Description

What distinguishes Ahern's instrument from other charge detection and measurement apparatuses used in electrostatics demonstrations, such as the conventional leaf electroscope and the tonal electrostatic voltmeter, is that Ahern's device responds to the movement of positive (+) and negative (-) electric charge toward or away from a sensing electrode. As such, the instrument is probably most closely akin to some of the electrostatic charge monitors now commercially available and routinely installed in electronics assembly areas to detect conditions that might cause damage to sensitive electronics components during assembly.

The instrument is small and portable, it operates on a 9 volt battery, and the output is visual. Furthermore, its construction requires only minimal skills with a soldering iron and other simple tools, and it can be packaged very conveniently in a small, shielded box. Refer to the photo at the left. A short, telescoping cellular telephone antenna mounted so that it retracts into the box when not in use, serves as a sensing electrode.

Use of the moving charge sensor

The instrument is sensitive primarily to the movement of charge rather than to its presence. Thus, when the approach of negative charge is sensed, the green LED comes on, while the red LED lights if positive charge is approaching. As negative charge starts to recede from the sensing electrode, the green changes to red, while red changes to green as positive charge recedes. If a condition of balance can be achieved between positive and negative, then both LED's can be made to light up; this balance is exquisitely delicate due to the very high input impedance of the cmos gates. In the absence of any charge, neither LED lights up.

The instrument is much more sensitive than most hand-held charge detectors. During demonstrations of electrostatic phenomena -- for example, triboelectrification -- the moving charge sensor can be used to sense the charge and to determine its sign. The circuit draws very little current, so the instrument can be operated for extended periods of time without draining the battery.

The circuit

The heart of this inexpensive circuit, the schematic of which is shown below, is a quad cmos NAND gate integrated circuit (4011). Experienced circuit designers will smile when they recognize that this circuit beneficially exploits the notorious static sensitivity of cmos IC devices. Only three of the four NAND gates in the IC are actually used in the instrument; the input of the fourth gate should be grounded. The 3 MW series resistor protects the chip from ESD damage, while the capacitors serve the primary function of biasing the gates to a quiescent state. Resistors in the ~10 GW range may be used instead, but the capacitors -- Ahern specifies tubular ceramics -- with their very small but not negligible leakage current, work just as well and are definitely cheaper and more readily available. In addition, the charge storage feature of the capacitors introduces some incidental phase lag between the two detection circuits that permits both LEDs to remained lighted when both signs of charge are detected simultaneously.

A small, inexpensive telescoping antenna -- of the type used in cell phones and available at electronics supply stores -- serves the function of the input charge-sensing electrode. But an even cheaper solution for the sensing electrode is a length of rigid wire formed into a loop so as not to present a hazard. One must make sure to remove the insulation from this wire. If the circuit is packaged in a plastic box, shielding is crucial. Aluminum or copper foil taped or glued to at least one side of the box and connected to the circuit ground is essential. In addition, if the telescoping antenna is used, it may be a good idea to shield it from the circuit on the inside of the box.

If the image of the circuit schematic is poor, please contact T. B. Jones.

Reference

Electrostatics Society of America, Electrostatics Newletter, Issue #142, Jan/Feb., 1999.