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TRANSISTORS AS SWITCHES
TRANSISTORS AS SWITCHES
Common Formats
Advantages:
Size Efficiency
Durability
Cost
Silicone Chips now contain over a million transistors for only a few
pence each.
How Transistors Work……..
The transistor is a bipolar device made up from 3
layers of semi conductor materials
P type (good Conductor)
N type (poor Conductor)
P type
How Transistors Work……..
The transistor layers are termed:
P
Collector
N
Base
P
Emitter
How Transistors Work……..
C
Apply Signal
+0.6 [Volts]
P
B
N
Resistance of N type Semi-conductor
Decreases
P
E
How Transistors Work……..
C
Apply Signal
+0.6 [Volts]
B
P
N
P
E
The Transistor becomes an
excellent conductor
Across the collector and
emitter
How Transistors Work……..
The BC 108 is common general-purpose transistor. The diagram below
shows the position of the legs when viewed from underneath the
case.
The transistor has to be connected into circuits correctly. The
arrowhead on the emitter indicates the direction of ‘conventional’
current flow  that is, opposite to the electron flow.
How Transistors Work……..
Or apply
Voltage
Directly
c
If you apply
Voltage
Directly
b
e
0v
How Transistors Work……..
c
b
e
The Transistor must be protected from short circuit by
placing loads at the base and the collector
How Transistors Work……..
c
b
e
Build the following circuit using crocodile clips
How Transistors Circuits are built….
Transistor
Circuit Group
Using input transducer devices that convert a change in physical conditions into a
change in resistance and voltage. This can then be processed in an electrical
network based on a voltage divider circuit.
How Transistors Circuits are built….
Voltage Divider
Circuit Group
Example 1: a simple voltage divider can be used to fine tune the signal to the
transistor
How Transistors Circuits are built….
Thermistor
Divider Circuit
Group
Example 2: a thermistor in the voltage divider can be used to trigger the transistor
at a given temperature
How Transistors Circuits are built….
‘LDR’ Divider
Circuit Group
Example 3: a LDR in the voltage divider can be used to trigger the transistor at a
given light Level
Build a circuit on a bread board
Problem: Mr & Mrs Beverage go regularly to their local clubby. When coming
home, usually too worse for drink. They stot and fumble in the dark trying to
open the door for ages, causing him to swear loudly and wake up Mr
Walker next door.
Brief: Design Mr Beverage a system that will bring on a Light when it gets
dark and has an adjustment that can alter for summer time- leaving Mr
Beverage more money for drink.
Investigate: LDR voltage divider configuration to provide a “darkness” sensor
Design & Build: A proto type using a breadboard