Monday, June 24, 2013

Auto Light Up Circuit

Let's make circuit to light up a LED when the surrounding is dark.

This might not be a very practical one but it can help us learn two important concepts.
1. Using Transistor as Switch.
2. How we can use LDR.

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 environment Resistance is about 5K-80K and in Light drops to around 1K.



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

State 1(Bright Room): Room or experimental environment has enough light. So the resistance of 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(While exposed to high intensity of light). Now what will happen is what we know from voltage dividing rule. The base of the transistor can now be treated as grounded or directly connected to 0V as current through the Variable resistor will go to ground via LDR. Thus the transistor is not going to turn on. As a result no current will flow through the transistor(Collector to Emitter) as well as the LED, so it won't light up.

State 2(Dark Room): Suppose the light of the room or experimental environment is off now. 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 current through the Variable resistor will now go to ground via both the LDR and the transistor causing the transistor to turn on and the LED will start glowing.

Variable resistor can be used to control the sensitivity of this circuit.

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 heavier loads. You can also use Relay to drive larger loads like a main line voltage device.

A simple LED flasher can also be built with this. Which I hope to write about in a future post.


1. Transistor Datasheet. 
2. Visit My full Blog. 
3. Darlington Pair.

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