Lab 4 - Transistor Switching

Objective:
To understand the usage of a transistor within a circuit.

The Circuit with a Switch:

Right Image: Breadboard Schematic
Left Image: Circuit Schematic

Components:

R1 = 180Ω

R2 = 10k Ω

R3 = 680 Ω

D1 = LED

S1 = Switch ( Push button)

Q1 = Motorola 2N3904 Transistor

With this circuit setup, the switch is used to determine if the circuit will work.  This means that if the switch closes the circuit, turned on, the LED will light and it will not light when the switch is open. Below is a video demonstration that the LED brightly lights up when the switch is closed.

Fingertip Switching: 

The circuit is modified to remove the Pushbutton switch and replace it with a fingertip to act as the switch.

Below is video demonstrations of the fingertip switch working properly.

The LED still lights up but the intensity of the LED is lower than the previous circuit.  This is difference is due to the resistance of each individuals skin on his or her fingertip, so the LED differs in intensity for each person.

The concept being illustrated by both lab demonstrations are a transistor amplifies any changes in current that one apply to its base.

The Circuit with a Potentiometer:

The circuit is modified to use a potentiometer in place of the switch.  This is to see how a transistor functions when potentiometer is placed to adjust the voltage of the base of the transistor.  The images below demonstrate the current flowing through the two ammeters that are connected to the potentiometer and the transistor. 

Measured Values of Current through Ammeter 1 and Ammeter 2: 

Graph of Measured Values: 

From the graph, the relationship of the current running through the Ammeter 1  and Ammeter 2 has a somewhat linear relationship.  It can be assumed that as current flow through Ammeter 1 increases, the current in Ammeter 2 also increases.  This is possible due to the usage of the potentiometer controlling the amount of current flowing through the circuit.