Modular Electronic Systems
Introduction: Electronics  A systematic approach
The systems represented in this section make use of E&L systems boards. Centres
using Alpha systems will be able to adapt the materials as appropriate. In all areas of
technology, including electronics it is useful to have a systems approach to problem
solving. This makes it easier to understand problems and enables you to solve these
electronic problems in a structured manner.
All electronic circuits and systems have an input, a process and an output. Identifying
these basic parts is the first step in solving electronic circuit/system problems.
Input
Process
Output
Modular boards
Modular circuit boards provide a simple means of solving both simple and complex
electronics problems. It is easy to identify the input, process and output stages. The
boards are, however, bulky and expensive.
Standard Grade Technological Studies: Electronic Systems Revision Notes
1
Analogue and digital signals
All components in electrical and electronic circuits are either receiving or transmitting
electrical signals. These signals can be either analogue or digital.
Analogue devices
An analogue signal varies according to the physical surroundings. For example, the
E&L light-sensing unit will send out a voltage that is proportional to the amount of
light falling on the LDR.
TP
Light sensor
(ORP 12)
+
+
S
0V
S
0V
-
-
0v
E & L INSTRUMENTS Ltd
This E&L unit is called an input transducer because it converts the change in light to
a change in voltage.
Light
Intensity
LDR
Varying
Voltage
The graphs of analogue input transducers are typically a sloping line or a curve.
Typical analogue input transducers are:
 input voltage units
 light-sensing units
 temperate-sensing units
 moisture/rain sensor units
 sound-sensing units.
2
Standard Grade Technological Studies: Electronic Systems Revision Notes
Digital devices
A digital signal is one which has only two settings, on or off. In electronic terms it has
only two levels, high or low.
The push switch unit below is a typical simple digital transducer.
O/P
IND
+
+
S
0V
S
0V
TP
-
-
0v
ACTIVE
HIGH
E & L INSTRUMENTS Ltd
This E&L unit is called an input transducer because it converts the change in physical
movement to a change in voltage.
Mechanical
Movement
Switch
High or Low
Voltage
The graph of typical digital input transducers is shown below.
Time
Typical digital input transducers are:
 switch units
 magnetic switch units
 pulse generator units.
Logic
In digital terms:
 on or high is Logic 1
 off or low is Logic 0.
This is why modern appliances have switches marked in this way.
1
0
Standard Grade Technological Studies: Electronic Systems Revision Notes
3
Transducer driver board
All systems require a transducer driver. This will normally be the second last
board in the system, that is the board before the output board.
COLLECTOR
+V
EMITTER
+
TP
S
0V
-
BASE
+
S
0V
-
0v
0v
E & L INSTRUMENTS Ltd
The transducer driver is a small transistor amplifier that provides the output devices
with enough power for them to operate. The signal (current and voltage) from the
input is otherwise too weak to power the output.
The terms in the above diagram are explained in the ‘Component Electronic Systems’
section.
Output transducers
Output transducers take an electrical signal and change it into a physical output. They
include the output boards in modular systems or output components in any electronic
system.
The main output transducers are shown below.
ACTIVE LOW
+
+
S
0V
S
0V
-
+
-
-
S
0V
+
+
S
0V
-
S
0V
-
E & L INSTRUMENTS Ltd
+
S
0V
-
-
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
4
+
+
S
0V
S
0V
-
E & L INSTRUMENTS Ltd
+
+
S
0V
-
S
0V
-
E & L INSTRUMENTS Ltd
Standard Grade Technological Studies: Electronic Systems Revision Notes
The Inverter
The inverter is often referred to as a NOT gate. This is because the output from the
inverter is NOT equal to its input.
If the input signal is high (or logic 1) then the output signal is low (or logic 0).
1
0
Inverter
If the input signal is low (or logic 0) then the output signal is high (or logic 1).
0
1
Inverter
From the diagrams above, you can work out that there are two input combinations.
These can be represented in a table called a truth table. Copy the symbol and table and
then complete the table.
A
0
1
Symbol for NOT gate
Z
1
0
Truth table
Switches
Switches are the most common types of input device. Most electronic systems use
switches. As shown earlier in this area of work, switches are digital devices.
Off (Logic 0)
On (Logic 1)
The switch used in task 6 above is a normally open switch. This means it is normally
off, that is at logic 0 state. You have to press the switch to turn it on and its spring will
return it to its normal position when the pressure is removed.
Standard Grade Technological Studies: Electronic Systems Revision Notes
5
AND gate logic
It can be worked out that there are four input combinations. These can be represented
in a truth table. Copy the symbol and table and complete the table.
A
Z
B
Symbol for AND gate
A
0
0
1
1
B
0
1
0
1
Z
0
0
0
1
Truth table
OR gate logic
It can be seen that there are four input combinations. These can be represented in a
truth table. Copy the symbol and table and complete the table.
A
Z
B
Symbol for OR gate
A
0
0
1
1
B
0
1
0
1
Z
0
1
1
1
Truth table
6
Standard Grade Technological Studies: Electronic Systems Revision Notes
Pulse Generator
+V
INC
FREQ
ACTIVE LOW
+
POS
SIG
+
S
PULSE
0V
GEN.
-
0V
TP
+
S
0V
S
0V
-
-
NEG
TP
+
+
+
S
0V
S
0V
S
0V
-
-
-
0v
+
0V
RANGE
+5V DC TO +8V DC
+
EXT.C
0v
PULSE
IND
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
The pulse generator is used to give a signal which constantly switches off then on.
The speed of this switching is controlled by the potentiometer on the board.
The pulse generator is most commonly used with an AND gate as shown below. This
means that the bulb here will only flash when the pulse generator is pulsing AND the
switch is pressed.
Push
Switch
Pulse
Generato
r
Power
Connectio
n
And
Gate
Transduce
r driver
Bulb
Unit
Latch Unit
INPUT
IND
OUTPUT
IND
+
TP
S
0V
-
+
S
0V
LATCH
A latched system is one that remains on until it is reset.
It is often called a memory system as it ‘remembers’
that the switch has been pressed until the reset on the
latch is pressed.
-
TC4011BP
864BHB
EXT
RST
E & L INSTRUMENTS Ltd
Note: the inverter is needed because the latch unit only
works as the voltage signal is dropping – this is called
negative-edge triggering. The inverter allows the
system to work when the voltage signal is rising, as
happens with a normal switch unit. This converts the
overall system to positive-edge triggering.
In any exam question you will use positive edged triggered latches.
YOU DO NOT NEED TO PUT AN INVERTER IN FRONT OF A LATCH IN
AN EXAM QUESTION
Standard Grade Technological Studies: Electronic Systems Revision Notes
7
Relay Unit
The relay unit is used to switch on a separate circuit, usually one which has a higher
voltage.
+
+
S
0V
S
0V
-
The ‘clicking’ sound at the output is caused by the contacts
of a switch inside the relay closing. The contacts are moved
by the electromagnet energising.
-
SOFT IRON
ARMATURE
PIVOT
SOFT IRON
CORE
E & L INSTRUMENTS Ltd
COIL
SPRING
TERMINALS
CONTACT
TERMINALS
COIL
TERMINALS
Delay Unit
INC.
DELAY
+
TP
SA
0V
+
S
0V
-
-
DELAY
NEC556
0862
B
2
+
0v
+
E & L INSTRUMENTS Ltd
This task uses a delay unit. The delay unit reacts to an input signal by waiting a few
seconds before it sends out a high (logic 1) signal to the next sub-system.
In any exam question you will use positive edged triggered delay units.
YOU DO NOT NEED TO PUT AN INVERTER IN FRONT OF A DELAY
UNIT IN AN EXAM QUESTION
8
Standard Grade Technological Studies: Electronic Systems Revision Notes
The comparator
The comparator board receives an input voltage from a sensor and compares it with
another voltage called the reference voltage.


If the input voltage is smaller than the reference voltage the comparator gives out
a ‘low’ signal (logic 0).
If the input voltage is greater than the reference voltage the comparator gives out a
‘high’ signal (logic 1).
The comparator may be used as an error detector in closed loop systems, where it
compares a feedback voltage to a reference voltage.
COMPARATOR
REQUIRED
TEMP
OUTPUT
DRIVER
OUTPUT
TEMP
MOTOR
TEMP
SENSOR
VREF
INC
REF VOLTS
+
+
S
0V
S
0V
-
REF
-
FEEDBACK LOOP
The output signal from a comparator is digital, that is, it is
logic 0 or logic 1. The feedback signal that is fed into the
comparator is usually analogue. Therefore the comparator can
be thought of as an analogue-to-digital converter.
TP
MAGIC
THINGY
0V
The potentiometer on the comparator board sets the reference
voltage.
WITH
REF VOLTS
E & L INSTRUMENTS Ltd
VREF
+V
+
POS
SIG
TP
S
0V
-
0V
NEG
INC
REF VOLTS
+V
+
+
+
+
+
S
0V
S
0V
S
0V
S
0V
-
-
REF
-
-
TP
0V
TP
+
+
S
0V
S
0V
I
N
0V
TP
-
-
TP
+
S
0V
-
0v
0V
RANGE
+5V DC TO +8V DC
O
U
T
0v
0v
MAGIC
THINGY
0V
WITH
REF VOLTS
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
E & L INSTRUMENTS Ltd
The temperature sensor sets the input voltage signal (feedback) to the comparator.
The comparator takes the analogue signal and gives a digital output.
COMPARATOR
Standard Grade Technological Studies: Electronic Systems Revision Notes
9
NAND and NOR gate boards
There are two modular logic boards that are very useful in building systems. These are
the NAND gate and the NOR gate.
+
SB
0V
-
+
TP
O/P
IND
SA
0V
-
Inverter
+
S
0V
-
NAND GATE
And Gate
THIS IS A
CHIP BUTTY
0v
E & L INSTRUMENTS Ltd
NAND gate
This board is like a combination of AND gate and inverter (NOT gate) boards. This
may simplify some problems or give alternative solutions.
NOR gate
This board is like a combination of OR gate and inverter (NOT gate) boards. Again,
this may simplify some problems or give alternative solutions.
O/P
IND
SA
0V
-
Inverter
+
SB
0V
-
+
TP
+
S
0V
-
NOR GATE
Or Gate
THIS IS A
CHIP BUTTY
0v
NOR
E & L INSTRUMENTS Ltd
10
Standard Grade Technological Studies: Electronic Systems Revision Notes