Emergency Light & Alarm
Source: RED Free Circuit Designs
Powered by two AA NI-CD batteries
Four switchable options
R1____________220K 1/4W Resistor
R2____________470R 1/2W Resistor
R3____________390R 1/4W Resistor
R4______________1K5 1/4W Resistor
R5______________1R 1/4W Resistor
R6_____________10K 1/4W Resistor
R7____________330K 1/4W Resistor
R8____________470R 1/4W Resistor
R9____________100R 1/4W Resistor
C1____________330nF 400V Polyester Capacitor
C2_____________10΅F 63V Electrolytic Capacitor
C3____________100nF 63V Polyester Capacitor
C4_____________10nF 63V Polyester Capacitor
D1-D5________1N4007 1000V 1A Diodes
D6______________LED Green (any shape)
D7___________1N4148 75V 150mA Diode
Q1,Q3,Q4______BC547 45V 100mA NPN Transistors
Q2,Q5_________BC327 45V 800mA PNP Transistors
LP1____________2.2V or 2.5V 250-300mA Torch Lamp
SPKR___________8 Ohm Loudspeaker
B1_____________2.5V Battery (two AA NI-CD rechargeable cells wired in series
PL1____________Male Mains plug
This circuit is permanently plugged into a mains socket and NI-CD batteries are trickle-charged. When a power outage occurs, the lamp automatically illuminates. Instead of illuminating a lamp, an alarm sounder can be chosen. When power supply is restored, the lamp or the alarm is switched-off. A switch provides a "latch-up" function, in order to extend lamp or alarm operation even when power is restored.
Mains voltage is reduced to about 12V DC at C2's terminals, by means of the reactance of C1 and the diode bridge (D1-D4). Thus avoids the use of a mains transformer.
Trickle-charging current for the battery B1 is provided by the series resistor R3, D5 and the green LED D6 that also monitors the presence of mains supply and correct battery charging.
Q2 & Q3 form a self-latching pair that start operating when a power outage occurs. In this case, Q1 biasing becomes positive, so this transistor turns on the self latching pair.
If SW3 is set as shown in the circuit diagram, the lamp illuminates via SW2, which is normally closed; if set the other way, a square wave audio frequency generator formed by Q4, Q5 and related components is activated, driving the loudspeaker.
If SW1 is left open, when mains supply is restored the lamp or the alarm continue to operate. They can be disabled by opening the main on-off switch SW2.
If SW1 is closed, restoration of the mains supply terminates lamp or alarm operation, by applying a positive bias to the Base of Q2.
- Close SW2 after the circuit is plugged.
- Warning! The circuit is connected to 220Vac mains, then some parts in the circuit board are subjected to
lethal potential!. Avoid touching the circuit when plugged and enclose it in a plastic box.
This circuit was awarded with publication in ELECTRONICS WORLD "Circuit
Ideas", September 2001 issue, page 708.