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Monday 21 July 2014

Turn On an LED During Day and Turn Off in Night Using LDR and BC547

by realfinetime  |  in transistor at  12:24

          We have already seen the Circuit to Turn On an LED During Night and Turn Off the Same During Day Using LDR and the Circuit to Sense Light using Arduino Mega and LDR. In this blog, we will see the circuit to turn on LED during the day time and turn off the same at night. Circuit is simple as shown in the following diagram.

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 and LED. This will increase current abruptly which will damage the transistor, LED and power supply.

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

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. 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 LED ( through ACEF ). This will turn on the LED. If the transistor don't have sufficient base current, transistor turn off. So current will not flow through the transistor and LED will turn off.

         LDR controlls the current through the base of transistor. This circuit will turn on LED during day and turn off LED during night. During day time, resistance of LDR will be less which will drive more current to base. This will turn on the transistor and collector current will flow through the transistor to ground ( through CEF ). This will turn on 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 not flow through the transistor which will turn off the LED.


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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.

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