SWITCHING ACTION OF TRANSISTOR USED AS NOT GATE

Transistor is a semiconductor device (derived from name TRANSfer of resISTOR ) which is used to amplify signals or for switching operations. Transistor was invented by william shockley in bell laboratory. the invention that makes possible microprocessors and controllers. perhaps the development of such a large electronic industry would not have been possible without the invention of transistor. To make the circuit working as a switch, it operates in two modes. These are

• saturation region
• cut off region

In saturation region , the current through the base and collector is maximum and  also the Voltage across the collector emitter junction is minimum. we can idealize the transistor as a closed switch in saturation mode

In cut off region , the current through base and collector is minimum and also the voltage across the collector emitter junction is maximum , we can idealize the transistor as a open switch in cut off mode.

CIRCUIT DIAGRAM AND WORKING

In the figure shown below  , when the switch is open (not pressed) the current through the base  is minimum and so consequently the collector current is also minimum. Thus the transistor operates in cut off region

When switch is pressed , the current through the base and collector is maximum , forcing the transistor to operate in saturation mode , thus the voltage across collector emitter voltage is minimum as shown below

OBSERVATIONS

• WHEN transistor operates  in saturation mode Vce is minimum (0.38V).
• When transistor operates in cut-off mode Vce is maximum (12 V In above case).

now we are going to glow led on the basis of our above observations and placing LED at right place.

switch is not pressed and led is on

transistor in cut off mode

switch is pressed and led is off

transistor in saturation mode

in the circuit diagram the question arises that "why 100 ohms resistor is used with the resistor however we have already placed 1k resistor for LED protection"

answer to the above question is that " transistor is not ideal.when transistor is in saturation mode, the small voltage across the emitter collector junction may glow the LED with light intensity very less but sometimes can be wrongly considered as transistor in cut off mode. so to avoid this problem we are using 100 ohm. we can chose any value but should be in range of 80-200 ohms.