In this tutorial I’ll be explaining how to make a Thermometer using a microcontroller (RPi3B) and a thermistor without using an ADC.
I bet you have seen millions of tutorials online on how to make a thermometer using a microcontroller, thermistor, and ADC. Some microcontrollers even have ADCs integrated on their SOC. Well, here you can learn how to make a thermometer just using a microcontroller and basic circuit components.
For this project you will need the following:
- Microcontroller (I’ll be using a Raspberry Pi 3 Model B)
- Thermistor
- 2*Resistor ~220ohm
- NPN transistor (PN2222a)
- Capacitator ~100uF
- Bread Board with at least 6 lanes
- Jumper wires (like 4)
First, I’ll describe how it works. Then I’ll show you how to set up the hardware. (The boring part) I will describe the maths needed to make it work. Finally, I will explain the program to make this work.
How it works
The special element here is the capacitator. We know the capacitator blocks voltage across it as it charges and the charging speed depends on the initial voltage across it. The thermistor resistance decreases as its temperature increases. So if the initial voltage increases the capacitator takes a shorter time to charge, this is considering the capacitator and the thermistor are connected in series. This is how we will find out the temperature by measuring the difference of time needed to charge the capacitator.
The raspberry pi has a couple of GPIO pins that output 5V DC power we will use those to power the circuit. Like most digital connections LOW voltage is considered to between 0V and 3.3V, so we will measure the time it takes the capacitator to drop the voltage to 3.3V.So the time it takes for the voltage to go below 3.3V will vary with temperature. Below is a proportionality table of what happens when Temperature changes, the independent variable.
| ΔT | ΔRt | ΔiVc | ΔTtC |
|---|---|---|---|
| + | – | + | – |
| – | + | – | + |
(where: ΔT is the change in temperature, ΔRt is the change in resistance of the thermistor, ΔiVc is the change of initial voltage across the capacitator, and ΔTtC is the change of the time taken to reach a 3.3V voltage)
How to set it up
