The 555 Fan

1 Using a potentiometer

A potentiometer is a component we can use in our circuits as potential divider.
Before you learn about the potentiometer, you need to learn what is meant by a potential divider.

Open Circuit Wizard and create the circuit shown below.
When opening Circuit Wizard, you will always select the 'Circuit with GENIE flowchart' option.
The symbol with an V in it is an Voltmeter - which will be found under 'Test instruments' at the bottom of the options list.

Now press play and the three voltmeters should show potential difference across each of the resistors and across the pair of them.
Because both resistors have the same value, the potential difference supplies by the battery is equally divided across each resistor.
Now let's change the value of one of the resistors.

Complete the table below by changing the values in your circuit diagram.
Take a screenshot of each of your circuit results to upload to www.bournetolearn.com
Note - 500mV is = to 0.5V

If we know the resistor values and the total potential difference provided by the battery, it is easy to work out what the potential differences across each resistor will be.

$V_{bottom} = frac{R_{bottom}}{R_{top}+R_{bottom}}.V_{total}$

$V_{top}=V_{total}-V_{bottom}$

Why is this important?
By changing resistor values we can influence the timing of the 555 timer chip. This will speed up or slow down the time the capacitor takes to charge.
Set up the circuit as shown below.
Note, you will find the NE555 timer in the 'Integrated circuits' option in Circuit Wizard.
SW1 is a switch - which will be found under 'Switches', IT IS THE FIRST 'LATCHING SWITCH' OPTION. Third one down - SPST.
C1 and C2 are capacitors - which will be found under 'Capacitors'
D1 and D2 are diodes - which will be found under 'Diodes'
Q1 is a transistor - which will be found under 'Transistors' > NPN Transistors
The symbol with an A in it is an Ammeter - which will be found under 'Test instruments' at the bottom of the options list.
VR1 is the potentiometer - which will be found under 'Resistors' - you will need to change it to 10k (the default is 100k).
MT is a motor - which will be found under 'Electromechanical'

Simulate the circuit and then start adjusting the potentiometer.
If you hover the arrow over the motor, you can see the rpm of the motor and the current flow.
Change the value of the potentiometer and see how it affects the speed and current flow through the motor.

Draw a graph to display how changing the value of the potentiometer changes the speed of the motor.
To learn more about the operation of the 555 timer, watch the video below:

It would be inconvenient to have to swap in and out resistors to alter the potential difference reaching our 555 chip. This is where a potentiometer comes in handy.
Imagine you had a length of resistive wire - as shown below.

But we can place a third contact point midway along the wire. This now divides it up into two resistors.

Now we can vary the size of the two resistors by moving the third contact point up and down the wire.

This is how a potentiometer works. A coil of wire is used and the third contact point is normally moved by rotating a dial.

Simulate the circuit and adjust the potentiometer to see how it effects the LED.

Adjust the values C1 and VR1 and write up an explanation as to how the potentiometer and the capacitor affect the LED output.