AC mains fails when over load is connected and this problem is common in now days. Here is the simple circuit using optocoupler Digital Mains failure and resumption alarm, for indicating AC mains fails or resumes by producing alarm sound.

 

Circuit Description of Digital Mains Failure Alarm

 

       The circuit digital mains failure alarm is build around optocoupler. The resistor R1, capacitor C1 & C2, with diode D1 &D2 provide sufficient voltage to glow internal LED of optocoupler. Here the IC2 CD4011 is used as oscillator to generate low frequency of 0.662 Hz to 1.855 KHz controlling with preset VR1. Audio sound is generator by timer IC NE555 (IC2). The generated frequencies from IC2 vary from 472 Hz to 1.55 KHz controlling with preset VR2. For sensing mains fails position of switch SW1 to point 1 and for sensing mains resumption change the position of switch SW1 to point 2.