Once again the most popular timer IC NE555 proved that its applications are limited only to the ingenuity of the user.
The circuit of power resumption alarm shown here is very simple, where IC1 is wired in a special kind of mode. When power supply resumes, pin 2 of IC1 is grounded through C1 and pin 3 goes high. This makes reset pin 4 to go high for about 10 seconds. When the capacitor C1 charges to two-third of the supply voltage, output of IC1 goes low, grounding the reset pin 4 and thus disabling the entire circuit.
The alarm circuit is wired around a second timer IC, IC2. Here it is used as an astable multivibrator, the frequency of which is about 1 kHz. This IC, IC2 is enabled when the output of IC1 goes high.
Thus, the speaker produces an alarm for about 10 seconds. After 10 seconds, IC1 restore its stable state. Capacitor C1 discharges quickly through pin 7, and the circuit becomes ready for the next detection, since pin 2 of IC1 is low. Thus, when power interrupts again an alarm is produced. The LED serves as an indicator.
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 1 MΩ
R2 = 1 KΩ
R3 = 68 KΩ
R4 = 330 KΩ
C1 = 10 µF/12V
C2, C3 = 0.01 µF
C4, C6 = 0.1 µF
C5 = 100 µF/12V
IC1, IC2 = NE555
LS1 = 8Ω loud speaker