Ions are defined as electrically charged atoms. Positively charged ions have a deficiency of electrons, and negatively charged ions have a surplus of electrons. An ion can also be classified as an atom or molecule with an electrostatic charge. Another classification of an ion is a charged particle that is formed when one or more electrons are taken from or added to a previously neutral atom or molecule.

The Ion Detector described in this article can be used to detect the presence, and indicate the relative amount, of free ions in the air. The Ion Detector, a handheld unit about the size of a pack of cigarettes, is designed to indicate ion emissions from ion generators high-voltage leakage points, static-electricity sources, electric-field gradients, and in other situations where the presence of their relative flux density is required.

The front cover features, a sensitivity control with on-off switch, a high flux indicator lamp, and a panel meter. An antenna, mounted on the top of the unit, serves an external ion collector. A strip of metallic foil on the outside of the plastic enclosure touches the users hand and is used to ground the unit. For fixed applications, the strip can be replaced by a wire connected to the ground.

Circuit Description. Figure 1 shows a schematic diagram of the Ion Detector - a rather simple circuit consisting of three transistors (two PN2907 PNP units, and a single PN2222 NPN unit), three resistors, an antenna, and an LED.

In that circuit, a telescoping antenna is used as the pickup. In the presence of an ion field, ions accumulate on the antenna, causing a minute negative current to flow to the base of Q1. Capacitor C1 and resistor R1 form an RC network, whose function is to eliminate any rapid fluctuations. Once the negative current becomes large enough, it causes Q1 to turn on, connecting the negative terminal of battery B1 to the base of Q2. That forward biases Q2, causing it to turn on. That, in turn, couples the base of Q3 to the positive terminal of the battery, forward biases Q3 - whose collector is in series with current-limiting resistor R2 and meter-sensitivity control R3 - causing it to conduct.

With Q3 turned on, meter M1 indicates (in a non-linear manner) the relative level of ion flux, while LED1 (which is connected in series with Q3's emitter) lights to give a visual indication of strong ion fields. It should be noted that in order for the unit to operate properly, some sort of ground is usually required.

Metallic tape is used in the prototype to provide a convenient contact for the users hand, thereby providing a partial ground. If possible, such as when the unit is used as a monitor at a permanent location, the detector should be grounded to a water pipe, or some other convenient grounding point.

The detector is set up to detect negative ions. It can be made to detect positive ions by simply reversing the polarity of the transistors that comprise the circuit, i.e., PNP units become NPN units, and NPN transistor is replaced by a PNP unit. It should not that the performance of the detector is seriously affected by high humidity. Damp or moist air tends to impair the circuits ability to detect ion flux.

The Ion Detector can be used to give a quick indication of the presence of a negative ion field, aid in identifying its source, and indicate its relative strength, but it is not designed to provide an absolute measurement of flux intensity. The circuit can also be used to aid in making adjustments to ion sources, by noting the meter's needle deflection as you attempt to increase or decrease ion emissions. The Ion Detector can also be used to ferret out residual ion fields, check for ion leakage (in shielding tests, for example), or to test for static charges (in people's clothes, fluorescent lighting, plastic containers, certain winds, etc.), along with a host of other applications.

Fig. 1. The Ion Detector is a rather simple circuit consisting of three transistors, (two Pn2907 PNP units, and a single PN2222 NPN unit), three transistors, a telescoping antenna (which is used as a pickup), and a LED.

Construction. The author's prototype of the Ion Detector was assembled on a section of perfboard, using point-to-point wiring for inter component connections. Pay close attention to the orientation of the polarized components (diodes, transistors, electrolytic capacitors, etc.), as well as the polarization of the DC source that will power the circuit when assembling the circuit. It is very important that you verify all your interconnecting wiring.

It is highly recommended that the circuit be enclosed in a plastic project box. Once the circuit is completed, a 1/2 inch wide strip of aluminum is attached to the side of the enclosure, and is then connected to the circuit board (at the junction of C1, the positive lead of the panel meter, and the positive terminal of the battery) as shown in Fig. 1. The aluminum strip serves as the circuit's grounding point. The grounding strip can be replaced or supplemented by a wired alligator clip for connection to a "true" earth ground ( a water pipe, for instance).

The author used a telescoping antenna as the ion pickup in his prototype unit; however, a piece of stiff wire (a wire hanger, for example) would also work. In either case, the antenna must be electronically isolated; i.e., it should not be connected to ground in any way. Note that S1 (the on-off switch) is piggy-backed to potentiometer R3 (a 5K potentiometer that serves as the meter's sensitivity control). You can also use a potentiometer with a piggy-back switch or use two separate components.

For meter M1, the author used a small 100-mA panel meter; using a meter with a rating other than that specified may affect the performance of the unit. It is also important to remember that any leakage around the input of Q1 will reduce the circuit's sensitivity. To help prevent (or at least reduce) leakage, the circuit can be coated with a high-quality varnish. If you decide to coat the circuit, make sure that the unit is completely clean and dry before applying the varnish.

Use. To demonstrate the unit's sensitivity, run a plastic comb through your hair, and place it near the antenna of the Ion Detector. Making sure that the unit is grounded (either by the user touching the aluminum strip or by connecting an earth ground to the circuit), bring the comb near the antenna. As the comb is brought near the antenna, you'll note a needle deflection on the meter (indicating the presence of ions), and LED1 lights. As the detector is brought closer to the ion source, the meter needle should deflect harder. If the needle deflects too hard (pegs), R3 can be adjusted to bring the meter reading on scale. That's all there is to it. While the Ion Detector is not a precision instrument, it can come in handy in your workshop or laboratory.

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