Insect Repellent (II) (BM016)

Figure 1 shows a circuit that uses two transistors to produce a tone in the range determined by C1 and C2, and adjusted by P1. It consists of an astable multi vibrator driving a piezoelectric transducer or a small loudspeaker.

Figure 1 – Using two transistors

Another circuit using two complementary transistors is shown in Figure 2. This circuit can drive a loudspeaker from 5 to 10 cm (4 or 8 ohm) when powered from 3 to 12 volts.

Figure 2 – Using complementary transistors

For voltages up to 6 volts Q2 must be replaced by a BD136 (or a TIP32) mounted on a small heatsink.

In this case, the output power will be up for some watts, ideal for experiments or applications involving large ranges.

The frequency depends on C1 and can be adjusted by P2. Replacing C1 by capacitors in the range of 0.4 to 2.2 µF, the circuit will produce very low-frequency pulses, such as a metronome.

Experiment to see the effect of this kind of audio stimulation on insects an animals.

Using the 555 IC

Figure 3 shows a tone generator using the 555 IC.

Figure 3 – Circuit with the 555 IC

This version can drive a piezoelectric transducer and, adding a transistor, a small loudspeaker.

The basic frequency is adjusted by the potentiometer P1 and depends on C1. With the components shown in the circuit, the frequency can be adjusted from 100 Hz to 1 kHz, representing the normal range of sounds that repel or affect most small insects.

The reader is free to change the values of C1, generating sounds of any frequency, including supersounds.

Of course, when generating supersounds, an adequate transducer must be used, because small piezoelectric transducers have the reproduction range limited to 10 kHz.

A suitable transducer can be found in piezoelectric tweeters. In this case, the small transformer found inside the loudspeaker must be removed and the transducer directly coupled to the circuit.

Another idea would be to use a transistor stage to allow the circuit to operate with low impedance loads. This circuit is shown in Figure 4.

Figure 4 – Using a transistor drive stage

This circuit can be used with common loudspeakers if operating in the audible range or it can use piezoelectric tweeters if operating in the supersound range.