Transistors can be used as switches and amplifiers and play a big role in circuits. So how do we connect a TRANSISTOR so that it can be used as a switch in a circuit?
First, why are transistors used as switches in circuits?
There are many different kinds of switches, listed below are various switches: pushbutton switches, rocker switches, slide switches, DIP switches, pushbutton switches, toggle switches, knife switches, they function the same as transistors, they are connected in a circuit To the load on the output side of the switch, the circuit below uses a single pole switch to turn the load (LED) on or off.
If these switches above serve the same purpose, why are transistors often used as switches in circuits?
The reason is that transistors are electrical switches.
Unlike the mechanical switches above, transistors are turned on or off by current flow. Mechanical switches, such as single-pole switches, push-button switches, require manual switching. However, the transistors are turned on and off not by human intervention, but by current flow.
Both have their own uses. Mechanical switches are often used outside of Electronic circuits, where one needs to control various functions such as ON-OFF switches for turning the device on or off, volume control, etc.
Transistors are used when we only want to turn a device on or off by its on-off state. As a prime example of how a transistor works perfectly as an electrical switch, we’ll cover it below.
How to connect a transistor as a switch in a circuit?
Now that we know why a transistor is used as a switch, now we discuss how to connect a transistor to use it as a switch in a circuit.
The transistor is a three-pin device consisting of the base, collector and emitter of a bipolar junction transistor (BJT). The emitter is the first pin, the base is the middle pin, and the collector is the third pin.
To connect a transistor as a switch in a circuit, we connect the output of the device that turns the transistor on to the base of the transistor. The emitter will be connected to the ground of the circuit. The collector will be connected to the load where the transistor will conduct and the supply voltage of the circuit.
There are several different parts in this circuit. The part that detects motion is the PIR motion sensor. When this sensor detects motion, it converts the motion energy into electrical current. Many electronic devices do this. They convert machinery into electrical current. PIR motion sensors can do just that. Once motion is detected, it outputs current to its pin 3 output pin. Since this output is current, it can be used to turn on the transistor.
Since the PIR motion sensor outputs current, and the transistor is a switch, it is an ideal switch to work with a transistor. A mechanical switch is a switch that is used when a person needs to press it to operate, and a transistor is a switch that is used when the current is turned on. So transistors are used when we want current to control the state of switches in a circuit.
When the PIR sensor doesn’t detect motion, it doesn’t output current, so the transistor doesn’t conduct. When the base of the transistor is not receiving enough current, no current can flow from the emitter to the collector to power the load, which in this case is a motor.
Even though the collector of a transistor needs a positive voltage (for an NPN transistor) to work, it won’t turn on just because there is a voltage. This is because when the transistor does not receive enough base voltage it acts as an open circuit. When the transistor is open, no current can flow to ground. Therefore, the +9V DC voltage supplied to the DC motor has no potential. The voltage across the motor is +9V. There is a definite potential only when the transistor is on and current can flow to ground.
When the motion detector detects motion, it outputs current from its output pin to the base of the transistor. This current turns the transistor on, so the transistor can now power its load, which is the motor. In this circuit, transistors act as switches and amplifiers. If a PNP transistor is used, a negative voltage is supplied to the collector.
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