Detect charged bodies with electronic electroscope

-June 28, 2013

Editor's Note: The last electroscope I used employed gold leaves or pith balls to indicate charge, but here's an electronic version with no moving parts. Though one of our reviewers felt the design was not robust or repeatable, I thought it was interesting and unusual enough to warrant inspection by our readers. What do you think?

Detect the type of charge on charged bodies using this simple electronic electroscope which uses an ordinary CMOS CD4011 chip. The high input impedance of the NAND gates has been put to use to detect the potential difference due to a charged body. The schematic circuit diagram of this scope is shown in Figure 1.

By simply bringing a charged body near the input of a CMOS gate, it is possible to detect the polarity of charge on the body. However, this type of trivial arrangement lacks a well defined quiescent state. The Design Idea here provides this quiescent state by making use of a transistor as an attenuator rather than an amplifier of current.

A reverse-biased silicon diode has a current of the order of 1-10nA. Here, it is desired to produce a biasing current smaller than this, so by making the reverse-biased diode current as the emitter current it is possible to have a base current that is much smaller than 1nA. Further, it is desired that when the charged body is brought near the detector, the LED indicator is stable for a sufficiently long time. It is also desired to have control over sensitivity, and return of the detector to its quiescent state once the charged object is withdrawn. All these qualities are achieved by employing Q1, Q2, C1, & C2 as shown in Figure 1.

Figure 1 Schematic circuit diagram of electronic electroscope. C1 & C2 determine the sensitivity while Q1 & Q2 (100 < β < 200) provide high impedance biasing. +/- are separate detector plates.

Q1 & Q2 provide the biasing required to return the circuit to its quiescent state once the charged object is withdrawn. The gain of the transistors determines how long the indicator LEDs glow and how quickly the circuit resets once the charged body is withdrawn. C1 and C2 decide the sensitivity – i.e., how much charge is required to raise or lower the voltage at the inputs of the gates. This in turn corresponds to how strongly the test object is charged. Transistors with very high gain (β>200) have the disadvantage of making the electroscope slow to return to quiescent state once the object is withdrawn. And, without C1 and C2, the circuit becomes extremely sensitive, such that a person (human beings are positively charged) standing at a distance can easily trigger the instrument. This extreme sensitivity is often more than desired. In testing, a BC177 and a 2N3904 were used for Q1 and Q2 respectively. This circuit has been tested satisfactorily by the author, as demonstrated below:

Some observations:

  • The circuit achieves quiescent state i.e., both LEDs go off in the absence of a newly charged object.
  • The normal positive charge on a person was detected.
  • Two bodies rubbed against each other had consistent opposite charges.
  • The LEDs glow for a sufficiently long time and then go off (i.e., a balance is set up with the external potential difference).
  • It will be seen that when the object is withdrawn, the LED indicating the opposite charge will glow for a short time.
  • When a positively charged body is brought near the detector plates, the green LED glows.
  • When a negatively charged body is used, the red LED glows.

Figure 2 Suggested housing for the electroscope.

Figure 3 Suggested layout for the electroscope.

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