Monday, June 24, 2013

Auto Light Up Circuit

Welcome back everyone. This post will be about a small circuit that will light up a LED when it will be subjected to a dark place. To achieve this the two important components are the LDR or Light dependent resistor and a the transistor.

Transistors can act like switches. In this circuit I'm going to use that particular property of it.

LDR is such a device that's Resistance is inversely proportional to the amount of light falling on it. So, More light less the resistance, Less light more the resistance. In dark it's Resistance is about 5K-80K & in light its resistance is around 1K. So, it is a light controlled variable resistor and its resistance decreases with increasing incident light intensity that means it exhibits photo-conductivity.


Indeed a very simple diagram.

Here we are using a voltage dividing circuit consists of a variable resistor & a LDR. Let's think about two different state.

State 1: Room or experimental environment has enough light. So the resistance of the LDR will be very low. We have to set the variable resistor's resistance in such a manner that it stays at much higher level than the LDR incident to high intensity of light. Now what will happen is what we know from voltage dividing rule is the base of the transistor can now be treated as grounded or directly connected to 0V. Thus the transistor won't turn on. As a result no current will flow through the transistor as well as the LED, so it won't light up.

State 2: Room or experimental environment is in darkness or low light. So this time the resistance of the LDR will be higher and it will go much more higher than the resistance of the variable resistor. So the transistor's base will get a small amount of positive voltage. As a result it will start conducting and the LED will start to glow.

Where It can be used?

You can do a lot of experiment with this. You can use Darlington pair transistors for more output current and drive powerful loads. You can use Relay to drive larger loads like an high voltage alternating current device. Or it can be used to turn on a generator or something.

Another thing that can be done is to make a LED flasher. If the LED's light fall on the LDR its resistance will go lower that means the light will turn off again and as soon as the light is off the LDR's resistance will go higher and turn on the LED again. So a simple LED flasher can be made using this.

The variable resistor can change the sensitivity of the circuit.

For Transistor datasheet click here.
Read my other posts here.

Good day and happy experimenting.

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