The article describes a very simple homemade emergency light circuit that can be used during power failures and outdoors where any other source of power might be unavailable.
Efficient, Automatic, White LED Emergency Light Circuit Description:
Referring the shown simple LED emergency light circuit, we see that the circuit is actually very easy to understand, let’s evaluate it with the following points:
The transformer, bridge and the capacitor forms a standard Power supply for the circuit. The circuit is basically made up of a single PNP transistor, which is used as a switch here.
We know that PNP devices are referenced to positive potentials and it acts like ground to them. So connecting a positive supply to the base of a PNP device would mean grounding of its base. Here, as long as mains power is ON, the positive from the supply reaches the base of the transistor, keeping it switched off. Therefore the voltage from the battery is not able to reach the LED bank, keeping it switched off.
In the meantime the battery is charged by the power supply voltage and it’s charged through the system of trickle charging.
However, as soon as the mains power disrupts, the positive at the base of the transistor disappears and it gets forward biased through the 10K resistor.
The transistor switches ON, instantly illuminating the LEDs.
Initially all the diodes are included in the voltage path, and are gradually bypassed one by one as the LED gets dimmer.
HAVE ANY DOUBTS? FEEL FREE TO COMMENT AND INTERACT.
Parts List for the proposed LED emergency light circuit
R1 = 10K,
C1 = 100uF/25V,
D1, D2 = 1N4007,
D3---D6 = 1N5408,
T1 = BD140
Tr1 = 0-9V, 500mA,
LEDs = white, hi-efficiency, 5mm,
S1 = switch with three changeover contacts.
In response to the suggestion of one of our avid readers, the above automatic LED emergency light circuit has been modified and improved with a second transistor stage incorporating an LDR trigger system. The stage renders the emergency light action ineffective during day time when ample ambient light is available, thus saving precious battery power by avoiding unnecessary switching of the unit.
Circuit modifications for operating 150 LEDs, requested by SATY:
Parts List for the modified emergency light circuit
R1 = 220 Ohms, 1/2 watt
R2 = 100Ohms, 2 watts,
RL = All 22 Ohms, 1/4 watt,
C1 = 100uF/25V,
D1,2,3,4,6,7,8 = 1N5408,
D5 = 1N4007
T1 = AD149, TIP127, TIP2955, TIP32 or similar,
Transformer = 0-6V, 500mA
The following circuit shows how a low voltage cut off circuit can be included in the above design for preventing the battery from getting over discharged.
Power Supply Circuit with Emergency Backup
The circuit shown below was requested by one of the readers, it is a power supply circuit which trickle charges a battery when AC mains is available, and also feeds the output with the required DC power via D1. Now, the moment AC mains fails, the battery instantly backs up and the compensates the output failure with its power via D2.
When input Mains is present, the rectified DC passes through R1 and charges the battery with the desired output current, also, D1 transfers the transformer DC to the output for keeping the load switched on simultaneously.
D2 remains reverse biased and is not able to conduct because of higher positive potential produced at the cathode of D1.
However when mains AC fails, the cathode potential of D1 becomes lower and therefore D2 starts conducting and provides the battery DC back up instantly to the load without any interruptions.
Parts List for an emergency light back up circuit
All Diodes = 1N5402 for battery up to 20 AH, 1N4007, two in parallel for 10-20 AH battery, and 1N4007 for below 10 AH.
R1 = volt/charging current (Ohms)
Transformer Current/Charging current = 1/10 * batt AH
C1 = 100uF/25
Using NPN transistors
The first circuit can be also built using NPN transistors, as shown here:
Efficient, Automatic, White LED Emergency Light Circuit Description:
Referring the shown simple LED emergency light circuit, we see that the circuit is actually very easy to understand, let’s evaluate it with the following points:
The transformer, bridge and the capacitor forms a standard Power supply for the circuit. The circuit is basically made up of a single PNP transistor, which is used as a switch here.
We know that PNP devices are referenced to positive potentials and it acts like ground to them. So connecting a positive supply to the base of a PNP device would mean grounding of its base. Here, as long as mains power is ON, the positive from the supply reaches the base of the transistor, keeping it switched off. Therefore the voltage from the battery is not able to reach the LED bank, keeping it switched off.
In the meantime the battery is charged by the power supply voltage and it’s charged through the system of trickle charging.
However, as soon as the mains power disrupts, the positive at the base of the transistor disappears and it gets forward biased through the 10K resistor.
The transistor switches ON, instantly illuminating the LEDs.
Initially all the diodes are included in the voltage path, and are gradually bypassed one by one as the LED gets dimmer.
HAVE ANY DOUBTS? FEEL FREE TO COMMENT AND INTERACT.
Parts List for the proposed LED emergency light circuit
R1 = 10K,
C1 = 100uF/25V,
D1, D2 = 1N4007,
D3---D6 = 1N5408,
T1 = BD140
Tr1 = 0-9V, 500mA,
LEDs = white, hi-efficiency, 5mm,
S1 = switch with three changeover contacts.
In response to the suggestion of one of our avid readers, the above automatic LED emergency light circuit has been modified and improved with a second transistor stage incorporating an LDR trigger system. The stage renders the emergency light action ineffective during day time when ample ambient light is available, thus saving precious battery power by avoiding unnecessary switching of the unit.
Circuit modifications for operating 150 LEDs, requested by SATY:
Parts List for the modified emergency light circuit
R1 = 220 Ohms, 1/2 watt
R2 = 100Ohms, 2 watts,
RL = All 22 Ohms, 1/4 watt,
C1 = 100uF/25V,
D1,2,3,4,6,7,8 = 1N5408,
D5 = 1N4007
T1 = AD149, TIP127, TIP2955, TIP32 or similar,
Transformer = 0-6V, 500mA
The following circuit shows how a low voltage cut off circuit can be included in the above design for preventing the battery from getting over discharged.
Power Supply Circuit with Emergency Backup
The circuit shown below was requested by one of the readers, it is a power supply circuit which trickle charges a battery when AC mains is available, and also feeds the output with the required DC power via D1. Now, the moment AC mains fails, the battery instantly backs up and the compensates the output failure with its power via D2.
When input Mains is present, the rectified DC passes through R1 and charges the battery with the desired output current, also, D1 transfers the transformer DC to the output for keeping the load switched on simultaneously.
D2 remains reverse biased and is not able to conduct because of higher positive potential produced at the cathode of D1.
However when mains AC fails, the cathode potential of D1 becomes lower and therefore D2 starts conducting and provides the battery DC back up instantly to the load without any interruptions.
Parts List for an emergency light back up circuit
All Diodes = 1N5402 for battery up to 20 AH, 1N4007, two in parallel for 10-20 AH battery, and 1N4007 for below 10 AH.
R1 = volt/charging current (Ohms)
Transformer Current/Charging current = 1/10 * batt AH
C1 = 100uF/25
Using NPN transistors
The first circuit can be also built using NPN transistors, as shown here:
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