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Section 1 - Overview
Electronic systems consist of three main parts: input , process and output. This can be represented in a
block diagram:
Electronic Systems
input process output
The output of an electronic system can either be analogue or digital.
Digital outputs can only have certain values (usually this is either ON or OFF).
Analogue outputs have a continuously varying value.
Analogue and digital signals can be identified from the waveforms produced on an oscilloscope.
Digital and Analogue Outputs
digital analogue
Section 2 - Output Devices
Analogue Output Devices
Motor electrical to kinetic (rotation)
Loudspeaker electrical to sound
Bulb electrical to light
Digital Output Devices
Solenoid electrical to kinetic (in a line)
Buzzer electrical to sound
LED electrical to light
Relay electrical to kinetic
7-Segment electrical to light Display
Choosing Output Devices
Output devices should be chosen for a particular situation according to what form on energy is required and whether the output needs to be digital or analogue.
The LED
An LED (Light Emitting Diode) converts electrical energy into light, but it will only do so when is connected the correct way round.
electron flow
A resistor is always placed in series with an LED to prevent it being damaged by too large a current passing through it.
Calculating the Series resistance for an LED
The value of the series resistance that must be used can be calculated as follows:
Firstly VR = Vs - VLED = 6 - 1. = 4.2 V Then IR = ILED = 10 mA = 0.01 A
So R = VLED / ILED = 4.2 / 0. = 420 Ω
The following circuit would allow an LED to light:
6 V
R
LED Data
ILED = 10 mA
VLED = 1.8 V
7- Segment Display
A 7-segment display consists of seven LED bars. Different numbers can be produced by lighting the appropriate segments:
a
g^ b
c d
e
f
Digit Segments lit 0 abcdef 1 bc 2 abged etc. etc.
Section 3 - Input Devices
Energy Changers
Many input devices are energy changers; they convert some form of energy into an electrical signal. (Note: These are all analogue devices.)
Microphone sound to electrical
Thermocouple heat to electrical
Solar cell light to electrical
Resistance Changers
Some input devices are resistance changers; their resistance depends on some external factor. These include thermistors , LDRs and variable resistors. (Note: These are also all analogue devices.)
The Thermistor
A thermistor has the following symbol:
The resistance of a thermistor depends on temperature. As temperature increases resistance decreases.
t
T emperature U p R esistance D own temperature
The LDR
An LDR (Light Dependent Resistor) has the following symbol:
The resistance of an LDR depends on light intensity. As light intensity increases resistance decreases.
L ight U p R esistance D own light intensity
Resistance, Current and Voltage
(Ohm’s Law)
resistance =
voltage
current
Ω^ A
V
R I
V
V = IR
R = V
I
I = V
R
Capacitors
A capacitor has the following symbol:
The voltage across a capacitor increases with time.
time
Charging Capacitors
When used as an input for electronic systems capacitors are usually connected in series with a resistor. This allows the time taken for the capacitor to charge up to the supply voltage to be controlled.
Increasing the capacitance, C, of the capacitor increases the time it takes to charge.
Increasing the resistance, R, of the resistor increases the time taken for the capacitor to charge.
Vs
R
C
Voltage Divider Circuits
A voltage divider circuit consists of two or more resistors placed in series and is used to split the supply voltage between the resistors.
^ V s R 2
0V
^ R 1
V 1
V 2
R 1
R 1 + R 2
V 1 = x Vs
In a voltage divider circuit the ratio of the voltages is equal to the ratio of the resistances. i.e. R 1
R 2
V 1
V 2
To calculate the voltage across each resistor the following equations should be used:
R 2
R 1 + R 2
or V 2 = x Vs
Note also that Vs = V 1 + V 2
Choosing Input Devices
Input devices should be chosen for a particular situation according to what form of energy is providing the information.
For any system requiring a time delay chose a capacitor and resistor.
Low Temperature Sensor
The following circuit will switch on the LED when the temperature decreases.
Explanation:
- Temperature decreases.
- Resistance of thermistor increases.
- Voltage across thermistor increases.
- Voltage at base of transistor increases above 0.7 V.
- Transistor switches ON.
- LED lights.
+Vs
0 V
t
High Temperature Sensor Sensor
The following circuit will switch on the LED when the temperature increases.
Explanation:
- Temperature increases
- Resistance of thermistor decreases
- Voltage across thermistor decreases.
- Voltage across resistor decreases.
- Voltage at base of transistor increases above 0.7 V.
- Transistor switches ON.
- LED lights
+Vs
0 V
t
Time Delay Circuit
In the following circuit when the switch is released the LED will come on after a time delay.
Explanation:
Switch closed:
- Voltage across capacitor is 0 V.
- Voltage at base of transistor is 0 V.
- Transistor is OFF.
- LED is OFF. Switch opened:
- Voltage across capacitor slowly increases.
- Voltage at base of transistor increases above 0.7 V.
- Transistor switches ON.
- LED lights.
Since the capacitor takes time to charge there is a delay between the switch being opened and the LED lighting
+Vs
0 V
C
S
R
Logic Gates
Logic gates are digital electronic devices that
have one or more inputs.
Logic gates have only two possible values (logic
levels) for their inputs and outputs:
logic ‘1’ - a high voltage logic ‘0’ - a low voltage
A truth table shows the output for all the
possible input combinations of a logic gate.
Each logic gate has it’s own unique symbol.
NOT Gate (Inverter)
Symbol:
Truth table:
Function: The output is the inverse of the input
input output
0 1
1 0
input A input B output
0 0 0
0 1 0
1 0 0
1 1 1
AND Gate
Symbol:
Truth table:
Function: The output is only 1 with both inputs A and B are 1.
A
B
input A input B output
0 0 0
0 1 1
1 0 1
1 1 1
OR Gate
Symbol:
Truth table:
Function: The output is 1 if either input A or input B is 1 (or both).
A
B
Logic gates can be combined together to perform simple logic circuits. The design of such circuits will depend on the inputs available and the output required.
e.g. A circuit that will switch on when it is cold (NOT hot) AND light
Truth tables can be drawn for these simple logic gates.
These truth tables should show all the possible input combinations, the logic levels at important points in the circuit and the output logic level.
Combining Logic Gates
Section 5 - Analogue Processes
Amplifiers increase the strength of electronic signals. The output signal of an amplifier has the same frequency but a greater amplitude than the input signal.
Amplifiers are found in many devices such as radios, televisions, music systems and walkie talkies.
Amplifiers
Power, Voltage and Resistance
power =
voltage^2
resistance
W^ Ω
V
P R
V^2
V^2 = PR
P = V
2 R
R = V
2 P
Voltage Gain
The voltage gain of an amplifier is given by:
voltage gain =
output voltage
input voltage
The voltage gain of an amplifier can be measured by:
Connecting the input of the amplifier to an oscilloscope and measuring the input voltage.
Connecting the output of the amplifier to an oscilloscope and measuring the output voltage.
Use the formula:
voltage gain =
output voltage
input voltage
Power Gain
The power gain of an amplifier is given by:
power gain =
output power
input power
