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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.


  • 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

  • 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.


  • 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.

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.