Transistor as a Switch CIRCUIT

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TASK
Look at the diagram above:
1. What components do you recognise?
2. What components are new?
3. Write the new components LDR and NPN Transistor into your Terminology and Meaning list.
Also include "Potentiometer: A resistor that can change it's resistance".
4. Write the heading amd draw and label the diagram.
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WHAT IS A TRANSISTOR?
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CHECK: DO ALL LEARNERS HAVE THEIR CONTAINER? THEY NEED A TRANSISTOR, POTENTIOMETER, 1K RESISTOR, AND A LIGHT DEPENDENT RESISTOR
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TASK
Pick up your transistor:
Identify the Collector, Base and Emitter. Make sure you are looking at the flat side.

HISTORY OF RESISTORS AND THE POTENTIOMETER

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TASK
Pick up your potentiometer:
Identify the A, B and C. 
Check it's value.
Can you see how it works.


DRAW THE TRANSISTOR AS A SWITCH CARD-TRONICS CIRCUIT
It is important to practice drawing this circuit before putting it on card. It's more complicated and there is potential for mistakes to occur.

TASK
In your exercise book copy the diagram below. Polarity matters with the LED and the transistor must have the correct terminals connected as shown. It's ok to draw the diagram freehand using an edge to guide you for the straight lines.

Once done get a classmate to check it.
Then bring it and get your Learning Leader to check it.
After these check draw the same diagram onto your card.

​CIRCUIT CONSTRUCTION
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WARNING: WILL BE USING A HOT SOLDERING IRON. POTENTIAL HAZARD 

​Materials needed:
1 x plastic container for components
1 x a piece of cardboard approximately 100 x 100 mm
1 x length of copper tape
1 x resistor 470 ohms
1 x resistor 1K ohms
1 x potentiometer 10K ohms
1 x LEDs (Light Emitting Diodes)
​1 x 9v battery clip
1 x NPN transistor
1 x Light Dependent Resistor
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SHORT CIRCUIT
Now that there are quite a few components on our circuit board there may be quite a number of wires on the other side. These wires must NOT TOUCH. Why? Because they will cause a short circuit, these means the electricity will flow in way that we didn't intend. This will usually cause the circuit not to operate as intended. 

USING PLIERS AND OTHER TOOLS
No tools can be removed from the room. All tools cost money which is not easy to come by. If any tools are removed that puts the viability of this course at risk.

​ATTACHING TRANSISTOR AND POTENTIOMETER
You need to be gentle and careful with the components. Take your time attaching them to the board. Pliers can be used to bend the wire ends. 
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USE PLIERS TO BEND WIRE IF NEEDED
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ATTACH COMPONENTS CAREFULLY SO AS NOT TO CAUSE A SHORT CIRCUIT


ADD THE COPPER TAPE AND FIT THE NEW AND TRICKY COMPONENTS FIRST
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​SOLDER IN THE COMPONENTS


​ALL COMPONENTS DONE. CHECK TO BE SURE
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ADD SOLDER TO JOIN ALL PIECES OF COPPER TAPE


ADD THE BATTERY AND TEST THE CIRCUIT
Initially, the circuit will not work. This is because the circuit needs to be ‘tuned in’.
For the LED to turn on and off the voltage level at the transistor collector needs to be ‘just right’ (0.7v).
The easiest way to do this is to turn the potentiometer all the way back and forth until you find the place where a small movement turns the LED on and off.
Make the LED only just turn on.
Really only just!
Shade the LDR and the LED should turn off.
Electronic magic! 

WATCH THE VIDEO
​The video explains a bit about the circuit and demonstrates how it works.ou need.

This circuit uses the transistor as a switch to turn on an LED. The LED could be an alarm based on a light sensitive or other component. Once the circuit is turned or 'armed' it would be active.


What Uses Could This Circuit Be Put To?
​This circuit has a variety of uses if we think about it. One might be a light dependant device of some sort. One that turns on when it get dark. This can be achieved by swapping the LDR and potentiometer positions. 

CHALLENGE
Try swapping these two components on the circuit to see what happens. 

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