Friday, January 23, 2015

GPIO Interrupts using WiringPi

My weather station has two devices that send simple pulses to the Raspberry Pi. The rain gauge pulses each time its "bucket" fills and the wind speed gauge pulses for each rotation. The best way to handle this is by using interrupts.

An interrupt is a signal to the computer to stop what it is doing and do something else.  These are used extensively by the operating system, but an application can use them as well.  If you have a GPIO input that needs to be responded to, you could poll the GPIO pin in a loop waiting for it to change, but this can be unreliable for very brief state changes. It is also wasteful of the CPU.  The better way is to assign an interrupt that will be activated when the GPIO pin changes state.

I code all my projects in C and use Gordon's WiringPi API.  This library makes it much easier to use GPIO.  Interrupts are now supported using the wiringPiISR function.  (The waitForInterrupt function is deprecated and should not be used.) Using interrupts takes only three steps:

  • Create a function to be called when the GPIO pin changes state. This must be defined like this: void myInterrupt(void)
  • Initialize the WiringPi library by calling wiringPiSetup
  • Call wiringPiISR to configure the interrupt

You can choose to have your interrupt called when the pin goes low, goes high, or both (INT_EDGE_FALLING, INT_EDGE_RISING, INT_EDGE_BOTH)

My interrupts simply increment a global variable. The value of this variable is then checked periodically (typically once per second) and the count is converted into the appropriate units for the device.

An example of how to use an interrupt is shown below.

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <wiringPi.h>

// Use GPIO Pin 17, which is Pin 0 for wiringPi library

#define BUTTON_PIN 0

// the event counter 
volatile int eventCounter = 0;

// -------------------------------------------------------------------------

void myInterrupt(void) {

// -------------------------------------------------------------------------

int main(void) {
  // sets up the wiringPi library
  if (wiringPiSetup () < 0) {
      fprintf (stderr, "Unable to setup wiringPi: %s\n", strerror (errno));
      return 1;

  // set Pin 17/0 generate an interrupt on high-to-low transitions
  // and attach myInterrupt() to the interrupt
  if ( wiringPiISR (BUTTON_PIN, INT_EDGE_FALLING, &myInterrupt) < 0 ) {
      fprintf (stderr, "Unable to setup ISR: %s\n", strerror (errno));
      return 1;

  // display counter value every second.
  while ( 1 ) {
    printf( "%d\n", eventCounter );
    eventCounter = 0;
    delay( 1000 ); // wait 1 second

  return 0;

All of the code from my weather station can be found at

You can also get it using git:
git clone

Sunday, December 28, 2014

Raspberry Pi Weather Station

My latest Pi project involves creating a weather station in a way very different from most others. I will cover various parts of this project in my next several posts.
The weather station completed and mounted outdoors.
The  most common way that a Raspberry Pi (or any other computer) is interfaced is via a pre-built system that has a serial or USB connection. This is the fastest and easiest way to do this. But what fun is doing things the way everyone else does?

I had an old weather station that was relatively cheap (less than $100) and did not have any drivers for Linux. I ran it on an old PC for several years, but it finally stopped working. The wireless part was never very reliable and it was a pain to keep replacing batteries. I was able to salvage the anemometer (wind speed) and rain gauge and hack them into my new system.

For temperature and humidity readings I use the AM2315 from adafruit. For barometric pressure I use the MPL115A2, also from adafruit. Both of the devices interface via the I2C bus. I have to give a plug for adafruit. It's a great place to get parts to connect to the Raspberry Pi.

The interface board and Pi on a mounting plate.
The anemometer and rain gauge are simple contact closure interfaces. Each tiny bucket of rain and each rotation of the anemometer produce a single pulse. These are connected simply through GPIO pins and drive interrupts which count the pulses.

The interface board that connects to the Pi is actually very simple. It mostly just passes GPIO pins to screw terminals. The MPL115A2 is the tiny blue board next to the ribbon connector.

Adafruit also provided the perfect weatherproof enclosure. I will have to drill holes to feed wires in, but those will be sealed with silicone.

In the near future I will write up posts on some of the problems I had to overcome to complete this project.
  • Handling GPIO interrupts
  • Interfacing to the AM2315 via I2C
  • Interfacing to the MPL115A2 via I2C
  • Calibrating the rain and wind sensors
  • Logging to my MySQL database server
  • Logging to Weather Underground
  • Providing a nice web interface to display the weather data
Also, I will post my source code for others to use and/or learn from.

This has been a very fun project and I expect to use the weather station for many years to come.

Of course, Murphy had to strike - when I went to mount the system outdoors, it was raining!

Sunday, December 14, 2014

My New System Checklist

I recently had need to create a new Raspbian system for a project and decided to record all the things I did after the system image first boots. Frequently, I forget one or two of these, so this will become a checklist that I follow when I create a new system.

There are many guides to creating a system for the Raspberry Pi and this post is not an attempt to create another. I am putting this here for my own reference as much as to share.  To make it helpful to beginners, I have added some explanations.  You may prefer nano over vi as the file editor.

Please feel free to use the comments section to let people know what customization you like to make for your Pi systems.

Load image as usual
raspi-config runs first time
- expand file system
- Internationalization -> set locale -> TZ = US-Eastern
- Internationalization -> Keyboard = English(US)
- advanced -> hostname (RasPi-##-Purpose)
- advanced -> mem split 16 for GPU
- advanced -> enable SPI and I2C and Serial
reboot, and log in as pi (raspberry)

CPU overclocking would also be set up in raspi-config, but I haven't had any need to do this.

All of the following commands require root privilege.  You can either put sudo before each command or enter sudo -i and run a shell as root.

Create a new user for myself, give it sudoer privilege.
adduser ted 
echo "ted ALL=(ALL) NOPASSWD: ALL" >>/etc/sudoers

Update the package database and upgrade all installed packages.
apt-get update 
apt-get upgrade

Install some new packages.
apt-get install samba screen libmysqlclient-dev libi2c-dev

Configure Samba (Windows file sharing)
vi /etc/samba/smb.conf
uncomment "socket options = TCP_NODELAY"
delete all shares and add:
   comment = opt
   writable = yes
   locking = no
   path = /opt
   public = yes

Restart the Samba service
service samba restart  

Edit the SSH server config.  Turning off DNS reverse lookups will speed up the connection process when to log in through SSH.
vi /etc/ssh/sshd_config
add "UseDNS no"

Edit the netwrok configuration and set static IP address and wifi config.  The interface name for the wifi will be used below in the supplicant file.
vi /etc/network/interfaces
iface eth0 inet static
iface home inet static 

Edit the wifi supplicant file.  The "id_str" setting connects back to the name used above.
vi /etc/wpa_supplicant/wpa_supplicant.conf

Install Gordon's WiringPi library.  I use this extensively in my C programming.
cd ~
git clone git://
cd wiringPi
git pull origin
gpio -v
gpio readall

Edit the kernel module configuration to enable SPI, I2C, and 1-Wire.
vi /etc/modprobe.d/raspi-blacklist.conf
uncomment SPI and I2C devices

vi /etc/modules
add this
# SPI devices  
# I2C devices  
# 1-Wire devices  
# 1-Wire thermometer devices  

Finally, reboot the system again.  Then log on as the new user you created and  remove the default user.
userdel pi

If you don't do this last step and your system is accessible from the internet, then it will not be long (sometimes only hours or minutes) before a hacker finds it and does bad things.  My firewall log shows constant attempts to brute force a login via SSH and "pi" is a common user name that is tried.

Tuesday, December 9, 2014

Wifi Router Case Mod

In my previous post I showed how to use the case from an old LinkSys router as a case for a Raspberry Pi.  Today I decided that it needed a little improvement.  Bring on the blinking lights.  Isn't everything better with blinking lights?

I showed in the posts Server Box with Utilization Displays and CPU and I/O Utilization Display - Details how to use LEDs for a utilization display.  This project is a little smaller scale - only six LEDs instead of twenty.

The circuit is very simple.  The positive lead to each LED is connected to a resistor.  All of the resistors are connected to 5V.  The negative lead of each LED is connected to a GPIO pin.  A low signal on the GPIO pin turns the LED on.

This inverts the logic, but that can be handled in software.  This has the benefit of being able to push more current through the LEDs than would be possible if the GPIO line was connected to the positive lead of the LED.

This PCB layout shows a close approximation of how I made the circuit.  I didn't make a printed circuit board, but I have been designing some for work this week, so I did this drawing using PCB Artist.  I definitely plan to create some PCBs for my Pi hobby, once I decide what to make next.

All the wires are connected directly to a ribbon cable. I created a real power plug while I was at it.

The LED circuit is pushed into the front part of the case and through some holes that I drilled.  It is held in place by friction and a little tape.

My latest Raspberry Pi creation is stacked with the modem and router with all their blinking lights.

Now maybe it will not feel inadequate.

Sunday, December 7, 2014

Raspberry Pi Case From Wifi Router

You have most likely seen one of these somewhere before.

This case style is very common.  If you happen to get your hands on one, you can make a great case for the Raspberry Pi.

(Or, you can re-load the router firmware.  See for more information.)

I had two in my huge pile of junk, so I though I'd have some fun. This was literally a ten minute project.

The case pops apart easily, if you know the trick.  Grab the blue front part and pull apart from the rear part.  After that, the top and bottom will come apart. A couple of screws later and you have prime case material.

I drilled two holes to mount the Pi and trimmed the rear opening just a little. It needs a better power connector.

One good thing about these cases, they stack very nicely.

Now to search the pile of junk for a hard drive. There is enough room to add one in this case.

Works Great!

Sunday, November 16, 2014

Debouncing GPIO Input

Reading the state of a GPIO pin is very simple.  There are several methods to do this.  But what if you need to detect a very short pulse on a GPIO line or you need to respond very quickly to a state change without using a lot of the CPU to poll the state of the pin? This is exactly what interrupts are designed for.  I will cover how to use interrupts in a future post.  This post will show you how to deal with a common problem that must be addressed before interrupts will work correctly.

When the contacts of a mechanical switch close, they will almost always bounce a little.  This causes multiple spikes up and down on the connected line until the contact is firmly made.  The result is a series of pulses when a single state change was expected. This is, for obvious reasons, called "bouncing" and must be corrected either with hardware or software.

If that wasn't clear enough, maybe a picture will help. Imagine a push-button connected to a GPIO line. When the button is not pressed, a pull-up resistor holds the GPIO at 3.3 volts so that it reads a value of 1.  When the button is pressed, the line is connected to ground and the GPIO will read 0.  But on closer inspection, something else really happens. As this graph shows, the bouncing of the voltage level causes spurious state changes to the GPIO input. If you use an interrupt to increment a counter when the state changes on the GPIO, then a single button press can result in the counter incrementing two, three, or more times when the programmer is expecting only a single increment.

This can be dealt with in software, but that method tends to be unreliable in many instances. It is much better to correct for the bouncing using a simple hardware circuit.  This is called debouncing and one simple solution is shown below.

By connecting a capacitor as shown here, the result is a "smoothing" of the voltage curve. Before the button is pressed, the capacitor is fully charged and the GPIO will read 1. When the button is pressed, the capacitor will start draining through R2 at a rate determined by the size of the C1 and the R2.  The voltage on the GPIO pin will smoothly curve down from 3.3V to 0V and the GPIO will sense a single state change.

The values for C1 and R2 will determine how quickly the state changes.  If they are too small, then there could still be some bouncing.  If they are too large, then the circuit may respond too slowly for the application needed.  This could result in state changes being missed. You may need to experiment to find the proper values for a specific application. The values shown in this circuit are a good place to start.

A digital circuit that is also commonly used for debouncing is called the set/reset latch and can be constructed from two nand gates. To learn more about this option and everything else you might want to know about debouncing, read this paper.   A Guide to Debouncing

Now that you input is properly debounced, you can go on to using it to reliably drive an interrupt.

Wednesday, October 29, 2014

Improved Home Brewing Controller

This post is an update to my Automated Home Brewing post. See that post for more details on the project. After adding some new features haphazardly, I had an ugly conglomeration of old and new parts. It worked, but it wasn't pretty, and I sure didn't want to show it on this blog. So, I decided to build a whole new interface and include the new items:
  • thermocouple interface for flame detection
  • relay control for refrigerator
  • relay control for heater (these two are used for controlling lagering temperature during fermentation)
  • digital input for counting the bubble rate in the airlock during fermentation.
  • battery level monitor, since I usually run this on a small 12V battery
I also kept all the original controls:
  • relay for propane valve
  • relay for igniter
  • 1-wire bus for multiple temperature inputs
  • relay for circulation pump
The item that I struggled with the most was how to make a reliable flame sensor. This is covered in the Flame Sensor Update post. Note - the drawing for the amplifier circuit in that post has the resistor values reversed. Here is the corrected drawing.

The gain formula for a LM358 non-inverting circuit is

               gain = 1 + R2/R1

In this case, R1 = 1K and R2 = 200K, resulting in a gain of 201. This means that a thermocouple output of 5mv will produce an output of just over 1V. Using a gain of 500 would allow for better resolution of the temperature, but 200 is adequate for this purpose. I don't care about the actual temperature. I just need to see if the value is increasing to verify that the flame actually lighted.

I decided to build the interface on a circuit board that would plug directly into the GPIO pins on the Raspberry Pi and mount it with a Pi permanently in a project box. Here is the completed circuit board, in its enclosure, installed on the Raspberry Pi

Here is the project box with the Pi mounted inside. There are holes cut for the USB, network, and HDMI connections. I like this arrangement a lot and will use it again in the future.

Finally, this is the completed system with all the connectors labelled.

The "bubble detector" input was a last minute addition. I haven't actually built the detector yet but the design is straight forward. It will use an infrared LED and detector (sometimes called an IR gate) to see when a section of the airlock changes from water to air. Water is mostly opaque to IR light.

The pulses from the bubble detector will be very short and difficult to accurately count using a simple polling loop. I expect that it will need to be done using GPIO interrupts. I will post the code when I get that working.

Monitoring the "bubble rate" won't really produce any useful numeric data. It is only good for determining when the fermentation has completed. This is important since fermentation can stop early for various reasons and steps must be taken to correct the problem.

Finally, a warning: Working with flammable gas and boiling liquids can be hazardous. I take no responsibility for your use of the advice provided in this post.
BE CAREFUL!  And drink responsibly.