circuit that can be used during power failures and outdoors where any other
source of power might be unavailable.
The circuit uses LEDs instead of
incandescent lamp, thus making the unit very power efficient and brighter with
its light output. Moreover, the circuit employs a very innovative concept
especially devised by me which further enhances the economical feature of the
best, that is voltages which is around its forward voltage drop facilitates
the device to operate in the most efficient way.
the LED starts drawing more current, rather dissipating extra current by
getting heated up itself and also through the resistor which also gets heated
up in the process of limiting the extra current.
rated forward voltage, we could use it more efficiently. That’s exactly what I
have tried to fix in the circuit.
source is a bit higher than the forward voltage of the LEDs used here, which
amounts to 3.5 volts. The extra 2.5 volts rise can cause considerable
dissipation and loss of power through heat generation.
and made sure that initially when the battery is fully charged; three diodes are
effectively switched so as to drop the excess 2.5 volts across the white LEDs (because each diode drop 0.6 volts across itself).
subsequently to one making sure only the desired amount of voltage reaches
the LED bank.
highly efficient with its current consumption, and it provides backup for a much
longer period of time than what it would do with ordinary connections.
evaluate it with the following points:
supply for the circuit. The circuit is basically made up of a single PNP
transistor, which is used as a switch here.
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.
voltage and it’s charged through the system of trickle charging.
at the base of the transistor disappears and it gets forward biased through the
and are gradually bypassed one by one as the LED gets dimmer.
T1 = BD140
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.
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: