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Transistor Amplifier (Darlington pair) – Easy (simple "Touch Switch")

Building the circuit:

This circuit can be built entirely from parts in our starter kit; the parts list is as follows:

Ω      Two NPN transistors

Ω      One LED (any colour!)

Ω      One 120Ω resistor

Ω      One 9V battery clip

Ω      One 9V battery

Ω      Some Breadboard hook-up wire

Ω      One breadboard

The circuit:

Touch Switch Circuit Diagram

Above you can see the circuit diagram for this circuit, don’t worry about the part codes in this schematic. You can see the Darlington pair, where one transistor feeds directly into the second transistor. You can also see the LED above them, and another resistor. The transistors together are so good at amplifying the current that too much current will be drawn through without that resistor, and the LED will break. We call this resistor a ‘current limiting resistor’; again, this is used a lot in electronics to protect components. The two exposed wires are the ones you touch together to complete the circuit.

 

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

Touch Switch Breadboard Circuit Image

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:

Touch Switch Breadboard Circuit Image

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:

Touch Switch Breadboard Circuit Image

Finally add in the two connecting wires, one from positive voltage, and another from the middle pin of the second transistor (its base), also add in the battery connector and battery with the red line (positive voltage) going to the red track on the breadboard and the black line (negative voltage/ground) to the blue line on the breadboard. You should now have a complete circuit:

Touch Switch Breadboard Circuit Image

Hold each of the wires in one hand and the LED should come on! You are now acting as part of the circuit! If the LED doesn’t come on check back at your wiring and make sure you’ve got it right. Once you have got it working you can also try using a pencil line as part of your circuit, or the pencil itself. Here is a photo of it working for us:

Touch Switch Breadboard Circuit Image

Overview / how it works:

This is a very simple circuit to demonstrate how a transistor can amplify electric current. The circuit is completed by holding either end of the two exposed wires with your body acting as the cable between them. You can also use a trace from a pencil or a liquid such as water (dissolving salt in it helps it to conduct even better!!). The problem with using your skin to complete the circuit is your skin has a very high resistance (somewhere between 1KΩ and 1000KΩ for an arm) and so because of ohms law:

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

This means that:

I (current) = V (voltage) ÷ R (Resistance)

So if the resistance is 1000KΩ:

9V ÷ 1,000,000Ω = 0.000009A or 0.009mA

This isn’t nearly enough to power an LED (which usually takes 20 – 30 mA to light up well). So we use two transistors to amplify it (make it bigger).

A transistor is a very useful device because it can be used as an electrical switch, or an amplifier. In this circuit we use two to amplify the current. Transistors come in two types:

 

         

transistor circuit symbols

 

 

 

 

  NPN:

 

           

 

 

 

 

PNP:

 

 

 

 

Want an easy way to remeber which transistor is which on a circuit diagram?

NPN- the Arrow is   Not Pointing iN

 

 

In our starter kit we only have NPN transistors so you don’t need to worry about the differences. Transistors all have three legs, a collector, a base, and an emitter. The base is used to control the current flow from the collector to the emitter. A small current flow between the base and the emitter will trigger a bigger current flow between the collector and the emitter; this is how we use them to amplify.

NPN Transistor Circuit Symbol

 If no current is applied to the base of the transistor, it does not conduct, and the circuit is broken. In our circuit we have two transistors because one transistor cannot amplify the current enough. The emitter of the first transistor goes into the base of the second transistor, causing an even bigger current to flow from its collector to its emitter. This is called a ‘Darlington pair’ and is commonly used in electronics.

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