Sensing project

Project brief

A new greenhouse is being developed for gardeners who are concerned about overheating plants in their current greenhouses.

The new greenhouse has been built with opening windows but occasionally the windows of the greenhouse slips fully open and allow all of the trapped heat in the greenhouse to escape quickly.  This is a major concern for serious gardeners.  The test is to develop a sensing circuit that will sense whether the windows are fully open, so a gardener can be alerted of the problem.  


To begin we must know what a sensor is.  A sensor is an electrical component, which produces a signal in response to alteration in its surroundings, this maybe an alteration in a physical variable or by movement of objects.

The sensor planned for the sensing circuit ensuring the windows are not fully open is a rotary potentiometer.

A rotary potentiometer is a component, which has three terminals.  When the angle of rotation is changed on the component’s arm then the potential difference (often called voltage) is also changed.  Once the voltage reaches a specific point a separate circuit could activate an alarm showing that the window has slipped and is fully opened.  The potential difference is the difference between potential energy between two separate points.

Alternatively a different sensing circuit could be used.  A circuit with a thermistor (a component sensing change in temperature) could be used.

In normal situations when the window would be fully closed the temperature in the greenhouse would be high so the output voltage from this circuit would be low.  However if the windows were to slip open fully then the temperature inside the greenhouse would decrease so the output voltage would increase.  The circuit could be linked to an alarm of some type so as when the voltage increases past a certain point when the window is fully open, the alarm rings alerting the gardener to shut the windows again.

A circuit containing a LDR (light dependant resistor) could also be constructed and applied to the outside or inside of the greenhouse.  In this instance the light dependant resistor would have to be very high resolution, as the light intensity would not vary great amounts over the hours of sunlight.

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I think that the rotary potentiometer sensing circuit will be better for this project as temperature will vary a great degree throughout the day and if the circuit were too sensitive the sensor would keep activating.  The rotary potentiometer senses the occasions when the window slips so does not need to be as sensitive so will no activate continually.


  • 12V power supply
  • Wires
  • Rotary potentiometer
  • Voltmeter
  • Fixed resistor
  • Crocodile clips
  • Protractor
  • Pointing device (Lego)

Circuit diagrams

Method and set-up

This circuit is first constructed carefully ...

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This is an excellent piece of work and is five stars rating. Excellent introduction explains all options of investigating this problem and explains the choice of the rotary potentiometer clearly. Good predictions made and justified in detail. Good methods, diagrams and risks studied. Excellent results, calculations and conclusions. Work is presented in a very logical order and shows a good scientific understanding.