Jun 012015

Automatic night light circuit with one LED

This Automatic night light circuit with one LED circuit is simple, interesting and very easy to do. Surely you have seen the new lanterns made of the new high-capacity LEDs. The idea is to get something that looks like.

The project presented here uses a common red LED, but can be replaced by a high intensity LED without problem.

How the Automatic night light circuit works?

To be sure that our Automatic night light circuit will works fine, we use a phototransistor. So when daylight is gone, the LED will light. For the circuit to be transportable a button type battery is used. This battery is similar to those used in electronic devices such as calculators, watches, etc.

Automatic Night Light with One LED

Analyzing the diagram:

When daylight illuminates the phototransistor, it becomes conductive and the voltage at its emitter rises.  The emitter voltage goes up enough so the voltage level at the base of the PNP transistor (Q1) causes the transistor to go into cut-off region. If transistor is not conducting, the LED is off.

When night comes, the phototransistor turns off and the voltage at its emitter decreases. As if the phototransistor had disconnected. Thus, the transistor Q1 has a base connected to ground through resistor R, causing it enters into conduction and the LED lights.

To control the light level at which the LED is activated, you can change the resistor R by another of similar values. I do not suggest putting a potentiometer because it enlarges to much the final circuit. Although you can use a trim or a hollow shaft potenciometer if you want. The circuit requires about 13 mA when the LED is on and a few hundred uA when LED is off.

The diagram shows a proposed placement of the elements (it is 200%) and a suggested design of the PCB (the top of the page)

Automatic Night Light with one LED PCB

List of components for the Automatic night light circuit

  • 1  2N2905A PNP transistor
  • 1  phototransistor
  • 1  red LED
  • 1  3v button type battery
  • 1  1K resistor
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