Figure shows the water overflow indicating circuit . The core of the circuit is 555 and along with R3, R4, C1 a multivibrator is formed . The oscillator ‘s oscillation frequency f = 1.44 / (R3 2R4) C1, icons corresponding to the frequency parameter of about 1kHz. When the water is full , A, B was short between the state , so that BG1 triggers the oscillator circuit. The oscillation signal output drives the audio speaker Y to sound that the water has exceeded trhe limit and it overflows.