Simple & Cheap Thermometer

This example shows how to rapidly build a simple and cheap thermometer with a PIC, and to experiment in on an EasyPic2 or EasyPic3 board.
You will use :

• the PIC analog input and its ADC
• the PIC timer 0
• a 4 digits 7 segments LED display
• 3 buttons
• a simple silicon diode

Did you ever have to control a fan speed rotation ?
Did you ever need to have an idea about the air temperature in your fridge ? in your PC case ?  in your basement ? in your attic ?
Did you ever need to automatically shut down a device, to prevent it from overheating ? or to power it up, in order to pre-heat it ?
Is a 2°C accuracy good enough for your project ?
You don't mind to calibrate it once ?
You don't want to spend too much for it ?
You want to build it right now ?
GOOD ! if you have a PIC with an internal ADC, and a diode in your drawers, you are ready !

 1. Select a temperature sensor

Temperature sensors are widely used, here are most common families :

Family	Output	Accuracy	Integration	Cost
NTC & PTC Resistors (Thermistors)	ANALOG	GOOD	Easy	Low
Thermocouples	ANALOG	HIGH	Hard	High
Resistance temperature detectors (RTDs)	ANALOG	HIGH	Hard	High
IR Thermometers	ANALOG	HIGH	Hard	High
ICs	DIGITAL	HIGH	Easy	Medium
Bimetal	BINARY	LOW	Very Easy	Low
Diodes	ANALOG	GOOD	Easy	Very Low

You will learn that any silicon diode, and widely any silicon junction have an almost-linear temperature coefficient of about 2.3 mV/°C when a forward voltage is supplied across the junction.
I suggest to use a diode : you can't find cheaper sensor, and you surely have one. Any 1N4148, 1N4004 will do the job.

2. Build the circuit 

All that you have to do, is to insert the diode in the TS1 socket of your Easypic.
Plug the cathode of the diode into the upper hole of the socket, and the anode into the middle hole. The 10k pull-up resistor will bias the diode with a ~0.5mA current, almost what we need (1mA would be better).
This should look like this :




Your sensor is in place !




Then set up your EP3 board as follow :

• put a pic16f877a in the dip40 socket
• put a 8 Mhz crystal in the oscillator socket
• set JP14 jumper to select RE2 pin
• set SW2 switches 5 to 8 to ON, to enable 7segment LED display
• set JP12 to Down, to pull-down PORTD
• set JP20 to Up, to pull-up keyboard

3. Write a program

Or better : use this one (thermosensor.c) ! I wrote it specially for this purpose.

This C source code will allow you to experiment your new temperature sensor. You can compile it with the MikroC compiler, it fits within the 2k demo limit (in fact less than 800 bytes of ROM), so that you don't need a licence key to build the binary file.
If you don't have already MikroC compiler for PIC on your PC, then download it now for free :
http://www.mikroe.com/eng/downloads/get/29/mikroc_pro_pic_2011_v460_setup.zip
Open a new projet, select target device pic16f877a (or any 40 pins pic device), copy/paste the source code, build the binary file and write it into your pic.

4. Run the program

First, you will have to select either Fahrenheit or Celcius display for the temperature. Press the RD7 button to toggle between °F or °C.
Then you will have to calibrate your diode. The forward voltage of a silicon junction is around 0.6V at 25°C, but it may vary a lot from a diode to another.
You will have to know the temperature of your room : you will have to press RD0 to increase the temperature displayed, or RD1 to decrease it, until it is the same than your reference thermometer.
You will notice that temperature will jump by steps of around 2°C : that's normal ! Don't forget that the diode has a 2.3mV/°C temperature coefficient, whereas one bit of the pic ADV is 5000/1024 = 4.88mV ! Without extra-component, like an operational amplifier, we only have a 2°C accuracy. That's the price of the extreme simplicity of our circuit.
We suppose that all silicon junctions have the same temperature coefficient, and we don't need to calibrate or adjust it. If you think that it is not enough good for your project, then refer to the datasheet of your diode and apply the new coefficient in the program.

5. Try other silicon sensors

You can try with a NPN transistor : just tie its base and collector together to make it work in diode mode.




Here is a 2N2222 NPN transistor in diode mode as temperature sensor, you will have to re-calibrate the display with RD0 and RD1 buttons.
It works much more slowly than the diode, because of its metal case.



You can try with a MTS102 temperature sensor : it is nothing else but a transistor !




Here is a MTS102 as temperature sensor, don't forget to tie also its base and collector (it is not already internally done).