There have been many articles published that proclaimed that ultrasonics, either in the form of pulses or a sweep signal, can be used as an effective insect and rodent repellent. Well, they are right! Some circuits that cost less than $20 done perfectly their work, on some old farmhouses that used to be absolutely overrun with mice and roaches. Results were obvious within two weeks of testing.
Low cost Ultrasonic Pest Repeller-1:
The first electronic circuit of a popular ultrasonic pest repeller is presented in figure 1. Despite its simplicity, the device was remarkable effective.
Figure 1. A popular ultrasonic pest repeller. The device was remarkable effective
The circuit is simply a 555 timer IC connected as a squarewave generator. Its base frequency is approximately 45kHz, as determined by the values of R1, R2, and C1. The 45-kHz "carrier" is frequency modulated by a modified trapizoidal voltage waveform applied to pin 5 of the 555 timer. That modulating voltage is developed by a network consisting of C2, R3, and R4 connected across one leg of the bridge rectifier. A check with an oscilloscope will show a sweep of approximately 20 kHz on each side of the base frequency. That sweep is from about 25 kHz to about 65 kHz and it is fed on a 2 inch piezoelectric tweeter.
Low cost Ultrasonic Pest Repeller-2:
The circuit in Figure 2 is a pest repeller described in the French electronics magazine, "Le Haw Parleur". In the article, the author claims that frequencies in the range of 20 to 40 kHz cause highly uncomfortable cavities to form in brain fluids and blood vessels of mice and insects, causing them to beat a hasty retreat. Radiated power levels can be as low as 1/3 watt.
Figure 2. This popular pest repeller, is built around a CD4011 quad NAND gate.
Looking more closely at the circuit, a quad two-input NAND gate is connected as multivibrator operating at around 40 kHz. With the minimum of filtering used in the Power supply, a residual 120-Hz sawtooth on the line from the power supply modulates the ultrasonic frequency. Two Darlington-connected NPN transistors provide some power amplification and drive for the piezoelectric speaker.
Low cost Ultrasonic Pest Repeller-3:
That device emits ultrasonic sound waves that sweep between 65000 and 25000 Hertz. The sound is apparently rather irritating for most unwanted flying and crawling pests.
The repeller is designed around a 556 dual timer. One half is operated as an astable multivibrator with an adjustable frequency of 1 to 3 Hz. The second half is also operated as an astable multivibrator but with a fixed free running frequency around 45000 Hz. The 25-65 kHz sweep is accomplished by-coupling the voltage across C2 (the timing capacitor for the first half of the 556) via Q1 to the control voltage terminal (pin 11) of the second half of the 556.
Figure 3. A pest repeller, based on 556 -dual 555- timer.
Transistor Q1 serves two purposes; it isolates the timing circuit of the first half of the 556 from pin 11 and it controls an LED indicator. When the first half is operating, timing capacitor C2 continually charges and discharges between 1/3 and 2/3, of the supply voltage . Because the base of Q1 is tied to C2, the voltage across C2 will affect the operation of Q1. The voltage at the base of Q1 causes it to conduct, thereby turning on the LED and lowering the control voltage that is applied to pin 11. The lower control voltage causes the output frequency of that half of the timer to increase to around 65 kHz. As C2 is charged toward 2/3, volt, Q1 conducts less and less. That causes the intensity of the LED to decrease and the control voltage applied to pin 11 to increase, because Q1’s emitter approaches +V. The increasing control voltage causes the output frequency to decrease from 65 kHz to 25 kHz. That sweep will take from 1 to 1/3, second depending on the setting of R1. According to theory, the periodic adjustment of the sweep rate will prevent the pests from developing an immunity to the sound. The device that radiates the ultrasonic sound is a piezo tweeter.
