BJT transistor as a switch circuit & Experiment

BJT transistor as a switch circuit & Experiment


Today I will explain and show, practical demo of BJT NPN, PNP transistor working and noise pick up. Here I have shown 6 circuits using transistors. Same 6 circuits are assembled here. 1, 2, 3, 4, 5, 6 In all circuits I have connected one LED. These 1st 3 circuts, are assembled using NPN transistors. And remaining 3 using PNP transistor. 1st we will discuss these 5 circuits. 6th one we will discuss later. In 1st 2 circuits, no gate current is flowing. So these 2 transistors are OFF. I have written OFF. So this LED & this LED, both are OFF Here I have shown one red color, here also. This red color wire is this wire. This red color wire is this wire. This is connected for noise pick up. This also is connected for noise pick up. Now I will touch this point. say here using this, a wire cutter. LED is glowing. Means some current is flowing here. If I touch this, more light will come. IF I donot touch, less light will come. This is plastic. This is plastic. So when we connect some metal part here, This picks up the noise. Because of noise current flows, and transistor, collector current will start flowing. In 2nd circuit, I have connected one resistor here. This is to suppress the noise. If you connect here, noise, it will not pick up. See I will touch here. LED is not glowing. This LED is not glowing. If I touch, then also it is not glowing. So noise is coming here also, here also, but this resistor is suppressing the noise. So a resistor across base & emitter, is very much required to suppress the noise. This 3rd one is ON. Here I have connected this resistor. Base current will flow. Base current flows like this. Due to this base current transistor will become ON. Load current or LED current will flow like this. This LED is here. In this PNP circuit, This resistor between B & E was there. Simillerly, between B & E, one resistor is connected. This will not make any base current. Base current has to flow like this. Here nothing is there. So this transistor is OFF. This transistor is this. This LED is this. Transistor is here, black color. So it is OFF. Here, I have connected a resistor here, for base current. So this LED is glowing. This is this. Base current will flow like this. And load current, or collector current will flow like this. So this LED will glow. In these 5 circuits, I have connected load at collector. This is collector. here also, here alos, here also. here also. This LED is connected at collector. But here, I have connected load at emitter point. LED here Here also base current will flow. And here collector current will flow. But this LED will not glow full. Here light is less. And transistor will not become ON fully. This is called the linear working of the transistor. Here this ransistor will have some drop. I will measure the drop, using multimeter & show you. Now I have connected one multimeter. across this transistor between E & C. Black color transistor. This is reading 4.54 volt. This means, this is in linear range. When transistor is fully ON, this drop will be less than 1 volt, Depending upon the data of this transistor. Now we will have a close look of this board. You can see close look, of this assemblly. This wire and this wire, are connected for noise. This red color power supply connection. And this ground is green color. Today we will close here itself.

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