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The 555 Fan

1 Using a potentiometer

Learn It

  • 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.

Try It

  • 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.


  • Press play again, and see how the potential differences have changed.

Badge It - Silver

  • 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

  • Can you see a pattern between resistor values and potential difference?
  • Click on this link to complete the quiz which will badge your work.
  • You will also need to upload you screenshots of the circuits.

Learn It

  • 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.

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

  • If we want to find Vtop then we can just do the following


Try It

  • 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 motorĀ - 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.


Badge It - Gold

  • 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:

Learn It

  • 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.


  • We can consider this wire to be the equivalent of a single resistor.


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


  • This then makes it the equivalent to 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.


Build It

  • Let's place a potentiometer into a circuit.


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

Badge It - Platinum

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