Lab 4 - Sensors, Semiconductors


  • Potentiometers, photoresistors, variable resistance.
  • Transistors


  • TIP-120 transistor.
  • Bulb.
  • 10K potentiometer.
  • 1K and 10K fixed resistors.
  • Pliers, strippers, clippers.
  • Hookup wire.
  • Jumper kit (short pre-cut wires).
  • Breadboard.
  • Power Supply.
  • Multimeter.


Part 1 - Potentiometer

  • Locate your 10K potentiometer. It should say "10K" on the front of it.
    • What is the measure between "A" and "B"? does it change as your turn the knob?
    • Record and plot the rotation of the knob and resistance at a few points. Is the relationship between them linear? ("linear" or "expontential" taper)

Potentiometer labelled.jpgVariable resistance.png

  • Wire up a simple voltage divider using your potentiometer and a 10K resistor, conected to the 5V supply.
    • Observe how the output voltage changes as you turn the knob.(measure at the point between the potentiometer and the resistor) What is the highest voltage you can get? What is the lowest?
    • Replace the 10K resistor with a 1K resistor (as shown in the second diagram). Observe the voltage as you turn the knob, it should be different than before. What is the new lowest voltage you can get? (HINT: it should be much closer to 0V).

Pot divider 10k.png Pot divider 1K.png

Part 2 - Light Sensor

  • Photoresistor as variable resistance
    • measure the "light" resistance of your photoresistor
    • measure the "dark" resistance. (cover the resistor with your hand, or try shutting it in one of the lab cabinets)

Photo resistor Symbol.png Photoresistor 2.jpg

  • Set up a voltage divider with the photoresistor and a 10K resistor. You can take the circuit from the end of Part 1 and replace the potentiometer with the photoresistor. What range of output voltages can you get by covering or uncovering the photoelement?

Photoresistor v out.png

Part 3 - Transistor Driver

  • Get out your transistor. It should say "TIP 120" on the front of it.

Tip120-TO-220-3.jpg Tip120 pins.png Transistor EBC.png

  • Connect a transistor to control the brightness of a bulb by turning the knob of a potentiometer.

Transistor pot bulb driver.png

  • Replace the potentiometer with the photoresistor. You should be able to control the brightness with your hand/shadow now.

Transistor photo bulb driver.png

  • Substituting a photoresistor in place of the potentiometer, make a light "phobic" and light "philic" circuit, as in the diagrams below.

Positive feedback.png Negative feedback.png