Automatic Night Light using Phototransistor

Here is the simple Automatic Night Light circuit using Phototransistor.

Components Required:

1) Breadboard

2) 470 ohm resistor
3) 22 Kilo ohm resistor

4) LED (Any Color)
5) Phototransistor
6) 3904 NPN Transistor

6) 9Volt Battery 

Procedure:

• Connect the circuit as shown in the circuit diagram below.
• During day time the brightness will be more, which forward biases the phototransistor.
• Therefore the current flows through the collector to emitter of phototransistor.
• Hence there is no base current to forward bias the 3904 NPN transistor and the LED remains OFF.
• During night time the brightness goes down, which cannot forward bias the phototransistor.
• Therefore the current will not flow through the phototransistor.
• Hence there is enough base current to forward bias the 3904 NPN Transistor and the LED glows.

Circuit Diagram:

Figure 1: Automatic Night Light using Phototransistor Circuit simulation made in Multisim

Figure 2: Automatic Night Light using Phototransistor