TeirmiLab - Raspberry Pi prototype

This article describes the building of a TeirmiLab prototype based on a Raspberry Pi B board.
The goal using a Raspberry Pi is to have a machine that can be expanded later.

Shopping list

• Raspberry Pi B or Raspberry Pi B+ or Raspberry Pi A+
• RGB 16x2 LED display
• DS1820 (specifically this one)
• MCP23017 I2C GPIO expander
• I2C RTC clock (optional)

The idea is to use a traditional LCD display instead a more sophisticate touch screen display.
Mainly the reasons :

• is not expensive
• doesn't require to develop a graphic interface for it
• allows to focus on the purpose - to show a temperature
• can be easily placed inside a protective container (no touch capabilities)

Hardware

Here a initial schematic for the TeirmiLab-Pi.

The RTC clock module is actually necessary only for the enhanced version, for the data-log feature, however it will be tested also on the base version.
The idea is to use as much as possible "ready to use" modules, like the RGB display, the GPIO I2C expander and so on, in order to use already made code (see the Software section).

The encoder will be connected directly to the Raspberry GPIO as well as the interrupt signal from the MCP23017 (optional for now), in order to be able to detect faster changes from the keyboard.
The internal pullup resistors for the encoder will be enabled.

First tests on a breadboard

The second prototype on perforated board

The second prototype on perforated board

The second prototype with the keyboard and RPOf cable connected

The keyboard is working fine, however it would be better to have an easiest way to select an alarm or set up the offset.
A new version of the hardware will include a digital encoder to be used instead the keyboard (see below).

The prototype installed on a wooden platform for a more mechanical stability.
A monochromatic LCD is used instead an RGB one

The prototype installed on a wooden platform for a more mechanical stability.
The round PCB is hosting the RPOf to be piggy-bagged on a MSP430 Launchpad

The used board for the prototype is a Raspberry Pi B, however the final installation will use a Raspberry Pi A+ because the reduced dimensions and power usage.
A new interface board is under development.

GPIO Use

Here a table for the GPIO use :

Raspberry GPIO	MCP GPIO	Direction	Description
	GPA0	Output	LCD
	GPA1	Output	LCD
	GPA2	Output	LCD
	GPA3	Output	LCD
	GPA4	Output	LCD
	GPA5	Output	LCD
	GPA6	Output	LCD
	GPA7	Output	LCD
	GPB0	Output	LCD
	GPB1
	GPB2
	GPB3
	GPB4	Input	Keyboard 1
	GPB5	Input	Keyboard 2
	GPB6	Input	Keyboard 3
	GPB7	Input	Keyboard 4
GPIO4		Input/Output	1Wire protocol
GPIO17		Input	Shutdown input
GPIO18		Output	Shutdown feedback
GPIO22		Output	Buzzer
GPIO23		Input	Encoder A
GPIO24		Input	Encoder B
GPIO25		Input	Encoder switch

Sensor

With the sensor used (DS18B20) the TeirmiLab has these basic characteristics:

• Range -55 to 125°C (-67°F to +257°F)
• ±0.5°C Accuracy from -10°C to +85°C

Keyboard

The TeirmiLab-Pi needs to have at least 3 push buttons in order to do some settings.

• a Mode button
  Allows to select different modes, like "display temperature" or "Set alarm" or "Set offset"
• two + and - buttons
  Allows to increase or decrease a value, like the alarm temperature or the offset

Encoder

Instead of the keyboard, it is more easy to use a mechanical digital encoder with an embedded pushbutton for the selection.
The pushbutton acts as Mode button and rotating the encoder cause the values to change.

The video is showing the encoder operations

Power Supply

The instrument must be powered.
An USB wall wart, with at least 1 A, will be the power source for the Teirmilab.

A main switch will be necessary in order to correctly power up and power down  the instrument (see RPOf project)

Container

All the electronic will be placed in a transparent plastic box.
This will allow to reduce the drilling to the minimum, basically for the main power switch and eventually for some push buttons if not other means are used.
The display will remain totally protected but visible behind the clear plastic.

Raspberry Pi - connecting a traditional LCD display (Update)

Raspberry Pi is a powerful enough platform and  it is possible to use many recent touchscreen with a nice graphic on it.
However often a traditional LCD is more useful, and surely is more cheap than a touchscreen solution.

Here one way to connect a traditional LCD and use it in projects, like the TeirmiLab one.
This article describes how to connect a traditional LCD but with RGB capabilities to the Raspberry Pi B.

We need :

• Raspberry Pi B or Raspberry Pi B+
• RGB 16x2 LED display
• MCP23017 I2C GPIO expander

It is possible to hook a display without the need of the GPIO expander, however a normal LCD uses at least 6 signals. 9 if the LCD display is an RGB one like the one used for this project.
Considering that the Raspberry Pi B doesn't have many GPIO available and that usually the speed required to handle the display is not critical, using a chip like the MCP23017 make sense.
In this way it will be enough to "hook" the  chip to the I2C bus, leaving all the Raspberry Pi GPIO free for other use.

Let see a schematic.

Software

There are many ways to control the hardware.
For now I found a ready-to-use Python library published by Adafruit that is working nicely.
Again, the important thing is that control an LCD is not a critical task, so a python script is Ok.
Maybe in future I'll explore the possibility to control it directly in C.

Python 

To use our display we need to install some code in our Raspberry.
Here a quick step-by-step guide (see Adafruit tutorials for a more detailed and easy instructions) :

1. sudo apt-get update
2. sudo apt-get install build-essential python-dev python-smbus python-pip
3. sudo apt-get install i2c-tools
4. sudo pip install RPi.GPIO
5. cd ~
6. git clone https://github.com/adafruit/Adafruit_Python_CharLCD.git
7. cd Adafruit_Python_CharLCD
8. sudo python setup.py install

Before to be able to use it, we need to be sure a configuration file is correctly set-up.

Execute : sudo nano /etc/modules

and if the file exists, add these two lines in the end.

i2c-bcm2708 

i2c-dev

Save the file then open another one in edit.
Execute : sudo nano /etc/modprobe.d/raspi-blacklist.conf

In this file comment out the two lines showed below, adding # at the beginning:

#blacklist spi-bcm2708

#blacklist i2c-bcm2708

Reboot Raspberry : sudo shutdown -r now

or : sudo reboot

At this point everything should be installed in order to use the display.

First is better to run an utility to see if the I2C GPIO expander is seen correctly.

sudo i2cdetect -y 1

The command should display a table and in the table should be present a 0x20 address.

pi@raspberrypi ~ $ sudo i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --                         
pi@raspberrypi ~ $ 

If so, the chip is correctly seen by Raspberry.

To test if the display is working you can execute a test script :

1. cd Adafruit_Python_CharLCD/examples
2. sudo ./char_lcd_mcp.py

Some test messages should be displayed on the LCD display.

Troubleshooting

It is possible to have some problems installing the libraries.
Here a couple of tips and resolutions.

apt-get missing

If you start from a fresh installation of Raspbian, especially a lite version, before to start to follow the instructions above, be sure to execute :

• sudo apt-get update
• sudo apt-get upgrade
• sudo apt-get update

It is important because updating the first time, include more repo on apt-get.

python build fail

If during the python building of the library you have this errors :

python ./setup.py build

Downloading https://pypi.python.org/packages/source/s/setuptools/setuptools-3.5.1.zip

Extracting in /tmp/tmpw6UbTx

Traceback (most recent call last):

  File "./setup.py", line 4, in

    use_setuptools()

  File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 128, in use_setuptools

    return _do_download(version, download_base, to_dir, download_delay)

  File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 108, in _do_download

    _build_egg(egg, archive, to_dir)

  File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 57, in _build_egg

    with archive_context(archive_filename):

  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__

    return self.gen.next()

  File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 88, in archive_context

    with get_zip_class()(filename) as archive:

  File "/usr/lib/python2.7/zipfile.py", line 770, in __init__

    self._RealGetContents()

  File "/usr/lib/python2.7/zipfile.py", line 813, in _RealGetContents

    raise BadZipfile, "File is not a zip file"

zipfile.BadZipfile: File is not a zip file

this happens because for some magic reason :) the python script screw up the download of a zipped file.

In my case the file was setuptools-3.5.1.zip.

Doing an ls -l showed up to be 122 byte big !! Definitively NOT a zip file.
The workaround was quite easy actually.

• remove the file : sudo rm setuptools-3.5.1.zip
• manually download the file :
  wget https://pypi.python.org/packages/source/s/setuptools/setuptools-3.5.1.zip 
• Re-execute the build : sudo python ./setup.py build

Examples

Be aware about the type of kit you are using.
On the examples area there are different examples depending the type of display in use.

To test the basic display, use : char_lcd_mcp.py
To test the kit with RGB display and pushbutton use : char_lcd_plate.py