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S.G. Electronics
Revision Lesson
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Introduction

Electronic systems have 3 parts


Input
Process
Output

There are 2 types of systems:

Analogue
Digital


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Output Devices
These turn EE into some other form
Analogue
Motor, Bulb, Loudspeaker
Digital
LED, Relay, Solenoid
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Input Devices

2 main types


Change the size of the input
voltage
Switch, Capacitor,
LDR, Thermistor

and

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Input Devices

Energy Changers

Thermocouple, Microphone
Solar Cell

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LDR

As Light Intensity Increases
Resistance Decreases
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Thermistor

As Temperature increases the
resistance decreases
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Capacitors


Store Charge
V
–t

Provide timing delays
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Voltage Dividers
Vs
R2
VOUT = V S ×
R1 + R2
R1
R2
Vout
0V
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LDR in Voltage Divider

As light increases, resistance of
LDR decreases, voltage across it
decreases,
V out
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Thermistor

As temperature decreases, resistance of
thermistor increases,voltage across
thermistor increases, V out decreases
Vout
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Capacitor


Capacitor charges up
Vout increases
Vs
V out
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Switches


When switch is closed
V out is low
+5V
V out
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Process Devices

Analogue : Amplifier

Digital : Transistor Logic Gates
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NPN Transistor


V be > 0.7 V
Transistor is switched on
Collector
Base
Emitter
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Alarm Circuits







Light Intensity decreases
R ldr increases
V ldr increases
V be increases
V be > 0.7 V
Transistor switches on
Current flows through LED
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Alarm Circuits








Temperature increases
R thermistor decreases
V thermistor decreases
V resistor increases
V be increases
V be > 0.7 V
Transistor switches on
Current flows through LED
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Alarm Circuits







Capacitor charges up
Vc increases
Vbe increases
Vbe > 0.7 V transistor switches on
Current flows through LED
LED lights up
Closing switch discharges
capacitor / resets system
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Logic Gates

Digital Devices

And Gate

Or Gate

Not Gate
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And Gate
AB
0
1
0
1
Output
0
1
0
0
1
0
1
1
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Or Gate
A
0
1
0
1
B
0
0
1
1
Output
0
1
1
1
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Not Gate
Input
0
1
Output
1
0
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Clock Pulse Generator







Capacitor charges up,Vc increases
Input to NOT changes from 0 to 1
Output changes to 0 , (led on )
Capacitor discharges,Vc decreases
Input to NOT changes from 1 to 0
Output changes to 1, (led off )
Process repeats
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Amplifiers

Analogue Process Device

Frequency unchanged
Vout
Voltage Gain =
Vin
V in
V out
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Amplifiers

Amplifiers in CD players amplify
power
Pout
Power Gain =
Pin
2
V
P=
R
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S.G. ELECTRONICS
Now try the Revision
Questions
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