Gardeners in Britain use greenhouses so that they can grow their plants in a warmer environment than that outside the greenhouse. This is to try to protect the plants from frost and animals, whilst the seeds germinate and grow, and plants are stored. After this period, plants can be put into the garden. There can be problems with greenhouses though. One example of a problem would be in the height of summer when an unventilated greenhouse can reach internal temperatures that start to kill the plants.
Another problem can arise in late autumn, winter and early spring when the internal temperature of the greenhouse will decrease, also causing the photosynthesis reaction undertaken by plants to slow down, and even stop causing plants to die. I have been asked by a major British plant growing company to design a sensor that would be able to measure when the temperature is too cold inside the greenhouse for plants to survive and also when it becomes too hot for plants to survive. The specification that they have given me is as follows: * The temperature sensor should be able to detect when the temperature drops bellow 20i??
C and when the temperature rises above 30i?? C. The optimum temperature for growing plants is about 25i?? C. The company have a series of heaters in the greenhouses and underneath the soil, which could be used to raise the temperature within the greenhouses, when the growing conditions become too cold. The greenhouses that the company use also have windows that could be opened when the temperature inside the greenhouse becomes too hot. A thermocouple uses two pieces of dissimilar metals that have a contact potential between them, and this contact potential changes as the temperature changes.
The problem with using a thermocouple is that the output voltage is typically a few micro volts per i?? C. Normally, the output of thermocouples is amplified using an operational amplifier (op-amp) so that the output voltage is of a more useful magnitude. An application that a thermistor could be used for is to help the temperature compensation of a full Wheatstone bridge. The output voltage of a Wheatstone bridge is so small though, that it too needs amplifying. Due to the need for amplification, I shall not use a thermocouple or Wheatstone bridge to detect the temperature although they are both viable solutions.
This is because there is more to go wrong in a circuit containing an amplifier, than one without. Resistance Temperature Detectors (RTDs) are usually wire wound components, but can be made from a thin film as well. They work on the principal that when wire becomes hotter, its resistance increases. I could use a RTD in my sensor, but I do not have easy access to them so it is unlikely that I will use one Instead, I shall be using a negative temperature coefficient (NTC) thermistor. The name thermistor is combined from the phrase thermally sensitive resistor.
The resistance of an NTC thermistor decreases as the temperature increases, in a disproportional manner. I will use a thermistor because thermistors are reliable and cheap. They also only require a small amount of circuitry. Potential dividers Potential dividers, as the name suggests, divides the potential in a circuit. This is done using two or more resistors in series and works upon the principal that the current across both of the resistors is the same due to Kirchhoff’s law, so the potential difference across a component must be directly proportional to the resistance of the component.