24-07-2012, 02:49 PM
Smart Emergency Light
smart emergency light.pdf (Size: 198.66 KB / Downloads: 185)
Now you need not fear dark
nights when power breaks
down. Here’s a white LEDbased
emergency light that automatically
turns on when mains power supply
fails.
The circuit consists of power supply,
battery charger and switching sections.
The power supply and charger sections
are built around transformer X1, diodes
D1 and D2, transistor T1, resistors R1
and R2, and zener diode ZD1. The power
supply for the circuit is derived from
AC mains by using 9V-0-9V, 250mA
step-down transformer X1. Diodes D1
and D2 rectify the AC voltage into DC
voltage, which is smoothed by filter capacitor
C1. The unregulated DC voltage
is regulated by transistor T1 along with
resistor R1 and zener diode ZD1. The
regulated DC voltage, via resistor R2,
charges the lead-acid battery. Diode D3
connects the battery power supply to
the switching circuit when mains power
is unavailable.
The switching circuit is built
around an NE555 timer (IC1), which
is wired in monostable mode. When
a low voltage is applied at trigger pin
2 of IC1, the timer activates and its
output pin 3 goes high. It remains in
that state until IC1 is triggered again
at its pin 2.
Light-dependant resistor LDR1 is
connected between the positive supply
of the battery and trigger pin 2 of IC1.
Resistor R3 is connected between pin
2 of IC1 and ground. The resistance
value of LDR1 remains high in dark
(at night) and low in ambient light (in
daytime). This phenomenon is utilised
to control the switching circuit.
Working of the circuit is simple. In
daytime, when ambient light falls on
LDR1, its resistance decreases to make
trigger pin 2 of IC1 high. As a result,
output pin 3 goes low and the LEDs
(LED1 through LED7) remain off.