Voltage testing pen

Well, this guy made a few mistakes and while it does measure a voltage, it consumes power from the source it is measuring and thus is a really bad device.

Why do I say that? Because if you are measuring a voltage, if your measuring device draws current from the voltage you are trying to measure, you may change the value you are reading. Secondly, if you stick a diode in series with the measuring device, there is now a voltage drop across the diode. Thirdly, the diode voltage drop is a curve where the current through the diode indicates what the voltage drop across the diode.

This meter looks from the specs to have a 20ma current draw (those leds don't work without current). You might be okay measuring batteries with plenty of power, but don't try to debug your mini lathe power supply or a stepper driver or anything that might be a real device you would use.

I know, Jon posted the video... not blaming him, just warning folks from wasting money on this thing.

If it was powered from an external source, then you would not draw current from the measurement. But looking at the circuit board, I don't see how that could be done on the two wire version and the three wire version looks problematic as well. Yes, I can look at the circuit board and see these things - mostly that it can't be done because there is no way to separate the power to the meter from the signal to be measured...

If you want to do this, you would need an NON backlit LCD version to get the current draw way down (to like 10ua). Then, you would have to select a shottky diode to protect against reverse voltage and know the Vf characteristics to make the "meter" work. Here is what the Vf curve looks like:
Attachment 48904

That was from a SD101A diode made by Vishay that I found on Digikey.

OR - buy a decent volt meter and they will have built an autoranging volt meter that can measure AC and DC and a wide range accurately and safely.

Voltage testing pen - video