The discussed simple automatic emergency light circuit employs just a single IC 555 and yet is able to switch more than 20 LEDs directly, it will illuminate the LEDs only during the absence of mains power and ambient light.
The Circuit Concept
The proposed circuit is not only simple, it offers some very useful feature without involving too many components.
The use of IC 555 facilitates direct connection of the LEds across it's output pin#3, without needing an extra transistor driver buffer stage, although it may be incorporated in case more number LEDs are desired.
The IC is also configured as a light detector and furthermore a DC inverter.
The light detection feature ensures that the IC switches ON the LEDs only in the absence of adequate ambient light.
The level of darkness or the level of ambient light at which the IC triggers the LEDs may be set by adjusting the value of R2. This is an added feature which allows customizing the triggering threshold.
The introduction of C1 offers yet another novel feature to the design, it offers some delay before the LEDs are switched ON once the above specified conditions are met.
That means C2 can be selected for getting certain time delay before the LEDs get switched ON.
Using IC 555
Last but not the least, the IC also provides the facility which prevents the LEDs from illuminating for so long the AC mains stays active.
The reset pin of the IC is held at zero potential by the T1 during the presence of AC mains, the moment mains power fails T1 switches OFF connecting the reset pin#4 to battery positive, so that the IC is reset for the required triggering.
Just forgot to mention, the circuit also behaves like a trickle charger and keeps the associated battery fully charged and in a standby condition whenever in need.
Caution: The circuit is not isolated from AC mains, so be extremely careful while testing.
Parts List for the above simple automatic emergency light circuit using IC555
R1 = 2M2
R2 = 1M
R3, R5 = 10K
R4, R6 = 120K
R7----R13 = 330 ohms
LDR = any standard type having ambient light resistance around 30K and dark resistance to infinity.
D1---D4 = 1N4007
C1 = As required
C2 = 0.22uF/400V
T1 = BC547
LEDs = white, high efficiency, 5mm
Battery = 12V, 4AH