Warning: 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.
NOTE: Don't ever connect any LED directly to a 9-volt battery, it will
burn out the LED. Without the transistor to limit the current, a bare LED
needs a 1000-ohm resistor wired in series if connected to
the 9-volt battery.
Warning: Avoid touching the Gate wire of the FET. Any small sparks
jumping from your finger to the Gate wire can damage the transistor
Use three clipleads. Bend the
Gate wire of the FET upwards (see the small diagram above to see which
lead is the Gate, or check the diagram on the cardboard of the Radio Shack
FET.) The Gate acts as an antenna, so leave it unconnected. Use one
cliplead to connect the middle transistor lead to the red positive lead
for the 9V battery clip. Connect the remaining transistor lead to the
positive lead of the LED (the longer LED lead is usually the positive
one.) Connect the LED's remaining lead (the negative one) to the black
negative lead for the 9V battery clip. Check all connections twice, then
carefully connect the 9V battery to the battery clip. The LED should
light up. If the LED remains dark, try lighting it up by waving an
electrified plastic pen or ruler near the Gate wire (electrify the plastic
by rubbing it on hair.)
The 1-meg resistor helps protect the FET from being harmed by any
accidental sparks to its Gate lead. The circuit will work fine
without this resistor. Just don't intentionally "zap" the Gate
wire with an electrified object or finger.
To test the circuit, electrify a pen or a 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.
IF IT DOESN'T WORK, the humidity might be too high. Or, your LED might be
wired backwards, or the transistor is connected wrong, or maybe your
transistor is burned out. Make sure that the transistor is connected
similar to the little drawing above. Also, if the polarity
of the LED is reversed, the LED will not light up. Try changing the
connections to your LED to reverse their order, then connect the battery
and test the circuit again. If you suspect that humidity is very high,
test this by rubbing a balloon or a plastic object upon your arm. If the
balloon does not attract your arm hairs, humidity is too high.
1. SENSE E-FIELDS
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 electric 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. (But then, e-fields should extend to
On a very low-humidity winter day the circuit will respond
at a much greater distance. This happens because, when humidity is low,
the combing of your hair then generates a much stronger separation of
electric charge upon the comb's surface. Note that a metal comb will not
since any separated electric charge immediately weakens by spreading to
and across your whole body. A plastic or hard rubber comb works well
because rubber is an insulator and the imbalanced charges can't leak off
Try simply TOUCHING a plastic pen briefly to hair. The FET
will detect even this tiny negative net-charge on the pen. The sensor
will usually not indicate the equal positive that appears on your hair,
since hair is made conductive by humidity, and the positive net-charge
leaks to your head. The polarity of the surface electric charge on the
plastic pen is negative. The rule for this FET is, negative electric
the switch (and the LED) off.
2. SENSE POSITIVE ELECTRIFICATION
This FET sensor is not an ideal educational device because it responds
differently to positive than to negative. Create some positive net-charge
by affixing a small tuft of hair or wool to the end of a plastic object
(pen or ruler), then rub the hair upon another plastic object. (If we
electrify some hair, we can avoid leakage losses by not touching it with
fingers or other grounded object.) Bring the positively-electrified hair
near the FET. Note that the LED becomes brighter, but when the hair is
removed, the LED goes dark and stays that way. Bring the hair close by
again, and the LED lights up again. Rules for this FET:
Turn the LED back on by simultaneously touching fingers to the "Gate" wire
and to some other part of the circuit. Or, touch a plastic pen to some
hair, then wave it near the sensor, and the LED will light up. Remember
this trick when doing other demonstrations.
electrometers do not suffer from this "reset" effect, but professional
electrometers cost several hundred dollars at the very least!)
- negative objects turn the LED off, it lights again when removed.
- positive objects make the LED bright, then dark when removed.
The MPF-102 is an "N-channel" transistor, and is turned off when
the movable negative electrons in the transistor body are pushed out of
the silicon, changing it to an insulator. You can also buy "P-channel"
transistors which work backwards: their silicon is full of movable
positive charges called "holes," and they are turned off by a positive
charge on the gate. Try buying a few 2N5460 transistors from the usual
3. ELECTRIC CHARGE IS CONSERVED
Mount a tuft of hair on a plastic rod, verify that it is completely
discharged and does not affect the FET. Take a second plastic rod (or
plastic pen!) and verify that it is also completely neutral. (Fondle the
whole pen with slightly damp hands if not.) Now hold the plastic handle
and touch the hair-tuft to the tip of the pen, separate them, then hold
them up to the sensor one at a time. You'll discover that the end of the
plastic pen is now negative and turns the LED momentarily off. The hair
tuft is positive and turns the LED on, then off.
Contact between the hair and the plastic
caused some assymetrical sharing of the equal positive and negative
"electricity" within them. When they separated, some negative
stayed with the plastic, leaving it with more negatives than positive (net
negative charge.) At the same time, the hair was left with fewer
negatives than positives, for a net positive charge. Atoms were torn
apart, "ionized", and pairs of electrons and protons were yanked apart and
separated to vast distances. Note: "static electricity" is not caused by
friction, it is caused by contact between dissimilar materials, followed
by separation. We could say that it's caused by "peeling".
4. PEELING CAUSES ELECTRIFICATION
The "peeling" effect can be demonstrated with a roll of plastic adhesive
tape. Peel a few inches of tape off the roll and hold it near the
circuit. The LED will show that the tape is strongly electrified. Now
use the sensor to test the tape dispenser. You will discover that the
roll of tape has an opposite polarity compared to the strip of tape. This
illustrates that "static" electrification does not require friction, it
only requires intimate large-area contact between dissimilar materials.
Matter is made of positive and negative electric charge, and the peeling
can separate the electric charges that were already there in the matter.
the plastic backing of the tape is a different material than the adhesive,
when they touch together there is assymetric bonding and electron-sharing.
This leads to separation of opposite charge when we peel tape from its
roll. Also, try taking two strips of tape, stick them back to front (fold
little tabs so you can separate them again,) pat them down with moist
hands to discharge them, then peel them apart. Hold each near the sensor.
One strip indicates strongly positive, the other is equally negative. The
strips will attract each other. Try other demonstrations from Sticky
Electrostatics, using the Charge Detector to show polarity of various
parts of the tape.
[NOTE: people have found that "Scotch" brand tape doesn't work as well
for the above activity. It contains some chemicals that prevent
electrification. Use some other, inexpensive brand of tape instead.]
5. JUMPING ELECTRONS, "VOICE CONTROL"
If you build a tiny compact version of the FET circuit (solder it to a
torn-open battery connector), you can try the following trick. Hold the
circuit in your hand, make sure the LED is lit, stand on a rug, then jump
up and down. The LED will flash on and off. Walk around, and the same
thing happens. As your shoe soles make contact with the rug and then peel
away from it, your entire body becomes electrified. This makes the sensor
respond. ANd when jumping, if you place your shoes back onto the
oppositely electrified footprints,
you cancel out the net electric charge and the sensor indicates another
change. Scuff your shoes, stomp up and down, jump around, and the sensor
will flash wildly. Demonstrate to onlookers that the sensor does not
respond when you shake it up and down, but it does respond when you jump.
On a dry day, you can control the sensor with the tiniest motion: scuff
one shoe, then lift the toe to turn the sensor on and off. Say "on",
"off" while moving your toe, and you have a "voice control" magic trick.
Let some poor fool examine the sensor, yell at it, etc. It will only
respond to your voice! (grin!)
6. CHANGE THE SENSITIVITY
The circuit amplifies tiny voltages, and we can change it's "gain."
Obtain a small capacitor with a value below 100 picofarads. Any value
will do. Connect it
between the FET's gate lead and one other FET lead (doesn't matter
which one.) This greatly reduces the
sensitivity of the device. In situations where the sensor is TOO
sensitive, this can make a big difference. Capacitors larger than 100pF
can be used, but they REALLY wipe out the sensitivity. The large the
value, the smaller the sensitivity.
The capacitor does this because it forms
part of a circuit called a "Capacitive voltage divider," a sort of
loudness control for invisible voltage fields.
Next, make the circuit MORE sensitive.
Obtain an alligator clip-lead, and connect it to the Gate lead of the FET.
hang loose without touching anything. You'll find that this has vastly
increased the sensitivity of your FET circuit. It does this by increasing
the capacitance between the FET gate and the source of the voltage signal.
dry day it will
respond to hair-combing from 20ft away. If a TV screen is present, the
sensor will act weird (especially when people walk between the screen and
the sensor.) The clip lead acts as an extra antenna, and the longer it
is, the more sensitive the FET circuit becomes.
7. FIELD DISTORTIONS
Electrify a plastic object, place it on an insulating support, place the
FET sensor near it, then make sure the LED is turned on. If you now wave
your hand near the object or the sensor, the LED will respond. Your hand
causes the e-field around the object to distort and change. Even though
your hand is not electrified, the FET responds. You've created a sort of
"DC Radar" system which sends out a signal and then responds when nearby
objects "reflect" the signal. Some types of industrial sensors
("proximity" or "capacitive" sensors) use this effect. Some burglar
alarms do as well.
8. VANDEGRAAFF SENSING
See at what distance your FET electrometer can sense the e-field from
an operating tabletop VandeGraaff electrostatic generator. Suddenly
discharge the generator by using a grounded sphere electrode, and watch
the distant FET respond. You are actually sending out radio waves with
nearly zero frequency when you do this. The FET does not actually
respond instantly, there is a speed-of-light delay (about one nanosecond
per foot of distance.) It takes a short while for the wave of vanishing
e-field to reach the sensor. Radio waves are simply propagating changes
in electric fields, so your VDG machine and FET sensor constitute a simple
radio transmitter and receiver.
9. HOMEMADE CAPACITORS
The FET circuit is so sensitive that it will detect the energy stored on a
tiny homemade capacitor. Build a simple capacitor out of aluminum foil,
styrofoam (from a coffee cup), and wires. Store energy in the capacitor
by briefly connecting it to a 9V battery. Now touch one capacitor wire to
the negative battery terminal of the FET circuit, and touch the other
capacitor wire to the Gate terminal (avoid touching the wires with
fingers, this will discharge the capacitor.) The LED will indicate the
stored energy. Use the 9V battery to reverse the polarity of the
capacitor, then test it again with the FET and note that the polarity is
indeed backwards. Note: don't use paper for your capacitor dielectric,
paper becomes slightly conductive when humidity gets high, and your stored
energy will mysteriously vanish because the paper offers a leakage path so
the separated electric charges can recombine. Another note: this
demonstrates that "static electricity" and battery circuits are the same.
The FET detects the potential difference created by the 9V battery, just
as it detects the much larger potentials in the space around electrified
objects. It is not too far wrong to say that "static electricity" is
simply "voltage." Everyday circuits are driven by the "static
electricity" produced by their low voltage power supplies.
10. DIPOLE ANTENNA
After you use this FET device for awhile, you'll get the idea that it has
just a single antenna terminal. However, like all voltmeters, it actually
has two. The rest of the circuit acts as the other terminal. To
demonstrate this, build a miniature version of the detector circuit onto
the top of a 9V battery. If you hold the battery as usual, the Gate does
act as the antenna, and negative objects make the LED go dark. Now
carefully grasp the Gate wire between fingers and lift the whole device
into the air. Avoid touching the battery. If you now hold a negatively
electrified object near the battery, the LED will get brighter instead of
dimmer. Polarity of operation has been reversed. If you lay the whole
unit down upon an insulating surface and approach it with electrified
objects, you'll find that the FET gate wire responds with one polarity,
while the battery and the rest of the circuit responds with the other. Try
connecting the gate wire to earth ground, then suspend the rest of circuit
with an insulating handle. If you hold up objects having various
polarities, you'll find that polarity of operation is opposite that of the
11. 'SCUSE ME, WHILE I SENSE THE SKY
All over the earth, thunderstorms are transporting negative electric
downwards and positive charge upwards. As a result, the earth is
electrified negatively everywhere, while the sky is positive. (Actually,
it's the conductive ionosphere which is positive.) The FET sensor can
detect this. Take it outdoors, away from trees or buildings. Hold it
high in the air, then lower it to the ground while watching the LED.
(Maybe get a tall adult to do this.) The LED will get darker when the
device is lowered, and get brighter when it is raised up. The earth is
negative! Maybe hang a cliplead antenna on the sensor wire to improve
sensitivity. (This polarity reverses when there is a thunderstorm directly
overhead, but I wouldn't suggest standing out in the open when there is a
chance that lightning may strike!)
12. BATTERY VOLTAGE (DANGER!)
WARNING! DANGEROUS VOLTAGE INVOLVED
This one is for science teachers and advanced experimenters only.
Voltages above 60V are a shock hazard! If you do not know how to
safely work with dangerous high voltage, then don't perform this
WARNING! DANGEROUS VOLTAGE INVOLVED
Get ten 9V batteries. (Old batteries are fine, as long as they still put
out 8V or more.) Form them into a chain like so: place five batteries
side by side on the table, so the connectors all are aligned the same way.
Then link them together by plugging the other five batteries upside down
into the first batteries.
This creates a connected block of batteries. It's a ninety-volt battery.
Now wave the antenna of the FET
circuit around this battery chain. DON'T LET THE ANTENNA TOUCH ANY OF THE
BATTERIES OR THEIR CONNECTORS! You'll find that the negative end of the
battery chain will make the LED go dark.
See what's happening? Batteries can create "static electricity" effects.
But it only happens at high voltage. The voltage of a single 9V battery
does not affect the FET sensor, because the e-field of a 9V battery is a
bit too weak. But a 90V battery creates an e-field ten times stronger!
13. UNTESTED SUGGESTIONS
Here are a couple of things to try out. I haven't tested them, I don't
know how well they work. You be first!
Electrify a large plastic object while no one sees, then have a group of
people with FET-based charge-detectors try to find which object in the
the imbalanced electric charge.
Have everyone build FET electrometers. Line them all up in a row,
electrify a plastic object, then sweep the object back and forth. You'll
be able to "see" the electrostatic field that surrounds the object. Hold
your hand near the row of detectors while standing on a rug. Jump up and
down and see what happens.
Use a piece of cloth to create a small electrified spot on a plastic book
cover. Use the FET device to find the spot. Draw an electrified shape
using the cloth as a paintbrush, then see if you can use the sensor to
figure out what the shape is.
Build many FETs and LEDs in a row on a wooden stick. Connect them all to
one battery. Place a negatively electrified object on a table in a dimly
lit room, then sweep the FET-stick rapidly past the object. Go back and
forth really fast, and you should see a row of red lines caused by the
moving LEDs. In the middle of the red lines will be a black splotch
caused by the electrostatic field surrounding the negative object! Repeat
this test, but this time use a bit of cloth to write the letter "A" on a
plastic book cover in invisible, negative net-charge. Can you see the "A"
when you sweep the stick back and forth? Mount your row of LEDs on some
sort of motorized propeller, and you'll have an automated "charge detector
WHERE IT CAME FROM
The circuit is an electronic version of an Electroscope.
Electroscopes are simple science instruments for measuring high voltage.
have been in use for hundreds of years.
This FET charge-detector circuit is based on a much earlier circuit called
"electronic electrometer" made with a vacuum tube. As a kid I found the
schematic in an old paperback book on ham radio projects from Pop.
Electronics magazine. It used a 6J7
tube and an NE2 bulb and 100K
resistor connected to the plate terminal. I
blew away my allowance for weeks to buy that tube (plus a 6.3V
transformer, plus a fancy bakelite box.) It used 120VAC line voltage
connected to the cathode and the LED (the tube then acts as a rectifier.)
The 6J7 tube has a terminal on
the top which connects to the tube's Grid terminal (a "grid cap," rather
than the more usual plate cap.) My 'antenna' came from a coffee can run
through the can-opener, plus the plastic can lid to protect the sharp
metal edge. When a negatively electrified plastic pen was
waved near the grid, the glowing NE2 would turn off. On winter
days the sensitivity was quite amazing.
Other modern devices also respond to nearby electrified objects. When
first were sold, I bought one and used an ohm meter to measure across
source/drain. Sure enough, when an electrified object was waved near the
unconnected grid wire, the ohmmeter reading went crazy. Also, if you have
an FET-type opamp chip (TL072, etc.), and you leave the input floating, or
if you have CMOS logic chips with the inputs floating, they will sometimes
act crazy when you wave your hand around them. Unless the humidity is
very high, your body usually has
enough DC charge to turn them on or off.
I tested several common FETs to find a sensitive one, and discovered that
MPF102 was much more sensitive than the original vacuum tube I had as a
kid. With a couple feet of gate terminal wire, I could turn it on and off
by waving an electrified plastic comb back and forth from over ten feet
(With longer antennas it started picking up 60Hz hum, and was overloaded.)
Later at the Museum of Science in Boston I designed an exhibit, an array
of many hundreds of these things, each with a small steel screw as the
antenna. See "Aura
Tester". Also see a bit more about the circuit.
HOW IT WORKS
A complete description of this device requires delving into the physics of
solid state electronics. Instead, here is a quick description based on
the fluid analogy for electric charge.
Metals act as conductors NOT because charge can pass through them.
Instead, they are conductors because
they contain charge which can move. Think of a metal wire as being
hose that's aways full of water. And remember, vacuum is an
insulator, even though it presents no barrier to charges. But vacuum
contains no charges which can move.
The "sea of charge" in a metal is not very compressible. To remove even a
tiny bit of it would take a huge amount of energy. In metals, the
"electricity fluid" is dense like water, since each atom
contributes one electron to an "electron sea", where the electrons don't
stick to single atoms but instead orbit all throughout the material. If we
could remove all the movable electrons from a metal, that metal would
become an insulator. Unfortunately, removal of electrons from even the
thinnest metal wire requires gazillions of Newtons of electrostatic force,
and develops gazillions of volts of potential difference. ("Gazillions"
means some huge number with way too many zeros!). Metals are
and we can't easily change that.
This is where silicon comes in. While a metal's electron-stuff
acts like a dense fluid, in silicon the mobile charges act like
compressible gas. In silicon, only very few atoms contribute an electron
to the "charge sea." In fact, the silicon doesn't really contribute
electrons at all, and ultra-pure silicon is an insulator. Instead, only
the tiny bits of impurities
in the silicon contribute movable electrons. If we only put a gazillionth
of a percent of impurities into the silicon mix, then the resulting
material's movable electron-stuff becomes much more compressible than the
"electron sea" within a metal. It acts almost like a gas. This reduces
the required voltage and force (by a
gazillion times!) that's needed to push the movable charges out of the
The electron-sea of a metal is not very
compressible. The electron-gas within silicon is very
So what? Well, if we can push the "electron sea" out of a conductor, we
can change it into an insulator. It would be like turning off a switch,
but almost no work is required to do it. Just apply an electrical
"push" in the form of electrostatic repulsion, the silicon becomes
insulating, and large currents
can be switched on and off.
The Field Effect Transistor is basically a tiny wafer of silicon with its
edges connected to the Source and Drain leads, and the Gate lead connected
to a metal plate layed upon the wafer. When the gate lead is electrified
negative, it repels the electron-gas out of the silicon and converts it
into an insulator. It acts like a switch that is turned off by pure
voltage. If we picture the silicon as being like a rubber hose full of
water, then the gate applies a sideways force which pinches the hose
closed. Placing a negative net charge on the gate wire causes the
"switch" to turn off and the LED to go dark. Merely holding a negatively
electrified object near the Gate lead will apply a force to the electrons
in that little lead wire, which pushes them into the metal plate, which
repels away the electrons in the silicon, which pinches the conductive
Interesting part: it really takes no energy to turn off the FET. It does
take electrostatic force, but force is not energy! And so, even a very
distant object with a feeble net-charge can affect the FET and
control the much larger energy directed to the LED.
The FET is not really turned off by negative net-charge. That is an
overly simplified description. It is really turned off by a DIFFERENCE in
the net-charge of the silicon and of the metal plate. You can either
electrify the metal plate negatively, or electrify the silicon (and the
battery, LED, and circuit wires) positively. Both will turn the FET off
by pushing (or pulling) the electrons out of the silicon. Think of the
rubber hose again: either you can squeeze it shut with fingers, or you can
lower the pressure of the whole water circuit, and the hose will be
collapsed by "suction" (by air pressure, actually.)
What are FETs good for? Well, most modern computers are constructed
almost entirely from FETs. The megabytes of memory are formed from little
grids of millions of microscopic FETs, each with a net-charge stored on
its gate lead signifying a zero or a one. The processor chips are built
of logic switches with Gate voltage as their input, and on/off switching
as their output. Other things: super-FETs can be built which actually
contain many thousands of small FETs hooked in parallel. These VFETs or
HEXFETS are often used as the main transistors of large stereo amplifiers.
A tiny vibrating voltage on their gate lead can route many amperes of
sound-frequency charge flow through the loudspeakers, and a handful of FET
wafers the size of your fingernail control the audio power for a whole