Sensor Project

I have decided to measure the temperature of a greenhouse using a thermistor in a potential divider circuit, and then when the temperature gets too cold, the relay will activate and turn on a heater. A thermistor is a temperature sensor in which its resistance will increase or decrease with temperature change depending on what type of resistor.

An alternative for this experiment could have been measuring light levels in a greenhouse using an LDR. A light sensor in a green house could tell you when and where in the greenhouse the most intense sunlight is, which you could you use to aid your growing skills.

I decided to design a temperature sensor for a greenhouse using a thermistor, as temperature can be critical in growing plants especially if it gets too hot or too cold.

As my thermistor is an NTC this means that as temperature decreases its resistance increases, and when temperature increases its resistance decreases.

Plan:

Circuit –

Explanation –

The water is boiled until it reaches a temperature of just above 90° Celsius.

1.93 (R1+124.8) = 124.8 x 5.0

1.93 (R1) + 240.864 = 624

1.93 (R1) = 624 – 240.864

1.93 (R1) = 383.136

R1 =  = 198.516Ω

Temperature-Voltage Graph

Safety –

The issue of safety is very important in any experiment. In this experiment there is no really serious risk. However even minor concerns cannot be overlooked.

The major safety points considered include

• Avoiding the use of wet hands to handle electrical apparatus so as to reduce/eliminate the risk of electric shocks.
• Being careful and conscientious with the hot water as hot water is potentially dangerous.
• Ensuring proper electrical wiring (avoid using faulty equipment, use insulated wires etc.) so as to reduce/eliminate the risk of electrical injury and damage to electrical equipment.

Apart from these few points there is nothing serious to be considered.

Improvements –

My sensor could be improved in a number of ways. Firstly, I could have measured the change in resistance with temperature and not voltage. This is because using resistance as my variable would have given me a much clearer linear logarithmic graph making deducing values much easier and reliable.

Also, most textbooks relate the change in resistance of thermistors with temperature and not the change in voltage that occurs as a consequence of the change in resistance. Therefore, I would have had more formulae at my disposal.

References –

Advancing Physics AS textbook

Advancing Physics website

Student Notes