1. Electronics / Modelling in Livewire & PCB Wizard
Automatic Switching:-
Light / Temperature Sensing -
Transistor Projects.
R.A. Moffatt.

2. Automatic Switching: Transistor Project
Aims:
To understand the concept of automatic switching /
sensing in electronic circuits
To practice modelling basic electronic switching circuits
using the Live Wire and PCB Wizard software
To construct (PCB’s) printed circuit boards

3. Automatic Switching: Transistor Project
You will have an opportunity to:-
Design a temperature/light sensing circuit.
To manufacture a printed circuit board.
Two of the circuits respond to a change in light levels and the other two respond to a change in temperature levels.

4. Practically every modern Intel Pentium processor main board features a temperature sensor developed at Delft University of Technology. The sensor detects when the processor starts to become too hot, and switches on a fan to cool it down. As soon as the temperature drops to an acceptable level, the fan is switched off. The temperature sensor was developed by Dr. Anton Bakker, who in April 2000…

5. Our Learning Intentions
know how to use the Live Wire and PCB Wizard software
how to design automatic control circuits
understand input sensors and transistor switching
an understanding of resistor colour codes using four and five band resistors
circuit building skills developed by using the soldering iron
practice at following detailed test procedures

6. WALT Electronics We Are Learning To
To understand the concept of a potential divider which provides automatic switching of electronic circuits
This is because ..
We can learn how to design electronic circuits and systems using automatic switching for our project work
Remember to:
refer to the water analogy to help understand how the Potential divider works

7. What I am Looking For WILF: Electronics
an understanding of the potential divider as
a sensor / automatic switch
the ability to sketch a circuit diagram of an
automatic temperature / light sensing
sensor circuit
to be able to model and test an automatic
switching circuit
to make a PCB and build a sensing circuit as
an example of a transistor-based control
system

8. Potential Divider Single-Pole-Single-Throw Switch
Preset – Variable Resistor
Component
symbols
Light Dependant Resistor 0V
Potential Divider

9. The Potential Divider Water Pipe Analogy: X B Y
Diagram extracted from Making Things Work by Ken Shooter & John Saxton

10. The Transistor
Transistors are semiconductor devices. This means that they have a resistance to the flow of electrons – current. However they become conductors if a small voltage is provided across two of the legs – the base and the emitter.
Collector (c)
Emitter (e)
Base (b)
Symbol for the n-p-n transistor
You can use the transistor as a high speed switch if a small voltage of 0.6 – 1.6V is applied to the base leg as shown below
Technology Education Centre 05.

11. The Transistor Large Voltage C 9V B 0.6V E Saturated = ‘fully on’
PNP Transistor B C E
Small Voltage in
symbol
Saturated = ‘fully on’

12. hfe = Ic Ib hfe Transistor Gain:
We know that a small current flowing at the base of the transistor
will ‘turn on’ a large collector current. This is known as current Amplification.
The ratio I I
is a measure of the amplification which we call the transistors current
gain. The symbol for the transistor current gain is c b
hfe
hfe = Ic Ib
( A)
(0.05A)
= 0.5 A A
= 200

13. The Transistor

15. Bulb-’off’ when Dark
BULB
‘off’
LDR
Transistor
V/Resistor

16. Bulb-’on’ when Light
BULB
‘on’
V/Resistor
Transistor
LDR

18. LED-’off’ when Light shines on LDRVR
Transistor
LDR

19. LED-’on’ when Dark

20. Ex 1: Potential Divider / Dark Light ‘on’
In the light the voltage between the base and the emitter leg of the transistor is 657mV or 0.65V. For this transistor the voltage is not enough to switch it on – therefore the bulb remains off.
‘OFF’
In the darkness the voltage between the base and the emitter leg of the transistor is 739mV or 0.739V. This voltage is high enough to switch on the transistor – therefore the bulb is switched ‘on’.
EX: 10
‘ON’
Model the circuit using Live Wire
as shown and test!
Draw this circuit diagram in your
folder and explain how it works?

21. Ex 2: Dark Sensor: Circuit 4 from 112 Circuit Designs – M. Lynch
TIP 121
Light level low LED will be ‘on’
Model this circuit and test using Live Wire
Draw this circuit diagram in your workbook and explain how it works?
Convert this circuit to a PCB layout using PCB Wizard

22. PCB: Printed Circuit Board
Real World
Normal
PCB Wizard

23. Printed Circuit Board

24. Circuit 5 A Light Sensing Transistor

25. Play the AVI file to help understand how this circuit works?
M. Lynch
Play the AVI file to help understand how this circuit works?

26. Double-Click the PDF icon to access
the data sheet
House plants can suffer if owners do not give them the right amount of water. Houseplants can die if they do not receive enough water, and they can rot if over watered.

27. Moisture Tester
‘on’
moisture level

30. Darlington Pair Driver
In a Darlington pair circuit, two transistors are wired together to
provide better Switching.

31. Electronics Year 9 Test
Make sketches of a 1K, 2k2, 680R and 470R resistor include the colour code for each
Explain the function of a resistor in an electronic circuit?
Draw the two component symbols necessary and in the correct order to make a potential divider that sounds a buzzer when light shines on the LDR? One of these component symbols have been drawn for you.

32. 4) Explain how the circuit shown below works?
5) Suggest two uses for this type of circuit?
6) What is the trigger voltage for a BS 108 Transistor?
7) Sketch the symbol for a transistor and label the legs: -
emitter, collector, base

33. 8) An electronic sensor is required for an elderly persons
room that sounds an alarm when the room temperature
drops to a certain level or gets ‘too cold’. Draw and
complete the circuit diagram using symbols to show the
type of sensor required?
9) Name each of the components used in your design?

34. 10) Name each of the component shown below and explain briefly their function
when used in an electronic circuit?