2019-11-29 Electrostatic Field Sensing, BJT Versus MOSFET

The Facebook group Electrostatic and Electromagnetic Field Detectors (I’m a member) has shown the very low current sensor made of three BC547 BJTs (bipolar junction transistors). The three BJTs are connected as Darlingtons (I called them Trarlingtons) so they multiply the base current by their betas or current gains. For three transistors, assuming each has a gain of 200, that’s 200x200x200 or 8 million! So for 8 mA to the LED, it takes only 1 nanoamp of base current!

These BJTs sense current flow and light an LED. The LED lights as long as there is a current flowing. The problem is that electrostatic fields have almost no current, that’s why they are electrostatic – they are static with a miniscule charge.

So using the three BJT sensor, if an Electrostatic field is sensed, the current flow may be so small that the LED lights too briefly to be seen or not at all. What is needed is a sensor that operates on voltage, not current. The ideal candidate is a MOSFET. Most MOSFETs are power MOSFETs, which need no gate current. But they have a gate capacitance that is hundreds or more picofarads. This is too high and a static field would not be able to charge it up enough to turn on the MOSFET. There are a few MOSFETs that are not power, just small signal, and a popular one is the 2N7000. It has a low enough gate capacitance to be turned on easily by a static field. And since it is a voltage operated device, the gate can receive a charge and turn on, and it will hold that charge for hours or days.

Here is how this super simple circuit is made. The 2N7000 source is connected to common negative (two AA or AAA cells). The anode of the LED is connected to the positive 3V. The cathode is connected to a 220 ohm resistor and the other end of this resistor is connected to the drain of the 2N7000. The gate is connected to a short length of wire, maybe 50 to 100 mm long.

I can touch a finger of my left hand to the +3V of the battery, and touch my right finger to the antenna and it will charge up the gate and the LED will light. It will stay lit for a long period when I take my finger away. If I touch the negative and the gate, it will discharge and the LED will go out and stay dark for a very long time after I remove my finger. The length of time it stays lit or dark depends on how well the gate is insulated from the source and drain. Any leakage from or to the gate will charge or discharge the gate and the LED will go on or off. So the gate and antenna must be kept clean and dry with no oily salty fingerprints to cause current to conduct.

The 2N7000 can be bought online, from sellers on eBay for example. I recommend getting 10 or more because the gate can be zapped by strong static charges. You can get a bunch for a few dollars. If you have the three BJT circuit already made up, you can add the 2N7000 source and drain in parallel with the emitter and collector of the third BJT, so you can compare how each operates.

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