Power Outage Warning

5 to 15V supply - LED indicator

Adjustable time detection

Circuit diagram:

Power Outage Warning


R1,R2__________100K  1/4W Resistors
R3_______________1K  1/4W Resistor (See Notes)
R4______________10K  1/4W Resistor
R5_____________220K  1/2W Trimmer (Carbon or Cermet)

C1_____________100nF  63V Polyester Capacitor
C2______________47µF  25V Electrolytic Capacitor

D1_______________Red LED (Any shape and size)
D2____________1N4002 200V 1A Diode

IC1_____________4093 Quad 2 input Schmitt NAND Gate IC or
                     4011 Quad 2 input NAND Gate IC

P1______________SPST Pushbutton


A circuit capable of detecting even a very short power outage, can be useful, mainly if embedded into existing appliances like mains powered counters, timers, clocks and the like.
At switch-on of the appliance, the LED illuminates, but pressing on P1 it goes off and remains in this state until a power outage occurs. When power supply is restored, the LED illuminates steadily until you press P1 again.
The circuit sensitivity can be adjusted by Trimmer R5. This means that, under the control of R5, the LED may not light if the mains is missing for a short interval in the 1 to 15 seconds range.

Circuit operation:

IC1A and IC1B NAND gates are wired as a set-reset flip-flop. R1 and C1 provide auto-set of the flip-flop when the circuit is powered, so pin 3 of IC1A goes high and pin 4 of IC1B goes low. This allows the outputs of IC1C and IC1D, wired in parallel as inverters, to go high driving the LED D1 on. The flip-flop is reset by pushing on P1.
As the circuit is intended to be powered from the same appliance that is monitoring, the supply is derived from the ac voltage available at the existing transformer secondary winding (see the upper box of the circuit diagram enclosed in the dashed blue line). The circuit will work with ac voltage values in the 5 - 15V range.
A simple diode-capacitor cell (D2-C2) is sufficient to provide the necessary dc voltage. A rather low value was chosen for C2 in order to allow the circuit to detect very short periods of power failure.
The resistance value of R4 + R5 controls the discharge time of C2: with R5 set to the minimum value, the circuit will signal power outages from 1 sec. onwards. If R5 is set to the maximum, the circuit will signal power outages from about 15 sec. onwards.