Homework Answers

Here are the answers to the cover work questions.  If you don't understand anything, scroll down the blog to get more information, or come to ask me at lunchtime or after school.

First of all the simulation task you should have completed in class:

The input of the system is made using a voltage divider with a thermistor and a variable resistor.  Because this is a heat sensor the thermistor must go at the top because as the temperature increases the resistance decreases. As the resistance decreases, less voltage is dropped over the thermistor and therefore a larger share of the supply voltage is dropped over the variable resistor and therefore the signal voltage/voltage out of the voltage divider increases. The base resistor protects the transistor by limiting the current. When the voltage between the base and the emitter of the transistor (VBE) reaches 0.7V the transistor saturates, allowing current to flow from the collector to the emitter and therefore through the lamp, lighting it. The variable resistor is used to adjust sensitivity. Its resistance value could be changed to make the lamp light at different temperatures.  In this case if was adjusted to 1.8k to ensure the lamp would be on above 25.  This could be increased to make the lamp come on at a lower temperature, or decreased to make the lamp light at a higher temperature.

The answers to the theory questions are below: