Simple mosquito repellent circuit given here, it uses timer IC555 and piezo buzzer element. This circuit will produce KHz range pulse and that is reproduced as sound by buzzer.
We human cannot hear that sound but this will irritates mosquito, keeps mosquito away.
The IC555 is connected with components to produce astable pulses, these pulse are reproduced as sound by buzzer, It can be varied by R3 resistance.
Variable Resistor 10KΩ