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:

VD01 schematic

Here's a picture of the PCB inside:

VC01 PCB pic

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.

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    \$\begingroup\$ Possibly from static electricity building up (or already exists on the device/your hand). Try tapping the devices on a grounded conductive material (say a metal pipe or Aluminum foil) and see if you can observe similar effects. \$\endgroup\$ – helloworld922 Jun 6 '13 at 22:07
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    \$\begingroup\$ @helloworld922: It also happens with the two I have here at hand if you drop them from about an inch onto a table, or tap their tip onto the table while holding it. \$\endgroup\$ – Warren Young Jun 6 '13 at 22:10
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    \$\begingroup\$ is the table electrically grounded? \$\endgroup\$ – helloworld922 Jun 6 '13 at 22:15
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    \$\begingroup\$ @helloworld922: No. It's a typical coated MDF office table. It does have metal legs, but they rest on office carpet. So, there may be tiny trickle currents to ground, but you'd probably need a megger to measure the resistance. And I can't do any more drop tests. The NCVDs in question don't belong to me and the owner just got mad at me for throwing his tools around. :) \$\endgroup\$ – Warren Young Jun 6 '13 at 22:23
  • \$\begingroup\$ Might the probe touch the inside of the plastic case on impact, due to inertia? Perhaps the plastic is picking up charge as it whips through the air. \$\endgroup\$ – Kaz Jun 19 '13 at 20:00

This device works on capacitive coupling, not inductive coupling, as the inductor you have labeled "R2 lead" suggests. Capacitive coupling has the advantage of working equally well regardless of how much current is flowing in the wire. You probably want to know that wire is live even though the light is off at the moment, so this is good.

This probably also explains why the R2 lead is bent. This serves to increase the effective surface area of that plate of the capacitor, increasing the capacitance. My tester (Klein Tools NCVT-1) has a solid, flat probe in the tip.

Rather, R2's lead is one plate of the capacitor, and your hand is the other:


simulate this circuit – Schematic created using CircuitLab

Recall that capacitance is the ratio of charge to voltage:

$$ C = \frac{Q}{V} $$

But you can re-arrange that as:

$$ V = \frac{Q}{C} $$

We don't know what \$Q\$ is, but we know that it's roughly constant, owing to the very high input impedance of U1A.

Further, we know that the capacitance is a function of the separation between the plates, so as your hand moves towards or away from the probe, C is changing. So if Q isn't changing fast enough to be significant, and C changing rapidly as you move your hand (one plate) closer or farther away from the other plate (the resistor lead), then \$V\$ must change. Beep!

You can in fact get this effect by moving the probe rapidly towards or away from any conductive object. I have many times observed my probe beeping briefly when I bring it near a circuit I have disconnected at the circuit breaker.

To validate this explanation, try this experiment: tap the probe against your hand, rapidly, and vigorously, until it's beeping frequently. Then, reduce your taps to still somewhat rapid, but gentle contacts, or even not-quite contacts, with your hand. You should find it to be quite sensitive. Now, hold it touching your hand for a moment. Ten seconds is sufficient with my tester. Now try the same gentle pattern of contacts. You should find it much less sensitive.

Here's what I expect is occurring: when you go through the initial round of smacking, you increase the charge on the capacitor by injecting a lot of noise through triboelectric effects, wildly varying capacitance to your hand, RF currents picked up by your body, coupled through that capacitance, microphonic effects, jarring the battery, and so on. Point is, when you are done, \$Q\$ is very likely non-zero. This increases the voltage swing as you vary the capacitance between the probe and your hand, and thus, the sensitivity.

When you hold the probe near your hand, the charge separation you initially established has a chance to equalize through leakage paths past the capacitor. Since we are talking about a very high impedance, and a low charge, there need not be much leakage to be significant.

I tried to reproduce your drop tests, but could not, reliably. I didn't want to sacrifice my tester to solder the contacts, so I tested with an unmodified tester. If I smack the probe against my hand, I can get it to beep. But if I smack the opposite end of the device, and I'm careful to not be holding it in any way to be close to the probe end, it does not beep. I suppose battery contacts bouncing could still explain this behavior, and by smacking the other end I'm just not jarring the battery as effectively. However, battery jarring does not explain the experiment I previously described. Perhaps both mechanisms are significant.

  • \$\begingroup\$ How does this explain the drop tests? \$\endgroup\$ – Warren Young Jun 17 '13 at 21:32
  • \$\begingroup\$ Re: Whether the R2 lead is one plate of a capacitor or not, it is also an inductor. Does that matter? I suspect not, because in some pics of competing devices, you can see that they use a solid electrode, not a wire loop, so it would have much less inherent inductance. I also realize that the plastic case of the device makes capacitors with parts inside. Do we have to model this with some complex CLC type circuit? \$\endgroup\$ – Warren Young Jun 17 '13 at 21:33
  • \$\begingroup\$ @WarrenYoung I don't suppose the inductance is significant unless it makes a resonant circuit, but I don't see why that would be necessary. Since my tester seems to work just as well on lines with high current vs. no current, and it doesn't seem to work at all on the neutral line, I suspect the inductance is not significant. See edits for an experiment and more explanation. \$\endgroup\$ – Phil Frost Jun 18 '13 at 3:19
  • \$\begingroup\$ @WarrenYoung also, if you modified it to be powered from a bench supply, you probably also provided a path for charge to Earth, through the leakage of your supply's output transformer and its power cord. Try running it from a battery with soldered contacts, and I bet you will not get the same effect. \$\endgroup\$ – Phil Frost Jun 18 '13 at 11:14
  • \$\begingroup\$ I'm accepting this answer because if I don't, half the points I offered for bounty go for naught. You seem to be closer to the answer than trav1s is, and I learned more from this answer. The thing is, substituting the 2xAAA holder for the bench supply doesn't make the effect recur. Worse, I failed to make my sacrificed tester blink by beating the cell holder around, or by intentionally making and breaking connections on it. I've also repeated the drop tests in a more robust way, with results above. So, neither answer can currently explain the test results. \$\endgroup\$ – Warren Young Jun 19 '13 at 19:22

Slapping, shaking, or dropping a device with a battery can jar the battery, temporarily disconnecting the internal circuits from the power source. This forces the device to run through start up until the power source becomes reliable enough. This can cause strange behavior of the circuit, as the internal nodes may go into unusual states during power up.

The response may be different between different devices because of different mechanisms securing the battery. Some batteries are more secure than others, and don't get jarred as much when slapped.

To test if this effect is causing a failure in a device, there are some tests that can be performed:

1) Make the battery housing and contacts more robust against mechanical shock and again perform the mechanical stimulus. If the strange behavior is no longer observed, it was likely due to the effect described above.

2) Connect an oscilloscope to monitor the internal on-board power supply. This should be done without affecting the test set up too much. That is, you want to keep the device as similar as possible to the state in which the failure or strange behavior was observed. Apply mechanical shock and observe if fluctuations occur on the power supply using the oscilloscope. If the strange behavior in the circuit is still observed, but the power supply is unaffected, there must be some other effect causing the strange behavior. If the strange behavior is observed and the power supply has significant fluctuations, the failure mode is likely to be as described above. If the strange behavior is not observed during the test, then the results are inconclusive.

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    \$\begingroup\$ Maybe he should test to see if the light turns on when he turns off the power and turnes it back on \$\endgroup\$ – skyler Jun 15 '13 at 3:54
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    \$\begingroup\$ +1: This explanation seems plausible in light of the test results reported in the edited question above. Soldering the hookup wires to the PCB and repeating the hits in a way that vibrates the PCB now shows that the effect no longer occurs. The + terminal of the AAA cell bouncing on its contact could indeed explain it. Still, I haven't proven this answer correct yet. \$\endgroup\$ – Warren Young Jun 16 '13 at 1:20
  • \$\begingroup\$ I added another test to my answer. It should help to prove or disprove my theory. \$\endgroup\$ – travisbartley Jun 18 '13 at 1:47
  • \$\begingroup\$ Re test 1, I already partially tested that with my bench supply tests. But, I now observe the same behavior with a 2xAAA battery holder soldered to the PCB via 1' long hookup wires so I can thwack the PCB around without disturbing the battery. \$\endgroup\$ – Warren Young Jun 19 '13 at 19:13
  • \$\begingroup\$ Re test 2, I don't see a way to connect the scope to the board without making that connection part of what's being tested. How do you drop the board without breaking the scope probe away? I guess I could sacrifice one of my few BNC cables, cutting the connector off one end and soldering the exposed wires to the PCB so the connection will survive the beating, but that doesn't appeal. \$\endgroup\$ – Warren Young Jun 19 '13 at 19:14

Can't say for sure, but the device is ground coupled through your body's stray capacitance to ground. Slapping it will change this capacitance, and maybe do something to the circuit. Try tapping it against a table or other nonconductive surface to rule out a piezoelectric source.

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    \$\begingroup\$ I don't think so. The drop tests above rule that out. Also, the sort of NCVD that depends on body capacitance usually has an exposed metal contact where the user is supposed to grip the device, doesn't it? None of those I've tried do. \$\endgroup\$ – Warren Young Jun 16 '13 at 0:58
  • \$\begingroup\$ At the impedances in this circuit, a wood table is a conductor. I'd want a better insulator to draw any conclusions. \$\endgroup\$ – Phil Frost Jun 18 '13 at 3:39

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