Automatic quick turn on LED in dark

Earlier I’ve showed how to reuse waste mobile batteries for LED lighting. There was darkness detector part built upon a switching transistor and a LDR.

Since the switching transistor was receiving linear voltage up/down according to light intensity on LDR, the LED was also glowing slowly in dark and was dimming slowly in light.

A big negativity was that if light intensity on LDR is not too high then LED was glowing slowly and consuming battery. This module will correct that.

This new circuit design is based on comparator, using the LM393N dual comparator IC, circuit diagram below.


Operating principle of the above circuit

Comparator is a device which compares two input voltages as V+ and V-. If V+>V- then output is nothing or positive if we use pullup resistor, else output is 0 or negative.

There are many popular comparator chips available in market as LM339(4 comparator), LM393(2 comparator), etc. In the following circuit, the IC is taken as LM393N which has 2 comparators inside.

We use one of them whose input is PIN2 (V-) and PIN3(V+) and the output is PIN1. So, in this case, if PIN2>PIN3 (voltage) the output will be negative.

There is a general purpose potentiometer(volume/preset) in PIN2 which can feed anything between VCC(3.0V or 3.7V)  and GND(negative) to the pin (adjustable). Hence the above conditionm occurs when LDR has light and has low resistance causing PIN3 to be low voltage near negative. Then the output is negative and can’t saturate the switching transistor and the LED doesn’t glows.

In the other hand when light intensity on LDR is low then PIN3 becomes near positive and PIN3>PIN2 (voltage) then output becomes nothing (neither +ve nor -ve). Then the 2.2K resistor(you can use 1K also) supplies +ve voltage to the base of the transistor and the LED glows.


  • Small heatsink on D880 transistor if using high power LED such as 1watt LED.
  • Adjust the preset/volume to adjust when light will glow/dim.
  • Use waste mobile batteries again in this circuit, which will save environment as we are reusing, also within the 3.7V, 0.2-0.3V is consumed by the transistor and LED will get 3.5volts, which is recommended.


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4 Responses

  1. subhajit sarkar says:

    here transistor can be used as a switch in the front section then a melody generator with a speaker for acknowledgement it would be more cheaper for the same.

  2. Renjan says:

    What is the role of 2.2K resistor connected between Base and VCC in Automatic quick turn on LED in dark Circuit ?

  3. Himardri says:

    Hello Arup Da,
    If we Connect A greater Ah battery to a load will it mean that the load will work for longer time?

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