How To Use Transistor As A Switch

Touch on circuit using bc547

In this article, we have explained the Use Transistor As A Switch in different applications.  A transistor is a three-terminal semiconductor device that can be used for switching applications, amplification of weak signals, and quantities of thousands and millions of transistors are interconnected and embedded into a tiny integrated circuit/chip. A Transistor switch, which is used for ON or OFF of a circuit, so it means that the transistor is commonly used as a switch in electronic devices. Transistors work as a switch when it is in cutoff and saturation regions. In this article, we will explain how to use a transistor as a switch.

Use Transistor As A Switch

Bipolar junction transistors have two junctions base-emitter junction, base-collector junction. Accordingly, there are four different regions of operation in which either of the two junctions is forward biased reverse biased, or both. But the BJT can be effectively operated in their different modes according to the external bias voltage applied at each junction. i.e. Transistor inactive region, saturation, and cutoff. The other region of operation of BJT is called an inverse active region. The operation of the transistor in these modes is explained below.

touch switch circuit

The transistor in Cut off region

In this region, both junctions of the transistor are reverse biased. Hence transistor is cut off does not conduct any currents except for small reverse saturation currents that flow across junctions and can be used as a practical application in pce san Diego repair shops. In cutoff condition emitter current is zero and the collector current consists of small reverse saturation currents. The transistor, when used as a switch, is operated in cutoff on condition and saturation regions which correspond to switch off and on condition respectively.
Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch as being, both junctions reverse biased, IC = 0 and VB < 0.7v. For a PNP transistor, the Emitter potential must be -ve with respect to the Base.

The transistor in the Saturation region

The saturation region is one in which both Emitter Base and Base Collector junctions of the transistor are forward biased. In this region, high currents flow through the transistor, as both junctions of the transistor are forward biased and bulk resistance offered is very much less. A transistor in the saturation region is considered as an on the state in digital logic.
A transistor is said to be in saturation if and only if
 β > Ic/I
This is due to the fact that as both junctions of the transistor are forward biased along with electron current flowing from emitter to the base inactive region there will be an additional component of electron current flowing from collector to base. Small changes in Collector to base forward voltage leads to large variations in collector currents.

The inverse active region of the transistor

In the inverse active region is just inverse or complementary to the active region. In the inverse active region, the base-emitter junction is forward biased and the Base Collector junction will be reverse biased.

use of transistor as a switch

The diagram shows the two current paths through a transistor. The small base current controls the larger collector current. When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.
When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off. You can build this circuit with two standard 5mm red LEDs and any general-purpose low power NPN transistor (BC108, BC182, or BC548 for example). It is a good way to test a transistor and confirm it is working. A transistor amplifies current and can be used as a switch, as explained on this page.