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P6D
Truth tables
You need to be able to recall and identify
the input and output signals in an
electronic system with a combination of
logic gates.
Typically this can involve completing a
‘truth table’ for a logic system with three
inputs (although Higher tier students
must also be able to do this for a logic
system with up to four inputs).
Input signals
You should be able to describe how to use
switches, LDRs (light-dependent resistors)
and thermistors in series with fixed
resistors to provide input signals for logic
gates.
Using a switch
This circuit diagram shows how a switch
can be connected to a logic gate.
An example with three inputs
The diagram shows a logic system made
from two logic gates, an OR gate and an
AND gate.
Note that you do not have to recall the
shapes of the logic gate symbols
Here is the truth table for this logic
system.
When the switch is open, as seen here,
the input to the logic gate is low (0).
When the switch is closed, the input to
the logic gate is connected directly to the
5 V supply, so the input is high (1).
Using an LDR
This circuit diagram shows how an LDR
(light-dependent resistor) can be
connected to a logic gate.
Note that output C from the OR gate is
also one of the inputs for the AND gate.
You don’t need to learn the example
above. Instead, make sure you understand
how the output E was determined from
the inputs, so that you can apply the idea
to other logic systems.
When it is dark, the LDR has a high
resistance, so the input to the logic gate is
low (0). When it is light, the LDR has a low
resistance, so the input to the logic gate is
high (1).
Using a thermistor
This circuit diagram shows how a
thermistor can be connected to a logic
gate.
When it is cold, the thermistor has a high
resistance, so the input to the logic gate is
low (0). When it is warm, the thermistor
has a low resistance, so the input to the
logic gate is high (1).
Input signals
A variable resistor may replace the fixed
resistor in the diagrams above.
This lets you adjust the point at which the
resistance of the LDR or thermistor has
decreased enough to provide a high input
to the logic gate.
The advantage of this is that you could
change the light level needed for the LDR
to provide a high input, or the
temperature needed for the thermistor.
Output indicators
The output current from a logic gate is
able to light an LED (light-emitting diode).
The LED will not light when the output is
low, but it will light when the output is
high.
A fixed resistor must be connected in
series with the LED. The resistor limits the
current flowing through the LED, so
protecting it from damage.
Relays
A relay can be used as a switch. A small
current in the relay coil can switch on a
circuit in which a larger current flows. This
is useful, for example, where a device
such as a bright security light needs to be
controlled by the output from a logic gate.
You need to be able to recognise and
draw the circuit symbol for a relay.
PICLogic gates are low-power devices.
They would be damaged if they were
exposed directly to mains electricity. The
use of a relay isolates the low voltage in
the sensing circuit (the part with the logic
gate) from the high voltage mains (the
part, for example, with the security light).