INEXPENSIVE FET ELECTROMETER William Beaty,1994 http://amasci.com/electrom/e-field2.txt 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. Insulated screws are 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 a large, charged person actually touch the gate wire. The earth-ground point in the circuit can actually be at any of the nodes, not just the one shown. You can even ground the gate and use the entire circuit as the "antenna," if the circuit is battery-powered. I use the point shown in the schematic because it allows the handheld version to easily be lit or darkened via the battery: Hold the gate lead, hold 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 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, 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 1" 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. One of the insulators was of the screw-insulator type, it was like a 6-32 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. The 6-32 washer is mounted from the rear, the 8-32 washer is placed over it, the screw is inserted from the front, then the nut and lug are screwed on the back. Keep the washers very clean, and install them while wearing gloves. If the washers get 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. ONE "ANTENNA" FROM THE SENSOR PANEL, EXPLODED VIEW: __ _- -_ /__________\ |--__| |--__| |--__| |--__| |--__| |--__| |--__| Screw, 6-32 |--__| |--__| |--__| |--__| | | |--__| | | | | |--__| | | | | |--__| | | | | | | Nylon standoff, 8-32 | | | | | | | | ======== =============== Hole in aluminum sheet || || || || || || || || Nylon screw-insulating washer, 6-32 || || ___|| ||___ |____| |____| _____ ________ \ \ "Ground lug" 6-32 ___ ___ \ / | | \ \ | | | | \____| |____/ Hex nut, 6-32 ASSEMBLED "ANTENNA": __ _- -_ /__________\ | | |--__| | | Front | | |--__| | | | ||||--__|||| | | ||||--__|||| | | ||||--__|||| | | ||||--__|||| | ========|||--__|||=========================== |____||--__||____| Rear -----|--__|------_ ___|--__|___ \ / |--__| \ \ Sensor circuit attaches to "ground lug" | |--__| | \ \ ___|--__|___ / |--__|