Non contact voltage test pen teardown


Alan Parekh of Hacked Gadgets writes:

If you are doing any electrical work, one of these Non Contact Voltage Test Pens can be quite handy. Just touch the wire that you want to make sure isn’t live and check that the tester doesn’t beep and start flashing. This test pen is on all the time monitoring for AC between 90V and 1000 V. I would have preferred the device to have an on/off switch which would allow the battery to last even longer but I guess they figured that the 1.5 year life that they rate this for when in standby was good enough. This impressive life is because they got the current draw down to under 10 micro amps! Even when operating it only draws a handful of milli amps.
The board is quite thin at 0.8mm, this is half the typical generic PCB thickness which is 1.6mm. In large quantities the thinner board might be cheaper but typically anything thinner or thicker than 1.6mm is a premium price at most board houses that I know about.
The main component of the board is a 74HC14D hex inverting buffer the data sheet (PDF) shows a maximum supply current to the chip of 2 micro amps which makes sense based on our current readings. You can see a schematic below that Warren Young reverse engineered from a similar device (not the same but very similar operation).

Check out the video after the break.

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  1. I’ve had one of these non-contact pens fail – luckily I noticed and rechecked the mains wire with a multimeter.

    I’ve also seen an electrician use one when deinstalling our air conditioning system and think the outlet was safe. When he returned a week later to reinstall the air conditoining system (we had a new window put in in the meantime :) it turned the outlet was still live as he was flung from his ladder across the room.

    I don’t trust them.

    1. Pro tip: Always check a known live circuit just before checking an unknown circuit. When checking the known live circuit, observe that the LED is bright and bahaving the way it should. Chances are slim that the tool will fail in the short period between these two checks. I would never assume that a circuit I checked A WEEK AGO!!! is in the same state.

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