Here is a simple power resumption alarm circuit that can be fixed inside the switch box itself. It gives beeps when the power resumes after a power failure. This circuit is ideal to monitor battery chargers, since the long duration power failure may sometimes remain unnoticed. The circuit is too simple and derives power supply using a capacitor.
Capacitor CX, Resistors R1,R2 and Fullwave Rectifier comprising D1through D4 forms the power supply section to drop 230 volt AC to low volt AC. Capacitor C1 filters the ripples from AC so that around 25 volts and 100 mA current will be available which is regulated to 12 volt DC by the zener diode ZD. LED in the circuit indicates that the circuit is getting power.
The alarm circuit uses a CMOS timer IC 7555 and a few components. IC 7555 is wired as a short duration monostable with its trigger pin 2 shorted to pin 6.So when the power is off, trigger pin is negative. When the power resumes after a break, output of monostable goes high and buzzer beeps. This keeps the Reset pin 4 high for a short period through D5. Capacitor C5 also help to keep the reset pin 4 till the output turns low. Then the capacitor C2 charges via R5 .
When the charge in C2 rises to 1/3 Vcc, trigger pin 2 becomes high and the output of monostable turns low. This takes around 30 seconds. For long duration beeps, value of C2 should be increased.When the output of monostable turns low, its reset pin 4 also turns low to inhibit the monostable from working.
Caution: This circuit is extremely dangerous since it is at mains lethal potential. Do not try to construct it unless you are experienced to handle high volt AC. Do not touch or troubleshoot, when the circuit is connected to mains.