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Transistor Night Light – Easy

Building the circuit:

The parts list is as follows: A pack with all the parts is available here

Ω      Two NPN transistors

Ω      One LED (any colour!)

Ω      One 120Ω resistor

Ω      One 220KΩ resistor

Ω      One light dependant resistor

Ω      One 9V battery clip

Ω      One 9V battery

Ω      Some Breadboard hook-up wire

Ω      One breadboard

 

The circuit:

night light circuit diagram

Above you can see the circuit diagram for this circuit, again do not worry about the part codes for all the different parts. You can see the LDR (the resistor with two arrows pointing towards it) and the other resistor in the voltage divider like we talked about earlier and the feed from them going into the Darlington pair. The Darlington pair controls the amount of current pulled through the LED. There is a resistor before the LED to limit the current (we call this ‘a current limiting resistor’) and stop the LED exploding!

All the components can be bought from Mallinson Electrical or ordered online!

Start by cutting and stripping your hook-up wire and placing it in the breadboard as follows:

fridge alarm breadboard photo

Then add in your two transistors with the flat side facing you, the first pin of the first transistor should go where the red wire starts, and the first pin of the second wire should go where the red wire ends:

fridge alarm breadboard photo

 

Then add in your LED and the current limiting resistor as follows, the long leg of the LED should be placed on the same track as the resistor, and the short leg on the same track as the first pin of the transistor. The resistor should connect the long leg of the LED to positive voltage:

fridge alarm breadboard photo

 

Next you should add the Light dependant resistor and the 220KΩ resistor, the LDR should go between the middle pin of the transistor and ground, and the 220KΩ resistor should go between the middle pin of the transistor and the power.

fridge alarm breadboard photo

Finally add in your battery clip and battery to connect the power. Now your circuit is complete!

fridge alarm breadboard photo

 

Move your circuit to a dark place and the LED should come on! You can even try controlling it with your hand!

If your circuit doesn’t work check your wiring to the example we have given you, if that doesn’t work check your battery! Remember it must be very dark for the LED to come on!

Overview/How it works:

This circuit uses the same Darlington pair that we used in a previous tutorial to amplify current. However, this time is not our bodies that control the amount of current the transistors, but an LDR (light dependant resistor).  A light dependant resistor is a component we use for detecting the light level, there is one simple rule you need to remember to use a light dependant resistor:

The higher the light level, the lower the resistance!

Because of this we can use them to control the amount of current going into our Darlington pair (remember our Darlington pair is used to amplify the current!) which then drives an LED.

However, if the resistance decreases when the light level is higher, surely the LED will come on in the day? Yes! But there is a solution to this: we create what we call in the electronics industry a ‘voltage divider’ with the LDR. This is where we put the LDR in series with another resistor from power to ground and take a feed from between the two resistors. In the picture to the right Vin is the power source, and the funny symbol with lots of horizontal lines is the ground symbol. The values of the resistor Z1 and Z2 are what changer the output!

This happens because of Ohm’s law:

V (voltage) = I (current) X R (resistance)

So when Z2 is lower resistance more voltage goes through it to ground. If the voltage is higher going through the resistor then naturally it is lower going to Vout. So if the resistance of Z2 is higher less voltage goes through it to ground and so more goes to Vout.

In our circuit we will replace Z2 with our LDR. And the Vout line of our divider will go to the Darlington pair. This means when it is lighter the resistance of the LDR will be lower, this will mean more voltage goes through it to ground and less will go into the Darlington pair. The reverse works when it is dark, the resistance of the LDR is much greater (almost 1000KΩ) so less voltage goes to ground and more goes to the Darlington Pair. In this way we can turn the circuit off when it is light and turn it on when it is dark.

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