Automatic Night Operated LED Lamp

This circuit is not only simple but very interesting and very easy to build. You might have seen the new flashlights manufactured with new high bright high efficiency LEDs.

By: Nidhi Chopra

The idea is to achieve something similar but with an added feature.

Functioning Details

To make our circuit operative after dark, a phototransistor is employed, so that when the daylight is void, the LED gets switched ON.

To make the circuit extermely compact one button battery type is preferred here, quite akin to those used in calculators, watches, etc.

Understanding the diagram:

As long as ambient light illuminates the phototransistor, the voltage at its emitter lead is sufficiently high for the base of PNP transistor Q1 to keep it shut off.

However when darkness sets in, the phototransistor starts losing conduction and the voltage at its emitter diminishes causing the phototransistor to slowly switch OFF.

This prompts Q1 to begin getting the biasing via its base/ground resistor R and it starts to illuminate brightly as darkness gets deeper.

In order to control the level of the ambient light for which the LED may be desired to be switched ON, he resistor R values may be varied until the desired level is satisfied. Putting a potentiometer may not be recommneded, just to ensure a compact and sleek dimension of the unit.

The circuit may consume approximately 13 mA when the LED is illuminated and just a few hundred uA when its switched OFF.

Simulation and Working

Bill of material for the discussed automatic night operated LED lamp.

- 1 PNP BC557A
- One compatible phototransistor
- 1 super bright white LED
- 1 battery 3V coin
- One 1K resistor

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Ram said…
Sir, please post one topic on Automatic Night Operated LED Lamp operated on ac and can take load up to 10 watt (5 watt will also ok as we use led bulb ). Transistor switching will be preferred instead of Relay.
If time allow please post both type of circuit for led load and ac load like this one
Swagatam said…
Ram, it's already been covered in my blog, please check out the following links:
Ram said…
Thank you.
Yes, I also found 2nd circuit at late night. Can we replace transfer with ppc capicator in 1st circuit using the concept of
I guess 2 parallel 150/400v ppc capacitor will sufficient ?
Swagatam said…
yes it may be done, one example circuit is shown here:
Sriram Kp said…
Hai, I made a circuit like this
But the problem is, pin 3 of the IC goes high when light falls on the LDR. But the pin3 should go high when no light falls on the LDR. I tired 100k and 1M VR. But the result is same. Pls help...
Swagatam said…
Hi, either connect the relay from pin3 to ground or

interchange the positions of the 10K preset and the LDR

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