Subsystem: P2

Task Title:
Seeing in the Dark
Darlington Pair

 

Introduction and Aims

This is a very useful and common system that automatically turns on a light when it gets dark. We will use a Darlington pair (instead of a single BC548B (BC108C) transistor), to control an output device - an L.E.D. in our circuit.
This is anOpen Loop Control system”. You might have already carried out the testing of the light sensor (IP1) in a previous project.

Subsystem Circuit and Block Diagram

 

IP1
Dark Sensor
P2
Darlington Pair
OP1
Sinked
L.E.D.
 


Investigating the System

Open the "Library of Subsystems" here and search for "P2 Darlington Pair Amp" and "IP1 Potential Dividers". Read the descriptions and then complete the following:

1, The Variable Resistor VR1 should have a value of ohms.(see IP1)

2, Tr1 which forms part of the Darlington pair can have the following name/code .(see P2)

3, If the two transistors are thought of as a single transistor then a voltage of volts is neeed to turn the pair on and make the LED work..

4, In the circuit above a multimeter can measure the voltage that turns the Darlington pair on. It is the same voltage that is seen across the . (see IP1).

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 P2 project guide.