Mar 012023

Automatic night light with one LED

This automatic night light circuit with one LED is simple, interesting and very easy to make. The circuit allows a better understanding of the operation of a phototransistor and thus to use it in other applications.

The project is powered by a button-type lithium battery (CR2025 or similar) and uses only one LED to make it as efficient as possible.

A common red LED is used in the circuit, but it can easily be replaced by an LED of another color.

How does the automatic night light work?

To be sure that our automatic night light circuit will work properly, we use a phototransistor. The response time of a phototransistor is much shorter than that of a photoresistor, so the LED is expected to turn on immediately.

So when the daylight is gone, the LED will light up. To make the circuit portable, 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

Analyze the diagram:

When daylight shines on the phototransistor, it becomes conductive and the voltage on its emitter rises. The emitter voltage rises enough that the voltage level at the base of the PNP transistor (Q1) causes the transistor to enter the turn-off region. When the transistor is not conducting, the LED is off.

When night comes, the phototransistor goes into its cut-off  state and the voltage on its emitter decreases, so the transistor Q1 has a base connected to ground through resistor R, causing it to go into conduction and the LED lights.

2N2905 PNP Bipolar Transistor Pinout

To control the light level at which the LED is activated, you can change the resistor R by another similar value. I do not recommend to use a potentiometer, because it increases the final circuit too much. However, you can use a trimmer or a hollow shaft potentiometer if you want. The circuit draws about 13 mA when the LED is on and a few hundred uA when the LED is off.

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

Automatic Night Light with one LED PCB
You may be interested in the Automatic LED Emergency Light Circuit Diagram with Rechargeable Battery

 List of components of the circuit

  • 1 2N2905A PNP transistor
  • 1 SFH309-4 NPN phototransistor or similar
  • 1 red LED
  • 1 3V button cell lithium battery (CR2025 or similar)
  • 1 1K resistor

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