The triggering pulses for the TRIAC should be in synchronization
with the zero crossing reference of the AC mains. A zero crossing detector is constructed to generate the pulses at zero crossings of the AC mains (Fig. 4). The output of step down transformer (16-0-16 V, 500 mA) is rectified using the centre tap full wave rectifier constructedusing the IN4007 diodes. The rectified output (pulsating DC) is fed to the base of the transistor Q1 (BC547). The pulses at the zero crossing of the AC mains are obtained at the collector of the transistor. The zener diode Z3 that has a reverse breakdown voltage of 10 V is used to
limit the output of the transistor to 10 V.