coll circuit

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. 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 unit.

More innovative circuits HERE.

Let’s learn the concept and the circuit more closely:

The concept:

We know that LEDs require a certain fixed forward voltage drop to become illuminated and it is at this rating when the LED is at it’s best, that is voltages which is around its forward voltage drop facilitates the device to operate in the most efficient way. 

As this voltage is increased, 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.

If we could maintain a voltage around an LED near to its rated forward voltage, we could use it more efficiently. That’s exactly what I have tried to fix in the circuit.

Since the battery used here is a 6 volt battery, means this 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.

Therefore I employed a few diodes in series with the supply 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).

 Now as the voltage of the battery drops, the diodes series are reduced to two and subsequently to one making sure only the desired amount of voltage reaches the LED bank.

In this way the proposed emergency lamp circuit is made 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.

Referring the figure, 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

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 circuit has bee 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

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 or similar,
Transformer = 0-6V, 500mA

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

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: