Homemade electronic mosquito repellent circuit

The mosquitoes that bite people are usually female mosquitoes carrying eggs. Male mosquitoes do not bite people. Female mosquitoes do not like to get close to male mosquitoes during the egg-carrying period. Once they sense the ultrasonic signal of 21-23kHz emitted by male mosquitoes, they will avoid and leave. This electronic mosquito repellent emits ultrasonic waves that simulate male mosquitoes, thereby driving away female mosquitoes and preventing mosquito bites.

Working principle:

The figure below is a circuit diagram for producing 21-23kHz ultrasonic mosquito repellent. Close the power switch SW, and the light-emitting diode VD indicates the working signal. The single junction transistor VT1, the resistor Rl, the potentiometer RP and the capacitor C1 form an ultrasonic oscillation circuit. The power supply GB charges the capacitor C1 through Rl and RP. When the charge reaches the peak voltage, the single junction transistor VT1 is turned on and discharged through the resistor R3. When the voltage across C1 drops below the peak, the single junction transistor VT1 is cut off. This repeated charging and discharging generates oscillation. Adjusting the potentiometer RP can change the oscillation frequency to about 21-23kHz. The oscillation signal is coupled to the amplifier circuit composed of VT2 through the capacitor C2. The amplified signal is output by the capacitor C3 and sends ultrasonic waves to the piezoelectric ceramic buzzer BC.

Schematic diagram of generating 21-23kHz ultrasonic wave

Component Selection

Unijunction transistor VTl: BT33C

Transistor VT2:9018

Piezoelectric ceramic BC: HTD-27A

Power supply GB: 4F22-DC6V laminated battery

Debugging and use

(1) All circuit components can be installed on a printed circuit board. Finally, fix the circuit board in a discarded pocket radio case or soap box. The piezoelectric ceramic sheet B is HTD-27A type, which can be directly glued to the original speaker position in the case with 502 glue.

(2) The mosquito repellent effect of this mosquito repellent device mainly depends on the oscillation frequency of the circuit; the potentiometer RP should be adjusted repeatedly to make the circuit oscillate at the frequency that female mosquitoes are most disgusted with. In addition, the output power of the circuit also has a great relationship. The larger the output power, the larger the mosquito repellent range and the better the effect; on the contrary, the smaller the output power, the worse the effect.

(3) To increase the output power of the circuit, the final output power can be increased. The best way is to add a power amplifier and change the 6V to 9V stacked battery to increase the radiation intensity of the ultrasonic wave.

Use a multimeter to identify the pins of a single junction transistor:

The unijunction transistor is composed of a PN junction and has three electrodes. Its test method is different from the general triode measurement method. The unijunction transistor has an E pole (emitter) and two bases (b1, b2). Therefore, it is also called a double base transistor. When reading the circuit diagram, to determine its polarity, just remember that the E pole in the circuit diagram is always between the b1 and b2 poles.