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Using a transistor as a switch can be very useful in many sensing circuits. By combining our knowledge of potential divider circuits and the use of transistor as a switch, useful circuits can be produced.

Task 1: The Transistor in sensing circuits

Sensing circuit

  1. Open Yenka file Model 1.
  2. Move your mouse over the ammeters labelled 1 and 2. Write down the values of IBE and ICE below.
    IBE = 550 μA, ICE = 55 mA.
    IBE has been amplified to a larger ICE.
  3. Move your mouse over the torch at the LDR. Adjust the LDR so it gets brighter. Do the values of IBE and ICE change? Describe what happens to the bulb.
    IBE = 5.65 μA, ICE = 565 μA.
    IBE is larger than ICE. Bulb turns off.
  4. Why do you think this happens?
    The potential divider creates less than the 0.7 V needed to switch the transistor.


The transistor is used to amplify the base current to a larger collector current to turn on the bulb.

Task 2 : Using a variable resistor

Transistor Circuit

  1. Reload the Yenka Model 1 using F5.
  2. Delete the 5 kΩ fixed resistor and replace it with a variable resistor from Electronic components > Inputs > Sensors. Change the value to 10 kΩ and adjust the slider to its maximum. Write down the values of IBE and ICE.
    IBE = 826 μA; ICE = 82.6 mA
  3. Adjust the torch from darkness to brightness. Can you make any observation about when the bulb 'turns off'?
    The bulb turns off very quickly. Only just starting to get brighter (8000 Ω).
  4. Adjust the variable resistor until it reaches 5kW. Can you make any observation about when the bulb 'turns off'?
    It needs to get much brighter until the bulb turns off (2400 Ω).


A variable resistor is used to adjust the sensitivity of the circuit.

Teacher Summary

  • As an introduction to this activity, the teacher may like to revise potential divider circuits and give an explanation as to the working of a variable resistor.