This circuit in this project is based on LDR which is used as detector to detect the shadow. When the shadow falls on the LDR, its resistance goes high so voltage across it increases. This voltage reaches at input negative pin of comparator LM324, where it is compared to the reference voltage which is at positive pin. So when negative pin (input) voltage is higher than positive pin (reference) voltage comparator gives high output and buzzer sounds.
Filed Under: Electronic Projects