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Mini Emergency Lamp




Emergency lighting systems for specific purposes do tend to get less common as the applications become more and more precise. This DIY details with an interesting low power consuming emergency lighting system. It charges from the main power supply and gets activated as soon as the mains supply is turned off. 

This Mini Emergency Lamp turns on when power fails to give cool white light in the room. It uses 1 watt White LED to provide sufficient light in the room. It is an ideal emergency lamp in the Child’s study room to avoid the panic situation of sudden power failure. The circuit and the battery pack can be enclosed in a plug-in type adapter box.


Mini Emergency Light

230 volt AC is reduced to low volt AC by the step down transformer X1 and rectified by the full wave diode bridge comprising D1 through D4. Capacitor C1 is the smoothing capacitor to remove ripples from the rectified DC. When the mains power is available, battery charges through R1 and D5.At the same time, base of T1 will be high through R1 and the PNP transistor T1 remains off. When the power fails, T1 gets forward bias and conducts. White LED connected to the collector of T1 turns on by using the battery power. The lamp remains on till the mains power resumes.
1 Watt White LED requires 3 volts and around 100-300 mA current for sufficient brightness. The 4.5 volt rechargeable battery pack used in cordless phone can be used. It is rated 4.5 V 1.5 Amps. Brightness of the LED can be controlled using R3. If a 100 ohms preset is used in series with R3, the brightness of LED can be controlled as per the requirements. R3 should be 10 ohms 1 watt resistor.


R1 100 Ohms, R2 470 Ohms, R3 10 Ohms 1 watt
C1 470 uF25 V
D1 – D5 IN4007
T1 BD140 PNP
1Watt White LED, Toggle switch, 4.5 volt battery pack



Circuit Diagram

Comments (3)

what is the battery ratings

what is the battery ratings in ah?



what is the watts rating of the R1 - 100 ohm resistor.


The voltage across a 1watt

The voltage across a 1watt LED is 3.3v   The emitter-collector voltage-drop across the transistor is 0.2v   This gives 4.5v - 3.5v = 1 volt for the 10R resistor. The 10R resistor will only allow 100mA to flow - not the required 330mA.  The circuit has never been tested. 


well i have tested this

well i have tested this circuit on circuit simulation its ok and working 

i am using nokia mobile charger as (ac to dc ) source

using 4v 500ma  sealed lead acid battery  

but the led is always on irrespective of ac source 

wel bd 140 is not working in this circuit .....

8550 did the job 



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