Transcript Logic gates - Sackville School

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A transistor is the basic
building block of
electronic components.
The average computer
may have millions of them
within its circuits.
Essentially, it is an
electronic switch.
This is the symbol for an
NPN transistor
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The three terminals are
called the Base, Collector
and Emitter.
C
B
When the “switch” is ON, a
large current will be
allowed to flow through C
to B and out of E
For this to happen, a
current must first flow
through B (from the left).
E
Current out = Current in + Current in
at E
at B
at C
Ie = I b + Ic
A simple light operated
switch.
•When the base voltage is high
enough, the current through it will
“open the gate”
•and allow a very large current to
flow through the bulb so that it
lights.
1. This will happen when the
voltage across the LDR is high.
2. So the resistance of the LDR
must be high for the light to be
ON.
3. The resistance of the LDR is
high in the dark.
4. So the light will come on when it
gets dark!
If two transistors are connected as
shown to make a switch…
A current will flow when it is ON
This can only happen if a current
flows into A AND B
If the current into either input A or B
drops, then the switch will be OFF
This table shows the
combinations of inputs
and the outputs produced
A
B
Output
High
High
ON
High
Low
OFF
Low
High
OFF
Low
Low
OFF
Logic gates are electronic
switches that process information.
They are called gates because
they open to produce a high
output signal only when they
receive the correct combination
of input signals.
Most logic gates have multiple
inputs, which are used to
determine a single output.
A large number of logic gates
can be incorporated in one
electronic chip.
This chip contains four
NOT gates.
Digital systems have two states, ON and OFF. These simple
electronic states are represented in binary code: ON is
called logic 1; OFF is called logic 0.
Microchips contain logic gates, which use this binary
code to send and store information.
Microchips can be easily damaged
by high voltages. Due to this the
voltages for the two logic states are
standardized.
Logic 1 is 5 V, while logic 0 is 0 V.
Logic gates receive multiple signals in these two states.
Different combinations of signals lead to different outputs.