Capacitor C5 and potentiometer R12 determine the frequency of the output signal at pin 3 of IC1, the 555 oscillator. The output signal is differentiated by C3 and C4 before it`s input to the base of power transistors Q1 and Q2 via diodes D1 and D2, respectively. The signal from !C1 is adjusted to 120Hz, because the flip-flop formed by transistors Q3 and Q4.divides the frequency by 2.
When Q3 is on, the base of Q1 is connected via R1 to the regulated 12-V supply. Then, when the flipflop changes states, Q4 is turned on and the base of Q2 connected to the 12-V supply through R2. The 100 mA base current allowsQ1 and Q2 to alternately conduct through their respective halves to the transformer"s secondary winding. To eliminate switching transients caused by the rapid switching of Q3 and Q4, capacitors C1 and C2 filter the inputs to the base of Q1 and Q2 respectively. Power for the unit comes from an automobile"s 12V system or from a storage battery. The power is regulated by IC2, a 7812 regulator. LED1, connected across the 12-V input, can be used to indicate whether power is being fed to the circuit. The neon pilot lamp, LMP1, shows a presence or absence of output power.