable may be live without breaking the insulation. They are not infallible, so all usual safety precautions should be taken.
    As with all tools you have to understand how they work and things that can influence their operation. That includes high voltage equipment in the vicinity like neon signs or overhead lines.

    They should also be tested before each use on a known live cable.

    This video features three circuit variants that can be built with discrete transistors.
    While fun to build, it’s more practical to buy one for work use, as a significant part of the unit is the case.

  4. Tomi Engdahl says:

    wow !! incredible idea that should be in every home | Simple Inventions | Homemade DIY Ideas

    Hello guys in today i will show you how to make Amazing Voltage Detectoror voltage tester ( Non contact tester) this Amazing Invention You Can Make At Home its very Simple this Inventions it is 100% Homemade DIY

  5. Tomi Engdahl says:


    0:00 – Intro
    0:24 – Why?
    1:27 – Features
    2:11 – High samplerates
    2:37 – ADDA filters
    3:35 – Why does this thing sound so good?
    4:24 – Do you need an RME interface?
    5:39 – Disclosure
    6:08 – Support me!


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