INEXPENSIVE FET ELECTROMETER ARRAY
1994 William Beaty, Museum of Science, Boston

The schematic depicts the basic 'cell' of the Inexpensive Visual Electrometer Array. The MPF102 is a Field Effect Transistor available from Radio Shack and many other places. Any LED will do, but diffuse wide-angle, high-brightness types like MV5753 (Active Electronics) are best. Screws with nylon insulators were used in the panel. With a 1-in. wire antenna, the circuit is small enough to be built atop a 9-volt battery clip, then used as a handheld probe. With a 24-in. wire antenna, the circuit is sensitive enough to respond to the waving of a charged comb or balloon across the room.

In the present circuit, the LED turns on when a positive charge approaches the antenna, and turns off when negative charge approaches. The 1-meg resistor limits spark current should any large, charged persons actually touch the gate wire.

The earth-ground point in the circuit can actually be at any of the schematic nodes, not just the one shown. And if the circuit is battery-powered, you can even ground the gate and use the entire circuit and battery as the "antenna." I use the point shown in the schematic because it allows the handheld version to easily be lit/darkened via the battery: Hold the gate lead, touch one battery terminal, then release the gate lead. Depending on the + or - terminal you touch, the LED goes on or off.

While extremely inexpensive, this particular circuit has a problem. It can be turned on and off! In other words, it suffers from inductive charging via the diode junction which is part of the FET gate, and this makes the circuit behave differently with positive signals than with negative. If a negative charge approaches, the LED turns off, and when the charge is removed, the LED turns on. The opposite is NOT true for positive charges. If a positive charge approaches and the LED is on, the LED will stay on, but the FET's internal diode will turn on and allow the gate wire to become charged. When the positive charge is removed, the LED will turn off and stay off! There is a "memory" effect involved with the circuit, and so the circuit does not always reflect the state of the e-fields in the space around it. However, it is amazing that such a simple device detects e-fields at all, and the price is so low that every student can have their own e-field sensor. For hard numerical data, an expensive electrometer instrument can be used instead. If you are building a much smaller array, you might want to consider using a FET input opamp circuit and red/green polarity indicator LEDs.

THE ARRAY ELECTROMETER

To protect the circuit from direct discharges, I placed a 10meg resistor in series with the lead wire coming from each antenna screw, and I connected an NE-2 neon pilot light between the FET gate and earth ground. The glass of the neon bulbs turned out to cause humidity problems (decreased sensitivity) on moist days, so I painted the base of each bulb near the lead wires with red-brown, high-voltage insulating spray paint. It's important to not touch the base of the neon lamps either before or after painting, since fingerprints become conductive when moist. Handle them only by the lead wires. If there are finger prints on them, scrub them off in a bowl of rubbing alcohol.

For the main panel I used sheet aluminum anodized with black color (which I picked because the black wasn't insulating,) bought from a decorative metal supplier. It was covered by a peel-off protective sheet which is seen in the JPEG image (and must be removed when complete.) If you use another material for the plate, be aware that if the panel is coated with a good insulator such as paint, it also becomes a good trap for ionized air, for "frictional" charges, etc. It invariably will become charged up and distort the light patterns terribly.

You might wish to experiment with placing the whole panel behind glass. This will protect it from direct discharges, but if the glass becomes charged (will there be fur or cloth provided?), the exhibit will go flakey on you. The conductive panel is connected to earth ground.

Each "antenna" screw was next to an LED. The screws were 6-32, and stuck out about 3/8" from the panel. I think I used 3/4" screws, to give adequate length out the back. I used black "socket cap" screws just to keep everything black for good contrast. The vertical rows of sensors were about 1" apart, the screws were about 1.7" spaced vertically, with each LED about 3/4" from each screw. I bought white nylon insulators from an electronics catalog (digikey.com.) One of the insulators was of the screw-insulator type, it was like a 6-32 shoulder washer, but with a little thin tube sticking out of the side. The other insulator was an 8-32 unthreaded nylon spacer 3/8" long. The two insulators meshed together with a slip-fit action, which totally isolates the screw from touching the grounded metal panel. Drill each hole with a #16 bit. The 6-32 washer is mounted from the rear of the panel, the 8-32 washer is placed over it from the front, the screw is inserted from the front, then the nut and solder-lug are screwed on the back.

Keep the nylon parts very clean. Don't open their bags until you're ready and install them while wearing gloves. If the washers get any salty fingerprint contamination on them, they can become conductive on humid days. If one or more segments stop working on humid days, it means that surface contamination is somewhere present on the washers, on the NE-2 bulb, or even on the FET itself.

To power the array I first soldered each FET to its adjacent LED, to the antenna lug, and to the NE-2 bulb. This leaves each element with two disconnected leads, a positive and a negative. I used horizontal wires to connect each row of elements together in parallel (I stripped small holes in the insulation to allow solder connection.) These wires are visible in the photo above: yellow for positive, bare wire for negative. Then I attached long thin pieces of copper circuit board vertically to the aluminum panel, then soldered all the horizontal wires to the circuit board strips.