If the current is large, the transistor is on and the light bulb is lit. In this case the current flowing into the base can be varied. Controlling Transistor Base Current with Potentiometer As a result, the current flows through the light bulb which is therefore emitting light. In this example, the two resistors are set so that the base of the transistor is at a sufficiently high voltage for current to flow into it and as a consequence, the transistor is on. In another case, there is current flowing into the base and so the transistor is “on” and the current can flow through it resulting in the light bulb being on. As a result, the transistor is “off” and no current can flow through the bulb. In the first transistor, the base is grounded and no current can flow into it. In this circuit, there are two transistors. Therefore, we can ignore the Q – Point and switch it between saturation and cut off areas. For switching purposes, we only need this device to operate either in fully-on or fully off region.
In the saturation region, it remains fully ON. This diagram depicts the three operating regions of the transistor such as saturation region, active region and, cut off region. Many output devices will require a transistor switching circuit to operate a high current requirement load such as relays, solenoids, and motors. If we want to connect a load that requires a higher operating current demand more than 3mA, it will burn microcontroller.
But the question that may come to your mind, Why do we need to interface transistor with a microcontroller? Because microcontroller pins can not provide output current more than 3mA and voltage more than 5V. In any application, we need to interface a transistor with a microcontroller.
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