AT Commander's

Invisible Joe Detector

Ever since the transparent Joes came to town, the balance of power has been irreparably damaged.  Just when one side begins to achieve a victory, an invisible force runs around and subdues the Joe team on the brink of their victory.  What can be done to halt this unseen menace?  Fortunately for the Joe team, one of the scientists deep in the laboratories of the Adventure Team Headquarters developed an Invisible Joe Detector.  It can detect - and almost pinpoint - the location of the invisible Joe from up to 12 scale feet away!

An electroscope can detect the invisible fields of voltage which surround all electrified objects.   This little circuit acts as a primitive  electroscope with lots of fun applications!

On a low-humidity day and with a short antenna wire, this little circuit will respond strongly when someone combs their hair nearby!  If a metal object is lifted up on a non-conductive support and touched to the sensor wire, the sensor can detect whether that object supports an electrostatic potential of as little as one volt!  Real-world objects become "electrified" when their pre-existing + and - charges are not equal. By the way, it won't detect Invisible Joe unless he's got a static charge from a berber carpet, wool sock, or something similar. It's the static that makes the LED light up (or darken, depending on the polarity of the charge).

Below is the circuit. Don't connect the battery until you are SURE you've hooked everything up exactly right. It's possible to burn out the FET or the LED if they are connected incorrectly. Don't let the transistor's wires bump together even briefly, or it will flash the LED and burn it out.   Don't ever connect any LED directly to a 9-volt battery, it will burn out the LED. 

A bare LED needs a 1000-ohm resistor wired in series to limit the maximum current from the 9-volt battery.  Avoid touching the Gate wire of the FET. Small sparks jumping from your finger to the Gate wire can damage the transistor internally.  You might also want to add a 1-meg resistor between the gate of the FET and the antenna, to help prevent the FET from being harmed by accidental sparks to its Gate lead.  To test the circuit, charge up a pen or comb on your hair, then wave it close to the little "antenna" wire.  The LED should go dark.  When you remove the electrified pen or comb, the LED should light up again. 

Connect the circuit to its battery, and the LED will turn on. Comb your hair, then hold the comb near the Field Effect Transistor (FET) gate wire. The LED will go dark. This indicates that the comb has an excess of negative charge, and the FET responds to the electrostatic field surrounding the comb. It acts as a switch and turns off. Remove the comb and the LED brightens again. Wiggle the comb, and find at how great a distance the circuit still detects it. It's amazing how far an e-field extends around an electrified object.

This FET sensor is not an ideal educational device because it responds differently to positive than to negative Potential Difference at it's "Gate" wire.

Rules for this FET:
    * negative objects turn the LED off, it lights again when removed.
    * positive objects make the LED bright, then dark when removed.

If you've been playing with it and can't get the LED to come back on, then simply touching your fingers to the "Gate" wire and to some other part of the circuit simultaneously and it will "reset."  Or, touch a plastic pen to some hair, then wave it near the sensor, and the LED will light up.

In situations where the sensor is TOO sensitive, add a small capacitor (with a value below 100 pf) between the FET gate lead and one of the other leads. This reduces the sensitivity of the device - and can a big difference. Capacitors larger than 100pF can be used, but they REALLY wipe out the sensitivity.

Obtain a clip-lead, and connect it to the Gate lead of the FET. Let it hang loose without touching anything. You'll find that this has vastly increased the sensitivity of your FET circuit. On a dry day it will respond to hair-combing from 20ft away.

If you hold the unit in your hand and watch the LED, you'll see that the Gate acts as the antenna, and negative objects make the LED go dark.  But if you hold the whole thing up in the air, but hold an negatively electrified object near the battery, the LED will get brighter - not dimmer; polarity of operation has been reversed. The FET gate wire responds with one polarity, while the battery and the rest of the circuit responds with the other. 

(Originally an article entitled "RIDICULOUSLY SENSITIVE CHARGE DETECTOR" (C)1987 William J. Beaty:  I edited this info for this project.  Check out the original website for more information at

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