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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