I have noticed that if I slap a non-contact voltage detector against my palm, the LED blinks. I have tried this with
four six different detectors, each a different brand, and each clearly a different implementation. (And no, none of them appeared to share an ODM.)
Why is this?
I bought a cheap NCVD that exhibits this behavior, and reverse engineered this schematic:
Here's a picture of the PCB inside:
As you can see, the lead of the resistor I am calling R2 extends into the tip of the probe. I initially thought this acted as an inductive pickup, but if you look at pictures of competing devices, you'll see that others use a solid plate here. It is more likely that this is acting as one plate of a capacitor.
The rest of the circuit looks like a fairly straightforward signal conditioning sort of thing. The main IC is a 74HC14 hex Schmitt inverter in SO-14.
I don't think I'm seeing a piezoelectric effect here. The three capacitors are ceramic, and so subject to these effects, but none of the three really strikes me as likely:
C1 is a bypass cap. Its ends are pinned by the power supply, and in any case tiny voltages applied across the 74HC14 power pins don't explain this.
C2 is tied on one side to a low impedance output, and the other to a voltage rail through the big R4 pull-up resistor. I would think either of these would eat any tiny piezoelectric voltages produced.
C3 might be capable of something like this, but I think the next point explains why it isn't responsible.
Once I had the PCB out of the "pen," I soldered some hookup wires to it so I could power it from my bench supply, then tried to reproduce the effect, and failed. I tried smacking the PCB against my work bench, holding on to the hookup wires you can see, working it like a lash. I have also tried smacking it with a toothpick, bending it like the arm of a crude catapult and letting it go so that my hand never touches the PCB.
Later, at Phil Frost's request, I repeated those tests using a 2×AAA battery holder, soldered to the board with 1 foot hookup wires, so I can strike the PCB and battery independently. The effect failed to recur in this configuration: neither battery contact bounce nor PCB vibration seem to be the cause for this effect.
The effect is not due purely to the tester being held by a human. If you take a tester subject to the effect and drop it onto an insulated surface, it still happens. If you don't mind risking your tester in the name of Science, drop it from a foot or so, and you will see that the light doesn't blink until impact. The effect isn't caused by the human letting go of the tester.
I can only conclude that there is something about the physical realization of the final product that causes the effect. For one thing, the plastic housing over the probe tip will modify the capacitance, since it will have a higher relative permittivity than air.