Theory

The main theory of the electronic animal and insect repellent circuit was how it generates variable frequency and how to control the frequencies generated. In order to perform the function of this circuit fully, a 555 Timer has been used. The reason 555 Timer used in this circuit was because it can convert an analog signal input (voltage) to digital signal output (frequency), which functioned as an ADC (Analog to Digital Converter).

555 timer based circuit is a kind of voltmeter, also an analog to digital converter, which converts the analog input voltage to digital output pulses. The output pulse width is proportional to the difference between the analog input voltage and the voltage across the capacitor. It can measure from +5V to +18V. The accuracy is high in the range of 6V to 18V. The readings are about the same with the 10 bit ADC readings. The accuracy depends on only the +5V supply voltage and the microcontroller's clock frequency.

Depending to the manufacturer, a 555 could have around 20 transistors, 15 resistors and a couple of diodes.

An Astable Multivibrator is often called also oscillator. A 555 can be used to generate clock pulses in a wide range of frequencies with enough output power to drive several ICs.

The oscillation frequency is calculated with the following formula:

F =	1
	0.67 (R1 + 2 x R2) x C

The same circuit can be used to control DC loads such as LEDs, lamps and DC motors. The idea is to use this circuit as a PWM signal generator. To do this, you need to replace R2 with a potentiometer. By altering the potentiometer's value, this results in changing the duty cycle output. The duty cycle is calculated as follows:

D = T_TOTAL / T_HIGH

D =	R1 + R2
	R1 + 2 x R2