## Circuit to Turn On an LED During Night - Automatic Street Light Demo

by realfinetime  |  in transistor at  08:15

An LED that turn on during night and turn off  during day is amazing. Normally a Light Depending Resistor is used to sense the presence of LIGHT. We cannot connect LDR directly to LED. Some switching methods should be adopted to make an LED to respond, when LDR is kept at LIGHT or DARK. Transistors are normally used for switching. This blog will help you to design a circuit that will turn on an LED during night and turn off during day like street lights. By some simple modifications in this circuit, we can easily reverse the operation.

Circuit Diagram

Connections are done as shown in the following circuit diagram. Transistor used is BC547. Pinout diagram of BC547 is given at right side top of following image.

Transistor - BC 547 is the transistor used here. It is an npn transistor and is suitable for small operations.

5V - Normally supplied from arduino or  5V regulator.

R1 -  This is a current limiting resistor. It helps to switch the transistor between ON and OFF states by controlling the current through R1 and hence through the base of transistor. If that resistor is not connected, there is a possibility that, base current will always remain high because, LDR alone cannot resist the current considerably to turn off the transistor.

R2 -  It is a pull down resistor. It will ground the base, if there is no external signals to the base of transistor. When an external signal is connected, beginners confuse whether this signal get grounded through R2. But this signal will not get grounded because of high resistance of R2 ( 10K ).

R3 -  If R3 is not connected, when transistor turns on, power supply get shorted through A,C,B,E and F due to the low resistance of transistor. This will increase current abruptly which will damage the transistor as well as power supply.

R4 -  When transistor turns on, for the collector current to flow through the transistor, resistance of  other path ( through CHJF ) should be high. If R4 is not connected, resistance of both parallel paths ( CEF and CHJF ) will be almost same and current will flow through both parallel paths equally, which may turn on the LED even if the transistor is ON.

Observations

LED will TURN OFF when LDR is kept at LIGHT.
LED will TURN ON  when  LDR is kept at DARK.

Tabular column.

Voltage across various components of the circuit, when LDR is kept at DARK and LIGHT conditions is given in the following table.

Explanation of the Circuit

"Transistor as a switch" principle is used to turn on and turn off an LED depending on the background light condition. When transistor act as switch, a small base current is sufficient to drive a large current from collector to emitter ( through ACEF ). Then transistor is said to be in saturation. If the base current is less than a particular amount, transistor will be off and current will not flow from collector to emitter ( through ACEF ).

LDR is connected in the circuit in such a way that, it will control the current to the base of transistor. Current reaching the collector pin of transistor have two options. Either it can flow through the transistor to ground ( through CEF ). Otherwise, it can flow through 1K resistor and LED to ground (  through CHJF ). Current always choose low resistance path. That is, if the transistor get sufficient base current, transistor turn on and current at the collector pin will flow from collector to emitter and then to ground ( through CEF ). If the transistor don't have sufficient base current, transistor get turn off. So current will flow through the 1K resistor and LED to ground ( through CHJF ). This will turn on LED. That is, if the base current is not sufficient, transistor will turn off and LED will turn on.

LDR controlls the current through the base of transistor. This circuit will turn on LED during night and turn off LED during day. During day time, resistance of LDR will be less which will drive more current to base. This will turn on transistor and collector current will flow through the transistor to ground ( through CEF ) because, the resistance of transistor is low compared to resistance of 1K resistor connected in series with LED ( CHJF ). This will turn off LED. During night, resistance of  LDR will increase which will block current flow to the base of transistor. This will turn off the transistor and collector current will flow through 1K resistor and LED ( through CHJF ) which will turn on the LED.

Calculation of the Base current during DARK and LIGHT condition

LDR in DARK

Voltage across R1, Vag  =  0.02V
Resistance of R1             =  1K = 1000 Ohm
Current through R1, Ir1  =  0.02 / 1000  =0.02mA
This current will be approximately equal to the base current, which is not sufficient to turn on the transistor. So LED will turn ON. Because collector current will flow through the LED ( CHJF ), not through the transistor.

LDR in LIGHT

Voltage acroos R1, Vag  =  0.18V
Resistance of R1             =  1K = 1000 Ohm
Current through R1, Ir1   =  0.18 / 1000  =0.18mA
This current will be approximately equal to the base current, which is sufficient to turn on the transistor. So LED will turn OFF. Because collector current will flow through the transistor ( CEF ), not through the LED.

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2. Can the input voltage be changes to say 3v ?

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4. You have written that at dark 2ma current is flowing through the base which is sufficient to turn off at the same time you are also writing that 18ma is sufficient

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## IMPORTANT NOTICE

All the circuits, published in this blog is only after testing and getting proper results in my private lab. When you try these circuits, you should check the supply voltage, polarity of components, presence of childrens nearby and shorts in the circuits. This website will not be responsible for any harm happened to you or your components caused by your carelessness.