Download gcse physics : electronics

GCSE PHYSICS : ELECTRONICS
M. Manser
Electronics and electronic systems are quickly becoming indispensable to everyday living. They are being used
more and more in our homes, schools and places of work.
Semiconductor devices and their use in integrated circuits (“chips”) have become invaluable for entertainment
(video games), convenience (automatic banking machines) and in medicine (pacemakers).
Mass production allows for these tiny devices to be made quite cheaply. The systems (of which they are a part) are
usually small, reliable, use small amounts of energy and respond very quickly in comparison to most mechanical
systems.
ELECTRONIC SYSTEMS
Input Sensor
Processor
Detects
changes
in
the
environment, and converts one type
of energy into an electrical signal :
LDRs, thermistors, microphones,
switches.
AND, OR, NOT gates
Output Transducer*
Takes the electrical energy from the
processor and converts it to another
form like heat or light or sound:
lamps, LEDs, loudspeakers,
motors, heaters, relays.
* A transducer is a device for converting one type of energy into electrical energy, or vice versa.
INPUT SENSORS
Some property of these devices must be affected by the environment. For example, a thermistor’s electrical resistance is
affected by the surrounding temperature. When included in a circuit, the current and voltage distribution in a circuit is
automatically and instantly affected by changing temperature.
A potential divider is usually used to provide a required voltage. Often an input sensor and a variable resistor are used. The
total voltage supplied (Vin) is then shared across the two resistors depending on the environment (ie temperature or light
intensity for example) and also on the value of the variable resistor. If either resistance is increased, the share of the voltage
across it also increases.
Vin
The potential divider equation : Vout = Vin x
R2 ,
(R1 + R2)
R1
Vout is then fed to the processor.
Vout
Changing the setting of R1, allows the circuit to respond at different
temperatures.
R2
0V
0V
Whatever device is connected across Vout will have the same voltage across it
as R2 since it is in parallel with it.
Consider the variable resistor to be set at a certain value. Then R1 alone will vary depending on the temperature. If it gets
hotter, then R1 will get smaller, so there will be a smaller portion of Vin across it. This means that there will be a higher
voltage across R2 , i.e. Vout will increase as the temperature increases.
The way that this circuit is used can be altered by taking Vout across R1 instead. In this case, as the temperature increases, and
the restistance of the thermistor drops, the voltage across it (Vout) drops too.
Vout will be in the form of an (electrical) analogue signal, i.e. it will have many different values as the environment changes.
PROCESSORS
Logic gates are tiny electronic devices that are manufactured from many transistors (semiconductors) and produce a digital
output from an analogue input (a voltage). They can be manufactured or set to work above certain voltages. If we consider a
particular logic gate designed to “work” with 3V, then any input below this can be thought of as “LOW” or “0”, then if the
voltage is 3V or above, the input is considered to be “HIGH” or “1”. Logic gates are used in a variety of combinations for
different jobs.
The NOT gate.
The OR gate
A
(inverter)
The AND gate
O
A
O
A
B
O
B
Truth table
Input
Output
A
B
O
0
0
0
1
0
1
0
1
1
1
1
1
Truth table
Input Output
(A)
(O)
0
1
1
0
Truth table
Input
Output
A
B
O
0
0
0
1
0
0
0
1
0
1
1
1
Using logic gates
Example 1 : A washing machine designer wants to ensure that the motor will not start until the on/off switch is ON and the
door is safely CLOSED. Very simple circuits can be set up so that when the switch is on, a HIGH voltage sent to the
processor (ie, one input = 1) , and if the door to the washing machine is shut, the second input is HIGH (or 1) as well.
(A)Power
on/off switch
on = 1
off = 0
MOTOR (O)
(B)
Door switch
1= motor on
0 = motor off
shut = 1
open = 0
A:
0
1
0
1
Truth table
Inputs
B:
0
0
1
1
Output
O:
0
0
0
1
Example 2 : This circuit is a safety feature. A buzzer sounds to remind occupants to fasten their seatbelts. If the ignition
switch is on, and the seatbelt is fastened, then the engine will turn ON and the buzzer will be OFF. If the ignition switch is on
and the seatbelt is not fastened, then the buzzer will sound (ON) and the motor in the engine will stay OFF.
A
1
1
0
0
B
1
0
1
0
Output 1
1
0
0
0
Output 2
0
1
0
0
(A) ignition switch
(B)seat-belt switch
on =1
off = 1
Engine : O 1
engine on = 1
engine off = 0
Buzzer : O 2
buzzer : on =1 off = 0
belt fastened = 1
belt unfastened = 0