Download Circuit Theory

oisture Tester
The Circuit
&
Components
Objectives
Understand the purpose of Resistors
Understand Resistor Colour Code
Understand how a Transistor works
Understand our Circuit
So what is a Circuit?…
A circuit is something that electricity flows around.
It is a bit like a
race track the track must
be complete for
the car to get
round, otherwise,
it cant go any
further.
It is the same
for electricity.
If the circuit is
broken, the
electricity cannot
get around the
track and nothing
will happen,
ie, the light
wont come on.
Resistors…
•
Resistors are used to control the flow of
electricity around a circuit.
•
Resistors are measured in Ohms (Ω),
and there are lots of different values.
What do you think
would happen if there
were no resistors in
your circuit?
You know that you can write:
1000 ml = 1 Litre
Well, you can shorten the value of
resistors too:
1000 Ohms = 1K Ω
Other values can also be shortened:
Examples:
1800 Ω = 1.8 KΩ or 1K8
49K Ω = 4.9K Ω or 49000 Ω
Transistors…
…An un-pressed switch
If there is no voltage (or power) running
through the base, it acts like an insulator and
does not allow electricity to flow through.
This means the light in your circuit will not
come on.
… A pressed switch
When a voltage is applied to the base of
the transistor, it becomes a conductor, and
does allow electricity to flow through.
This means your plant does not need water
so the light will come on.
Moisture Probes
Battery
PLANT IS OK
Here, the light is switched
on.
There is moisture making a
connection between the
probes, so the transistor is
an conductor, switching
the light on.
PLANT NEEDS WATER
Here, the light is switched
LED (light) off. There is NO moisture
making a connection
between the probes, so
there is no current running
Transistor
into the transistor (it is an
insulator here).
Moisture Probes
Battery
Resistor Colour Wheel
Take a resistor colour wheel sheet. Colour
this as it says, and then cut out each
wheel.
Assemble the wheel with a pin and put it
onto the bottom of your sheet.
Plenary
What was positive about the lesson?
What was negative about the lesson?
What was interesting about the lesson?
Can you describe how our circuit works?
What does a transistor do?
What does a resistor do?
What is the value of a brown black green
resistor?