Long ago a cat could keep the mice away, but nowadays most cats enjoy refined menus based on turkey, beef or fish, leaving mice and rats undisturbed in warehouses and basements. Fortunately, there is an electronic way to repel mice. It is based on the use of ultrasound frequencies. Mice and rats are able to hear ultrasounds but humans cannot.
Many experiments prove that mice are being disturbed by loud sounds at ultrasonic frequencies in the range of 17 to 30KHz. These frequencies are not perceived by people. Thus, many electronic rodent repellents are operating by producing these specific sounds. These devices are effective enough and indeed they are able to remove mice and rats away. However, there is a small detail: After the first few days of use, the effectiveness of ultrasonic repellents is reduced significantly. Rats in particular are pretty smart animals and they do not easily give up places where food is stored. Also, they get used to the sounds pretty soon and they no longer are intimidated.
So we decided to make an electronic rodent repellent which uses a mechanism to continuously change the frequency of the generated sound, in order not to get used by the rodents. Moreover, our device does not sound continuously but uses bursts. Every burst lasts for 3 seconds and then follows silence for another three seconds and so on. This way the produced sound is much more disturbing for the rats.
The heart of the circuit is IC2. IC2 is the popular 4046. It has an internal voltage controlled oscillator (VCO) and a PLL (Phase Locked Loop). We only use the oscillator whose frequency can controlled by the voltage applied to pin 9.
Oscillator’s frequency depends on the voltage applied to pin 9 and also on the capacitance of C3 which is connected to pins 6 and 7. By using a capacitor of 3.9nF and by applying voltages from 2 to 11V, respectively, at pin 9, we are able to set the oscillator’s operating range from 17 to approximately 30 KHz. The tuning range can be further adjusted by the R7 potentiometer. The oscillator may also be turned on or off by applying logic 1 and logic 0, respectively, on pin 5.
In order to produce ultrasounds for 3 seconds and then pause for another 3 seconds, we simply toggle the logic level on pin 5. The transitions are being produced by an astable multivibrator (a type of oscillator) which is based on IC1A. The frequency of the astable multivibrator is determined by the RC constant (product) of R4 * C1. Moreover, R1 and R2 are used to bias the non-inverting input of IC1A op-amp at about 6V so that the op-amp may normally operate from a single voltage.
R4 and C1 set the operating frequency of the unstable multivibrator to approximately about 1/6 Hz, so that the oscillator switches from on to off and vice versa, every 3 seconds. C1 and R4 not only determine the frequency of the multivibrator but they also act as a low pass filter (integrator). The integrator converts the bi-stable pulse train produced at the output (pin 1) of IC1A to a "sawtooth - shaped" signal. This is due to the charging and discharging of C1. This signal is then applied to pin 9 of IC2, via the voltage follower (buffer) IC1B and modulates the VCO’s frequency. In this way, the frequency of the VCO, varies within the limits of 17 to 30KHz, according to the "sawtooth-shaped" voltage.
Until now we have explained how the circuit produces ultrasound bursts of varying frequency. These ultrasound bursts are available at the output pin 4 of IC2. Then, we use an amplifier in order to amplify the sound and eventually lead it to a piezoelectric transducer (tweeter). The amplifier is based on power mosfets FT1 and FT2 and uses a topology which is widely known as the “half bridge” topology. Half bridge topology requires that the gates of the two mosfets are driven by two identical signals, being out of phase by 180o. Actually, each driving signal is the inverse replica of the other one and both signals are produced by IC3, which is a driver for half-bridge topology amplifiers.
You probably have noticed that the signals are not sinusoidal (harmonic functions) but rather bistable (on - off). This means that produced sound contains many harmonics and perhaps, this is even more disturbing for the rodents.
FT1 and FT2 receive pulses being out of phase by 180o and provide sufficient current to drive the piezoelectric transducer (tweeter). LD1 is a typical LED, connected in parallel to the transducer. That is to indicate whether an ultrasound burst is produced.
Finally, we must mention some details about P1 button and the C4. P1 is used for tests. Since we are not able to hear ultrasounds, we have added C4. When P1 button is pressed, C4 is connected in parallel to C3 and thus changes the tuning range of the VCO at lower frequencies which we are able to hear. Thus, we are able to test the circuit without using an oscilloscope or any other equipment.
The final amplifier stage (FT1 and FT2) is powered from 24V. The 24V voltage is produced from a small power supply unit which uses an 18V transformer, a diodes bridge for rectification and a smoothing capacitor (C9). Due to low power consumption, a small transformer which can provide up to 6VA is adequate.
Apart from the output stage, consisting of FT1 and FT2, the rest of the circuit is powered from 12V, being provided from a 12V / 1W zener diode (DZ1).
All the resistors used in the circuit are of 1/4W type, apart from R10 and R11, which are of 1/2W type.