It is done by using a light-dependent resistor (LDR). It facilitates to switch on the light provided a bright environment and switches off based on the dark environment.

One of the examples of these switches is light-bulb.

By varying the values of the frequencies of transistor the speed of the motor can be varied.

In order to turn ON the motor and OFF it this application is used.

The dc motors can be controlled and monitored by using this concept of transistors.

The relay operation can be controlled by making the necessary changes in the circuit so that any external device is connected with respect to the relay and gets controlled.

The most frequently used practical application that is used for the transistor as a switch is functioning of LED.

The applications of the transistor used as a switch are as follows: The above is the basic application of the transistor as a switch for the P-N-P and N-P-N bipolar junction transistors. In this case, the terminal base is connected to the ground The load here is connected to the respective ground of the transistor and then the transistor P-N-P switches power. If it is in ON condition the flow of current is observed otherwise it is OFF. But the criteria of the operation remain the same. The operating conditions of the P-N-P and N-P-N transistor differ in the application positive or the negative voltages. The load can be either the led or the resistor, load depends based on the requirement. Once the transistor is in ON condition the current generated tends to flow from source to the load. If its crosses it the transistor is said to be ON otherwise OFF. Between the region of the emitter and the base, the voltage applied must reach the cut-in voltage. Similar to the condition of the diode there exist the cut-in voltage. Once the voltage has been applied to the region of base based on it the operation of the switching is performed. This indicates that when the zero of the signal applied to the base the transistor will be ON otherwise it will be OFF. This operation of the transistor is known as a single pole single throw (SPST).

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