INEXPENSIVE FET ELECTROMETER William Beaty,1994 The schematic depicts the basic 'cell' of the Inexpensive Visual Electrometer Array. The MPF-102 is a Field Effect Transistor available from Radio Shack. Any LED will do, but wide-field, high-brightness types like MV5753 (Active Electronics) are desired. With a 1-in. 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. antenna, the circuit is sensitive enough to respond to the waving of a charged comb or balloon across the room. In the 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 screw, and 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 on moist days, so I painted the base of each one near the lead wires with brown high-voltage insulating spray paint. 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 eventually.) If you use another material, be aware that if the panel is a good insulator, it is also a good trap for ionized air, for "frictional" charges, etc. It invariably will itself become charged 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. The "antenna" screws were 6-32, and stuck out about 3/8" from the panel. I think I used 1" screws, to give adequate clearance. 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 insulator was a screw insulator, it was like a 6-32 washer, but with a little tube sticking out of the side. The other insulator was an 8-32 unthreaded nylon spacer. They meshed together with a slip-fit action, which totally isolates the screw from the grounded metal panel. Keep the washers clean, and install them while wearing gloves. If they get salty skin contamination on them, they can become conductive on humid days. 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" | |--__| | \ \ ___|--__|___ / |--__| .....................uuuu / oo \ uuuu........,............................. William Beaty voice:206-781-3320 bbs:206-789-0775 cserv:71241,3623 EE/Programmer/Science exhibit designer http://www.eskimo.com/~billb/ Seattle, WA 98117 billb@eskimo.com SCIENCE HOBBYIST web page