DS18B20 Bluetooth Thermometer

One morning I woke up and wanted to know what the temperature outside was, and instead of running over to Home Depot and picking up a $2.00 glass thermometer, I decided to build my own wireless temperature sensor. At the heart of the board is a PIC12F675 microcontroller in an SO8 package. The right-hand side of the board houses the linear power supply (LP2950), bottom-center is the DS18B20 1-Wire temperature sensor, and out in left-field you can see the Sure TTL Bluetooth Module.

Being a big fan of standard protocols, I opted to transmit the temperature data over a Bluetooth SPP (Serial Port Protocol) link as opposed to XBee or another similar wireless architecture. Though the Bluetooth radio sucks a fair bit of power in comparison to an XBee module, the advantage is that not custom hardware is required on the receiving end. Since nearly all modern laptops have an integrated Bluetooth chipset, this effectively means that no hardware is required on the PC end.

After looking through all of my boxes of miscellaneous junk, I found a Sure Bluetooth Module I had bought several years ago for another project. For prototyping, the surface-mount module had already been attached to a home-etched PCB with header pins so that it could be plugged into a breadboard. Since I don't have access to a hot-air rework station, I ordered another GP-GC021 module for the final project. The module shown below was the one I originally started out with, whereas the photo above shows my most recently acquired module.

I decided to use a PIC12F675 for the main MCU for no reason other than the fact that I had one sitting in my parts bin. I was originally planning to exploit the PIC's internal 4MHz oscillator, however I found out later that since the serial module is fixed at 9600 baud, the main system clock had to be jacked up to 8MHz to keep pace and transmit data at the right speed.

As with all my projects, the schematics and PCB layout were done in Altium Designer. Over the years I have tried my hand at a few different EDA (Electronic Design Automation) suites including Eagle, ExpressPCB, and EdwinXP, however I find Altium stands head-and-shoulders above the rest of the crowd. If you are just a hobbyist, Altium is probably out of your price range, however their student licensing is fantastic; and many colleges and universities maintain large license sets or even site licenses. At Okanagan College we are very fortunate to have access to the colleges' floating license, which allows students to use Altium on any computer we want, regardless of whether or not we are on campus. I should make it clear that while I do not work for Altium, I have fought many epic battles with CAD software over the years; and while Altium and I have had our disagreements, overall it is a pleasure to work with.

Footnotes:
- The schematics, firmware, and all other related files are all included in the attached ZIP archive.
- Programming is not one of my strong suites, and as such the firmware is written in PICBasic. A ready-to-burn HEX file is in the ZIP archive, however if you want to modify the firmware at all you will need to have a copy of ProtonIDE in order to compile it.
- If you are planning on making a PCB from the included Gerber files, beware that the land size for F1 (the fuse on the sensor VCC bus) is actually too small (sorry).
- Everything here is presented as-is with no warranty whatsoever.


'****************************************************************

'*  Bluetooth Thermometer - R1 B23.BAS                          * 

'****************************************************************



' MCU Selection

Device = 12F675



' 8.000Mhz Crystal

XTAL = 8           



' Define Symbolic Pin References

Symbol DQ = GPIO.0  ' 1-Wire Net Pin

Symbol TX = GPIO.1  ' Bluetooth TX

Symbol RX = GPIO.2  ' Bluetooth RX



' Configuration Fuses: External Oscillator (High-Speed), Watchdog off, Code-Protect Off

Config HS_OSC, WDT_OFF, CP_OFF



' Define Variables

Dim temperature As Word

Dim C As Byte

Dim CPerD As Byte

Dim sample_count As Word 

Dim sample_delay As Word

Dim tx_pace As Word

Dim temp_dec As Byte

Dim sign As Byte



' Inserts a small delay between the transmission of each character to prevent data loss

tx_pace = 80



' Sample Rate

sample_delay = 10000





' Set all pins as digital

ALL_DIGITAL = true



' Set pin directions

Output TX

Input RX





' 20-Second Boot Delay

DelayMS 20000





' Display "POST" information

SerOut TX, 84, tx_pace, [13, 10, 10, 10, "Bluetooth Thermometer - Power Up Sucessful", 13, 10]

SerOut TX, 84, tx_pace, [13, 10, "Hardware Version: R3.0"]

SerOut TX, 84, tx_pace, [13, 10, "Firmware Version: R1 B23"]

SerOut TX, 84, tx_pace, [13, 10, "Sample Spacing (seconds): ", Dec sample_delay/1000, 13, 10]



' SerOut Data Format: SerOut (pin)(Baudmode (9600 8N1 Inverted))(Pace)[Data]





' Blank Space

SerOut TX, 84, tx_pace, [13,10,10,10]



' Wait two seconds

DelayMS 2000 



' Initialize Sample Counter to zero

sample_count = 0





' Print "START" to signal the begining of the data collection to the host

SerOut TX, 84, tx_pace, ["START", 13, 10,10]





' Main sampling/transmission loop

acquisition:



OWrite DQ, 1, fault, [$CC, $44] ' Send calculate temperature command

' If no 1-Wire sensor is found, jump to the "fault" function

' OWrite and ORead functions are quite comprehensive - See the PICBasic Library for details





Repeat

DelayMS 50 ' Wait for the conversion to complete

ORead DQ, 4, [C] ' Read the counter value from the DS18B20

Until C <> 0



OWrite DQ, 1, [$CC, $BE] ' Send "Read Scratchpad" command

ORead DQ, 2, [temperature.LowByte,temperature.HighByte, C, C, C, C, C, CPerD]



  

' Figure out if the value is positive or negative; flip the sign 

' Bits 8-15 are 1 for a negative temperature and 0 for a positive

If temperature.8=1 Then



     ' If negative then drop the first bit, and invert the value      

     temperature=(temperature.LowByte ^ $FF) >> 1 

        

     ' If Count = 0 then increment Temp

     If C = 0 Then temperature = temperature + 1    

     ' Change the sign to a negative

     sign = "-"                                                           

Else

     ' If positive then drop the first byte

     temperature = (temperature >> 1)   

     ' And change the sign value to positive               

     sign = "+"       

 

     ' Invert the decimal value                                                   

     temp_dec = 100 - temp_dec                             

EndIf



' Each Count is 1/16 of a degree, therefore 100/16 = 6.25, and that's our decimal value (of the temperature)

temp_dec = (6.25 * C)  





' Increment the sample counter

sample_count = (sample_count + 1) 





' Transmit the data to the host over the Bluetooth Link

' Transmitting "" seems to prevent data loss

SerOut TX, 84, tx_pace, [""]



' Format: sample count, comma, sign, temperature whole, decimal place, temperature fraction, "c"

SerOut TX, 84, tx_pace, [Dec sample_count, ", ", sign, Dec temperature, ".", Dec Dig temp_dec, 1, 0, "c", 13, 10]





' Wait for the specified amount of time before sending the next sample

DelayMS sample_delay



' Restart the sampling acquisition process

GoTo acquisition

 

 

' If the 1-Wire sensor is not found, the first OWRITE command will fail and jump here to indicate the fault  

fault:

SerOut TX, 84, tx_pace, [13, 10, "ERROR: 1-Wire Network Fault Detected - Check Sensor Wiring", 13, 10]

DelayMS 1000

' Try again - If the problem is fixed return to normal operation

GoTo acquisition

DS18B20 Bluetooth Thermometer

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