Subsystem: P1

Task Title:
Touch Sensing
Transistor Switch

 

Introduction and Aims

In this task we will now use a transistor, instead of a mechanical switch, to control an output device. We will see that the BC548B (BC108C) transistor will amplify a very small input current to make a much larger output current. This will be used to create an "electronic switch" to drive our output - in this case an LED.

Subsystem Circuit and Block Diagram

 

IP1
Touch Sensor
P1
Electronic
Switch
OP1
Sinked
L.E.D.
 


Investigating the System

Open the "Library of Subsystems" here and search "P1 Current Amplifiers" and "OP1 Sourcing Sinking". Read the descriptions and then complete the following:-

1, Component R1 has a value of Ohms. (see P1)

2, Component R2 has a value of Ohms. (see OP1)

3, The 1K resistor is used to the transistor.

4, The transistor switches on when a voltage of volts appears on the connection of the transistor.

Modelling the System/Circuit

There are different ways to model and test circuits before components are soldered to a P.C.B.. Modelling is an important stage as it helps you spot any problems and make modifications more easily.

Method 1: Breadboard

Using this method, components are pushed into the interconnected holes as shown in the diagram below. The lines shown, indicate which holes are internally connected.

Print this page and by using the circuit diagram above, design the breadboard layout for this system.

Method 2: Computer Simulation

To begin, ask your supervisor if circuit simulation software is available on your PC. This method allows you to build up your circuits quickly and cheaply in a virtual environment but will not give you hands-on experience of the components you will be using when finally soldering your P.C.B..

Method 3: Using OnTrack modelling hardware

See P1 project guide.