LED Emergency Light Circuit With Battery Over Charge Protection

Posted on Feb 7, 2014

The following circuit was designed by me in response to the request sent by PP. The article describes an LED emergency light circuit with advanced features such as, over charge battery cut off, day time auto-disable, and need less to say that the circuit switches ON the LEDs automatically when AC mains fails and reverts to charging mode when power

LED Emergency Light Circuit With Battery Over Charge Protection
Click here to download the full size of the above Circuit.

is restored. IC1 which is our very own IC555 has been set as a comparator. During day time, the light over LDR keeps the LDR resistance low such that the potential at pin #2 of the IC is kept well over 1/3Vcc. This situation ensures that the output of the IC at pin #3 stays at logic high. Another factor that keeps T1 switched ON and T2 switched OFF, is the voltage from the transformer power supply stage. This function is implemented via the resistor R9. This also means that as long as mains AC is available, T2 is restricted from conducting and therefore the LEDs cannot light up. Now suppose the mains power to the transformer fails, and assume that this happens during night or complete darkness, pin#3 of IC555 reverts to zero and also theres no voltage from the power supply, means T1 has absolutely no base bias and therefore has to switch OFF. This instantly prompts T2 to switch ON and consequently the entire LED array also switches ON, providing the required emergency illumination to the surrounding. MAKE SURE THAT THE LIGHT FROM THE LED DOES NOT FALL OVER THE LDR, WHICH MIGHT TRIGGER A RAPID UNDESIRABLESWITCHINGOF THE LEDS. Thebatterycharging section consists of T3, T4 and the associated parts. P1 is set such that it switches ON T3 when the battery voltage reaches just above 14 volts. The moment this happens, T4 switches OFF, cutting of the negative supply to the battery and restricting any further charging of the...

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