The tester consists of a rectifier circuit and a multivibrator circuit. The ac voltage is half-wave rectified by diode D1 and stored in capacitor CI. Resistor Rl is used to limit the current through D1 to a safe value. The voltage stored across CI supplies ICi operating power. The IC, the versatile 555 timer, is configured to operate as a multivibration whose operating frequency is determined by C2, R2, and R3. The output of ICI, on pin 3, is coupled to a piezoelectric speaker (SPK), which gives an indication of the presence of ac.