Saturday, July 4, 2015

New Utilization Display

In my last post I showed the new case for my MySQL server. The original case had two 10-segment LED bar graph displays that showed CPU and disk utilization. Now I have added a similar display onto the new case.

I chose to only display CPU usage but after I do some testing, I may change it to show disk I/O instead. This is a simple change to the configuration file of the utilization app. One issue with showing CPU is that the Pi 2 has four CPUs and typically only one will be maxed out. In the image above two of the CPUs are maxed out so the utilization shows 50 percent. The most I normally see is 25 percent since I seldom have more that one client querying the database.

The hardware used is very straightforward. I used a MCP23017 GPIO extender with ten of its pins wired directly to the LEDs. This chip uses an I2C bus connection so it only requires four pins on the Pi - power, ground, SDA, and SCL.

A couple of notes on the build:

1 - a resistor network would have been much neater than 10 separate resistors. I didn't have one handy but I do have a huge amount of single resistors.

2 - don't use ribbon cable. Just don't. It looks nice but it is normally used with press on connectors and is not intended for soldering. As soon as you touch the soldering iron to the wire an inch of the insulation melts away. Even with my super nice soldering iron turned to the minimum setting it was a pain to use. I won't make that mistake again.

If you want to use my code for the utilization display, I will be putting it on github. Check the comments for the address.

Friday, June 5, 2015

Another Linksys Case Mod for a Pi

I have rebuilt my MySQL server system using a Pi version 2 and a new external hard drive. I did this one in a Linksys router case. These cases are really easy to work with and not hard to come by.

I skipped the USB hub on this build.  There isn't enough room and I found an easier way. Power is spliced directly into a usb cable from a connector on the rear of the case. I found that this would not backfeed the Pi like I expected. (This does work on some of my older Pi systems.) To overcome this, I added a wire feeding 5V into GPIO pin #2.

The Raspberry Pi is mounted on the base of the case with the USB and network connectors facing the rear. The rear has to be trimmed a little for the connectors to be clearly accessible.

The hard drive is mounted onto the top of the case. The USB cable has to exit the case and to plug in the the Pi.

In the process of building this, I updated the instructions on how to run from an external hard drive.

This case mod makes a neat little case that stacks nicely. Here it is stacked with my cable modem and firewall. (The firewall is a Pi too!)

Tuesday, April 21, 2015

Fermentation Detector

This is a follow up to my posts on automated home brewing:
Automated Home Brewing
Improved Home Brewing Controller

In the second post I mentioned that I would be adding a "bubble detector" to my interface. Now I have finally gotten the time to do it and am using it to monitor the fermentation stage of a lager. This sensor detects bubbles in the air lock and if you have ever seen an airlock in action you will understand why I call this a "bloop-ometer."

The sensor uses this photo interrupter with a simple circuit to trigger an interrupt on the Raspberry Pi which records how many bubbles per minute are produced. The photo interrupter has a gap with an infrared LED on one side and an infrared sensor on the other. Voltage is provided to the output pin until an object enters the gap and breaks the infrared beam.

In this case the object to block the beam is a small amount of water which is mostly transparent to infrared.  To overcome this the circuit has a potentiometer to adjust the voltage applied to the LED. It must be adjusted down until the sensor is just on the edge of triggering.  In this configuration, the water will block the infrared enough to trigger the device, but when a bubble passes, the infrared can pass.

The circuit is very simple - an interrupter and a potentiometer is all that is really needed. To make it easier to adjust, I added a red LED. I also added an inverter since the output from the sensor is not enough to drive an LED and trigger the Pi. The potentiometer that I used was 5K ohms and I added an additional 100 ohms for safety. If you use less than 100 ohms then too much current will flow into the LED and it will die. (I know this from experience.)

Here is the pinout for the GP1A57HRJ00F.

And here is the circuit as I built it.

And finally, a video of it in action.


Thursday, March 26, 2015

New Raspberry Pi GPIO Pinout Diagram

Since I have been using model B+ and model 2 a lot lately, I wanted an updated GPIO pinout diagram. Also, since I used WiringPi, the diagram had to include that numbering scheme. None of the diagrams I found satisfied me, so I made my own. I used the output from the WiringPi command gpio readall and did some formatting in excel to produce the following diagram. I hope you find it useful.

Sunday, March 8, 2015

Using the MPL115A2 to read Temperature and Barometric Pressure

This post is a continuation of the series on my weather station system.

My weather station uses the MPL115A2, available from Adafruit.  It is an inexpensive sensor for measuring temperature and barometric pressure.  It is only moderately accurate - the MPL3115A2, which only slightly more expensive, would probably have been a better choice.  Both sensors are interfaced using an I2C bus and are fairly simple to use.

In order to compute the correct pressure, several coefficient values must be read from the device first.  These coefficients are unique to each device and provide the calibration necessary to arrive at an accurate reading.  Since they don't change, they only need to be read once.

After reading the coefficients, the data capture and conversion are started by writing a zero to register address 0x12.  The program must then pause briefly to allow the conversion to complete.  The results are stored in the device registers and are read with the standard I2C commands.  The temperature value is taken directly from the register values, with some simple adjustments to get it to degrees Celsius.  The pressure is then computed by a formula that applies the coefficients read earlier as well as the temperature.  The value is in kilo Pascals.  For use by an American like myself, these are finally converted to degrees Fahrenheit and inches of Mercury.

Below is an example, in C, of the code similar to what I use.

    example to test mpl115a2 (temp/baro sensor) on i2c

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdint.h>
#include <wiringPi.h>
#include <wiringPiI2C.h>

int main(int argc, char *argv[])
 int i, n, fd;
 // command string to get coefficients
 unsigned char coef_request[3] = {3, 4, 8};
 // command string to request conversion
 unsigned char conv_request[3] = {0, 0x12, 0};
 // variables for the final results
 float baro, celsius, farenheit;
 // variables to hold the integer values of coefficients
 int16_t a0coeff;
 int16_t b1coeff;
 int16_t b2coeff;
 int16_t c12coeff;
 // variables to hold the floating point coefficients
 float a0;
 float b1;
 float b2;
 float c12;
 // some intermediate values
 int pressure, temp;
 float pressureComp;
 // open a file descriptor to the device on the I2C bus
 fd = wiringPiI2CSetup(0x60);  // 0x60 is bus address of mpl115a2
 if (fd==-1)
  printf("wiringPiI2CSetup failed\n");
  return 0;

 // get the coefficients.  This only needs to be done once.
 // Note on C language: the << and >> operators perform bit shifting
 a0coeff = (( (uint16_t) wiringPiI2CReadReg8(fd,4) << 8) | wiringPiI2CReadReg8(fd,5));
 b1coeff = (( (uint16_t) wiringPiI2CReadReg8(fd,6) << 8) | wiringPiI2CReadReg8(fd,7));
 b2coeff = (( (uint16_t) wiringPiI2CReadReg8(fd,8) << 8) | wiringPiI2CReadReg8(fd,9));
 c12coeff = (( (uint16_t) (wiringPiI2CReadReg8(fd,10) << 8) | wiringPiI2CReadReg8(fd,11))) >> 2;
 printf("%d   %d   %d   %d\n",a0coeff,b1coeff,b2coeff,c12coeff);
 // convert coefficients to floating point
 a0 = (float)a0coeff / 8;
 b1 = (float)b1coeff / 8192;
 b2 = (float)b2coeff / 16384;
 c12 = (float)c12coeff;
 c12 /= 4194304.0;
 printf("%f   %f   %f   %f\n\n",a0,b1,b2,c12);
 // start conversion and wait a tiny bit
 // get the results by reading the device registers
 pressure = (( (uint16_t) wiringPiI2CReadReg8(fd,0) << 8) | wiringPiI2CReadReg8(fd,1)) >> 6;
 temp = (( (uint16_t) wiringPiI2CReadReg8(fd,2) << 8) | wiringPiI2CReadReg8(fd,3)) >> 6;

 // compute temperature compensation for pressure
 pressureComp = a0 + (b1 + c12 * temp ) * pressure + b2 * temp;

 // get the pressure in kiloPascals
 baro = ((65.0F / 1023.0F) * pressureComp) + 50.0F;        // kPa
 // get the temperature in celsius degrees
 celsius = ((float) temp - 498.0F) / -5.35F +25.0F;        // C

 // convert kilo-Pascals to inches of mercury
 baro = baro * 0.295299830714;
 // convert Celsius to Farenheit
 farenheit = (celsius * 1.8) + 32.0;

 //show the results
 printf("%f    %f\n\n",baro,farenheit);
 return 0;

The device comes with a 6x1 header which allows it to be mounted simply on a circuit board.  I mounted it directly on the interface board that I built for the weather station.  This is in a weatherproof enclosure, but that should have negligible effect on the reading of  pressure since the enclosure is not completely air-tight.  This will, however, affect temperature reading.  Therefore, while I do record the temperature value, it is not the primary source for temperature that my weather station uses.  For that I use the AM2315 sensor, which is described in my previous post.

Thursday, February 26, 2015

Using the AM2315 Temperature/Humidity Sensor

This post is a continuation of my series on the weather station that I built.  On e of the sensors that it uses is the AM2315 temperature/humidity sensor

The AM2315 is very poorly documented.  This leads to several problems when attempting to use it. Thankfully, Sopwith has provided details of his experience with this device so we can move on more quickly to making use of it. Sopwith provides a more detailed explanation of how to set up and use this device. If you are using Python, then you must read his article, since he explains some additional problems caused by the way the Python library works.

The first problem you will run into is that it doesn't show up on the i2c bus. That is because the device stays in sleep mode until it is woken up to prevent generating heat that would affect the humidity sensor accuracy. If you run the command i2cdetect twice quickly, then the second time it will show up.

The second problem is that the i2c address is wrong in the datasheet. It is no longet at 0xB8, but is instead at 0x5c. This is not a big problem since you will see the correct address show up in the second i2cdetect results.

The third problem is really a side effect of the first - the device does not keep updated values in registers like most i2c devices. You will have to issue a read request command and then read the response.

Even though I use the i2c support provided by the WiringPi library, it is best to do raw reads and writes to the device because it does not follow the i2c standard very well. This is the sequence to follow:

  • Write a zero byte to it twice to wake it up
  • Write a read request to make it update the temperature and humidity values
  • Read the response to the read request
  • Do some manipulations on the data to get the values in degress Celcius and %RH

Here is an example program in C with comments that give more detailed explanation.
    example code to test am2315 (temp/humid sensor) on i2c bus

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdint.h>
#include <wiringPi.h>
#include <wiringPiI2C.h>

int main(int argc, char *argv[])
 int n, fd; 
 // read request - 3 is the read register command
 // 0 is the address to start at
 // 4 is the number of bytes to read
 unsigned char read_request[3] = {3, 0, 4};
 // buffer for the response: command byte, length byte, 4 bytes data, 2 bytes of checksum
 unsigned char response[8];
 // dummy data sent to wake up device
 unsigned char dummy[1] = {0};
 // the final results 
 float humidity, celsius;
 // open the am2315 device using WiringPi
 // 0x5C is bus address of am2315
 // fd is the "file descriptor" used in later read and writes
 fd = wiringPiI2CSetup(0x5c);  
 if (fd==-1)
  printf("wiringPiI2CSetup failed\n");
  return 0;
 // run until killed with Ctrl-C
 while (1)
  // send some data to wake it up
  n = write(fd, dummy, 1);
  n = write(fd, dummy, 1);
  // send the read request
  n = write(fd, read_request, 3);
  printf("write returned %d bytes\n",n);
  // very short delay to allow device to do data conversion
  // read the reaponse
  n = read(fd, response, 8);
  printf("read returned %d bytes\n",n);
  // sanity check on data returned
  // first byte should echo the read requst byte (3)
  // second byte should indicate 4 bytes of data returned
  // I don't bother verifying the checksum
  if ((response[0]!=3) || (response[1]!=4))
   printf("i2c response invalid\n");
   for (n=0; n<8; n++)
    printf("%02x ",response[n]);
   // (high byte * 256) + low byte
   // divide by 10 
   humidity = (256*response[2] + response[3])/10.0;

   // same as above but mask out the sign bit on the high byte
   celsius = (256 * (response[4] & 0x7F) + response[5]) / 10.0;
   // make result negative if the sign bit is set
   if ((response[4]&0x80)!=0)
    celsius *= -1.0;

   printf("   humidity = %5.1f%%\n",humidity);
   printf("temperature = %5.1f\n",celsius);

  // wait two second and loop again
 return 0 ;
The AM2315 is supposed to be a very accurate device, but after using it a while, I am very suspicious of the humidity values.  The plots of my data show the humidity doing things that are not possible. I suspect that it sometimes gets saturated with moisture from fog, dew or rain and then it reads 100% for a long time. Mine is mounted so that it is directly exposed to the outside environment. Best practice says that I should have a radiation shield protecting the temperature sensor from direct sunlight. I plan on creating one for the AM2315 and this may also allow the humidity sensor to be more accurate.

Friday, February 6, 2015

Log Data to the Weather Underground

The next topic for my weather station system is how to send your data to a "Personal Weather Station" on the Weather Underground web site. This provides a nice interface for the world to see your current (and past) conditions.

The PWS Overview explains what personal weather stations are. It also has a buying guide. For comparison, the total cost for my system was somewhere around $150 (That's just a very rough guess.)

See PWS Upload Protocol for the complete documentation for uploading data for a PWS. To summarize, you need to send a properly formatted string to a web address (a simple HTTP GET to a PHP script.)

So here is a simplified example of the code that I am using.

   Filename:   wunderground.c
   send current conditions to my Personal Weather Station on 
   The Weather Underground
   Uses libcurl to send the data via HTTP

   build with:
   gcc -o wutest wunderground.c -L/usr/local/lib -L/usr/local/ -lcurl -lwiringPi

  23-Jan-2015   Ted Hale  created this as an example for blog


/* system includes */
#include <stdio.h>
#include <stdlib.h>  
#include <stdarg.h>
#include <time.h>
#include <string.h>
#include <curl/curl.h>

#define TRUE 1
#define FALSE 0

// these are normally global varables that my weather system updates
// just put them here for this example
float outsideTemp;
float windSpeed;
float windGust;
float rainPeriod;
float humidity;
float barometric;

// my PWS ID and password
char *myStationID = "KVAWILLI99";
char *myStationPassword = "NOTMYPASSWORD";

// structure used by the libcurl write callback function
struct url_data {
    size_t size;
    char* data;
// write callback function needed by libCurl
size_t write_data(void *ptr, size_t size, size_t nmemb, struct url_data *data) {
    size_t index = data->size;
    size_t n = (size * nmemb);
    char* tmp;

    data->size += (size * nmemb);
    tmp = realloc(data->data, data->size + 1); /* +1 for null terminator */

    if(tmp) {
        data->data = tmp;
    } else {
        if(data->data) {
        printf("skynet_post Failed to allocate memory.\n");
        return 0;

    memcpy((data->data + index), ptr, n);
    data->data[data->size] = '\0';

    return size * nmemb;

// upload current conditions to Weather Underground
int UpdateWunderground()
 // URL format 
 char *myFmt = ""
     //   1           2           3    4    5    6       7       8  
  //              9             10       11        12
  //        13          14

 /* 1 ID
  * 2 passwd
  * 3 yr
  * 4 mon
  * 5 day
  * 6 hr
  * 7 min
  * 8 sec
  * 9 winspeed
  * 10 gusts
  * 11 Outdoor temp
  * 12 rain
  * 13 baro 
  * 14 humidity

 int   error = TRUE;
 time_t   now;
 struct tm  *dt;
 int   hour,minute,second,year,month,day;
 char   url[1024];
 CURL  *curl;
 CURLcode res;
 struct url_data response;
 dt = gmtime(&now);
 // build the URL string
 //                                      1              2
 snprintf(url, sizeof(url)-1, myFmt, myStationID, myStationPassword,
 //         3          4          5           6           7          8
  dt->tm_year, dt->tm_mon, dt->tm_mday, dt->tm_hour, dt->tm_min, dt->tm_sec,
 //    9           10         11        12             13        14
  windSpeed, windGust, outsideTemp, rainPeriod, barometric, humidity);

  // guarantee null termination of string
 url[sizeof(url)-1] = 0;
 curl = curl_easy_init();
 if (curl) {
  response.size = 0; = malloc(4096); /* reasonable size initial buffer */[0] = '\0';
  curl_easy_setopt(curl, CURLOPT_URL, url);
  curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "GET");
  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
  curl_easy_setopt(curl, CURLOPT_WRITEDATA, &response);
  res = curl_easy_perform(curl);
  if(res != CURLE_OK)
   printf("curl_easy_perform() failed: %s\n",curl_easy_strerror(res));
   error = TRUE;
  } else {
   error = (strcmp(,"Success") != 0);
  free (;
 } else {
  printf("curl_easy_init failed\n");
  error = TRUE;
 return error;

int main()
 // set some dummy data
 outsideTemp = 69.7;
 windSpeed = 2.3;
 windGust = 5.6;
 rainPeriod = 0.001;
 humidity = 75.0;
 barometric = 31.5;

 return 0;

Please let me know if you use this code.  I would like to hear how it works for you.