Basic Transistor Circuit – B1P3

We know that we can turn an LED ON/OFF using a switch and that we can use a potentiometer to control its brightness.

LED control options

Both these involve some manual intervention and in this video, we are going to learn how to do this electronically using a transistor.

NPN transistor symbol

This is the symbol for a transistor and it has three terminals – the base, collector and emitter.

LED control using a transistor

If we connect an LED as shown above, the transistor can then be used to control the LED. The transistor works in different modes depending on the voltage and current flowing between the base and emitter terminals.

Transistor in the OFF state

When no voltage is applied to the base, the transistor is in the OFF state also known as cut-off mode and the LED is switched OFF.

Voltage applied to transistor base

As we apply a voltage between the base-emitter terminals, it causes a small current to flow through the base-emitter, which in turn causes current to flow through the collector-emitter terminals.

Transistor in the linear region

As the base-emitter voltage reaches around 0.6-0.7V, the transistor starts conducting and it causes a sharp rise in the collector-emitter current, which switches ON the LED. In this region, the collector-emitter current is directly proportional to the base-emitter current and the transistor is said to be in the active or linear mode.

Transistor in the saturation region

Increasing the base-emitter current will cause an increase in the collector-emitter current and this will continue until we reach a point where the collector-emitter current becomes independent of the base-emitter current. The transistor is now said to be in the saturation mode.

Transistor datasheet page

As with any electronic component, there is a maximum limit to the voltages and currents that can be applied to the terminals without damaging it. All this information is contained in something called a datasheet which can be obtained by simply typing the part number into a search engine. You can use resistors to limit this current.

Transistor circuit simulation

Let’s take a look at the simulation above to get a better understanding of the above. We have a transistor like before and we’ve connected a 3V battery between the collector-emitter terminals and have added a 330-ohm resistor to limit the current. We have plotted the voltage and current flowing through the resistor for analysis. Similarly, we have added a voltage source between the base-emitter terminals along with a current limiting resistor. The voltage and current have both been plotted for analysis.

Transistor circuit simulation

When the base-emitter voltage is 0, both the currents are 0. As we increase the base-emitter voltage, the base-emitter current increases, which causes an increase in the collector-emitter current as well. When the base-emitter voltage crosses 0.6V, there is a sharp rise in the collector-emitter current which tells us that the transistor has switched ON.

Transistor gain

The base-emitter current is about 30.05uA while the collector-emitter current is 6.01mA which is about 200 times the base-emitter current. This means that the transistor has a gain of 200.

Transistor overview

When used in the linear region, transistors are said to amplify the base-emitter current. If we continue increasing the base-emitter current, we will then enter into the saturation region. The cut-off and saturation region represent the OFF and ON states of a mechanical switch. The ability to use transistors as switches and amplifiers has greatly contributed to the advancement in electronics.

Basic transistor circuit - schematic

Let’s take a look at the circuit diagram for this project. We have an LED connected like before along with a current limiting resistor but we only have a wire connected to the base terminal.

Basic transistor circuit - layout

Let’s use the breadboard layout to build the circuit.

Basic transistor circuit - breadboard

If we hold the jumper wire connected to the base with one hand and use the other hand to hold the jumper wire connected to the positive rail, then you will see that the LED starts to glow. A tiny current flow through our body which switches ON the transistor. The amount of current depends on the body resistance and we can decrease the body resistance by increasing the pressure applied which in turn will increase the brightness.

CAUTION
Please do not repeat this circuit with any other power source. The circuit above is safe because of the high body resistance and because we are using a small, 3V battery source. Using a higher voltage or higher capacity power source can be extremely dangerous.
Also, do not directly connect the two terminals together without using a current limiting resistor as this will cause a high current to flow through the base-emitter terminals which will damage the transistor.

We can now use transistors to build interesting circuits so let’s move on to the next post.