All cleaned up and re-mounted

A nice clean board (actually, i just flipped the old one over) and all components re-mounted.  System is up and running 24/7 now.

The Cleanly Re-Mounted Alarm/Automation System

Reading Temperature With Thermistors

My first interest for an analog input is a probe to read the temperature in my beer fridge. (I am a home brewer and occasionally need closely controlled temperature for fermenting lagers.) There were a couple of 10K ohm thermistors in my box of misc components, so I decided to try those. Something like the TMP36 sensor may be a better choice, and I will try some of those soon.

A thermistor is a special type of resistor that changes resistance with temperature.  A 10KΩ thermistor is 10KΩ at 25°C.  The first problem to solve is how to use the thermistor to produce a voltage that the MPC3008 analog to digital converter can read.  A simple voltage divider, as show in the diagram here, will do the trick.

The thermistor is R1, R2 is 10KΩ, and Vin is 5V.  This will make Vout = 2.5V at 25°C.  The chart for this thermistor shows that it should vary from 1.3V at 32°F to 2.9V at 95°F.  Setting the reference voltage on the MPC3008 to 3.3V will provide the best resolution for this range. I took test measurements by placing the thermistor in an insulated cup of water ranging from ice water to very warm.  Well, that was the plan anyway.  Reminder to self: when using a thermistor or other sensor in water, shrink tubing won't cut it.  I got a rude reminder of this when I first tried to do a calibration.

My solution to the waterproofing was to simply encase the thermistor and connecting wire in epoxy.  I formed a small mold using wax paper, filled it with freshly mixed epoxy, and pushed the sensor into the epoxy.  Once it dried, I trimmed it and it came out looking like this.

I took test measurements successfully and put the data into Excel. The plot of this data shows, as expected,  a very linear curve.  Putting this data into a Least Squares Fit function produced a slope of 0.13239 and an intercept of -14.402.  This gives me parameters to convert from counts to degrees with reasonable accuracy.  Once in use I will probably need to tweak the intercept value for individual sensors due to variations in the components.

The MPC3008 is a 10 bit ADC, meaning that it will output a value between 0 and 1023.  This corresponds to zero and reference volts.  For example, if the reference is 5V, an input of 2.5V would give an output of 512 counts.  To convert from counts to volts:

            V = Vref *counts/1024

Now that I have the data from my earlier calibration, I can simply use that to convert directly from counts to degrees F.

           Temp = -14.402 + (Counts * 0.13239)

My automation/alarm software has now been enhanced to read the analog data and convert it to real values using the provided conversion parameters.  I am now monitoring the temperature in my beer fridge and once I set up a large relay to control the power, I will be able to finely control the temperature and maybe brew a decent lager.

Analog Interface

The capability to read analog inputs is a feature that is greatly missed on the Raspberry Pi, but I agree with the decision to omit this capability in order to keep the price down.  Besides, if they did include an analog interface, many would complain that it isn't adequate for their purpose.  How many input channels do you need?  What resolution? 8 bit, 10 bit, 12 bit 16 bit?  What throughput rate?

Fortunately, there are many analog input chips that use the SPI or I2C bus, making it almost trivial to add analog inputs to the Pi.  I chose the MCP3008, an 8 channel 10 bit ADC available from Adafruit.com.  Add a bi-directional logic level converter and some connectors and you're ready to go.

The level converter is on my main interface board which provides two SPI bus connectors.  The analog interface is a simple board which includes a connector for SPI, the MCP3008 chip, a jumper to choose the analog reference, and screw terminals for the inputs.  I ended up adding several more screw terminals connected to 5V in order to power thermistors. 

The SPI serial bus is full duplex, but the way it works may seems odd to a programmers point of view.  (It makes perfect sense if you understand how the hardware works.)  You may be familiar with how full duplex works on an RS-232 line: data can be sent and received at the same time, but independently.  That independence is due to the fact that RS-232 is an asynchronous protocol.  SPI is a synchronous protocol; meaning everything is driven by a clock pulse.
Data bits will be sent out the MISO line on each cycle of the CLOCK line.  At the same time data bits are being read in on the MOSI line.  The number of bits out is the number of bits you will read back in.  This means that you may have to write more bits than expected for a given command and you may read bits that are unused.

Fortunately, we don't have to worry much about the ugly details at the lower level.  There is a device driver for SPI that is included with the recent versions of Raspbian and the WiringPi library provides support for SPI I/O.  The functions wiringPiSPISetup and  wiringPiSPIDataRW are all that is needed.  Here is the source code for a program that I used when testing and calibrating sensors.


#include <stdio.h> #include <stdint.h> #include <wiringPi.h> #include <gertboard.h> // read SPI data from MCP3008 chip, 8 possible adc's (0 thru 7) int readadc(adcnum) { uint8_t buff[3]; int adc; if ((adcnum > 7) || (adcnum < 0)) return -1; buff[0] = 1; buff[1] = (8+adcnum)<<4; buff[2] = 0; wiringPiSPIDataRW(0, buff, 3); adc = ((buff[1]&3) << 8) + buff[2]; return adc; } int main(int argc, char *argv[]) { int i, chan; uint32_t x1, tot ; printf ("SPI test program\n") ; // initialize the WiringPi API if (wiringPiSPISetup (0, 1000000) < 0) return -1 ; // get the channel to read, default to 0 if (argc>1) chan = atoi(argv[1]); else chan = 0; // run until killed with Ctrl-C while (1) { tot = 0; for (i=0; i<100; i++) { // read data and add to total x1 = readadc(chan); tot += x1; delay(10); } // display the average value printf("chan %d: %d \n", chan, (tot/100)) ; } return 0 ; }

Note: The SPI device is not loaded by default.  The easy way to get it loaded is to used the "gpio" utility that comes with the WiringPi library.  Just enter
      gpio load spi
and the drivers will be loaded and ready to use.

Dual power supplies is the way to go

You may notice in the picture of my setup (in the previous post) that the Raspberry Pi is being powered normally via the USB power connector.  This was done because the Pi was failing to boot when powered through the GPIO port as I had planned.  While I did not do any tests, the obvious cause is a lack of sufficient current to power the Pi.  It really does want to have a solid 700mA of 5 volt juice.  My planned configuration worked fine on the workbench but failed once I mounted it on the wall and connected the alarm sensors.  The sensors require 12V power and I was using the same power supply to drive the sensors and the power adapter for the interface board.  That 12V power supply was only 2A and that was apparently too little for all of this.

I could have gotten a larger 12V power supply, but I already have several of these 2A supplies. Realization finally struck me that using a single power supply would be a mistake.  If any of the sensor lines was compromised, shorting the power to ground, it would cause the Pi to shut down suddenly.  So now I have one supply just for the alarm sensors and another that drives the interface board, and through that, the Pi.

The system is up and has been running for over a week with no major problems.  I spent several hours improving my code for driving the X10 interface to make it as reliable as possible.  It seems to work as well as it ever has when using heyu or ActiveHome to drive it.  X10 is inherently unreliable, but is usually good enough for casual use.  If you have a fairly new house that is wired properly, then it can work fairly well.  My house, unfortunately, is old and poorly wired.  I have made some improvements over the years, but there are still areas of my house that the X10 signals simply will not reach.  I will cover X10 in more detail in an future post.

Mounted and Testing Begun

The interface is complete now with all (well, most anyway) of the kinks out of it.  Here it is mounted on the wall in a utility area of the house.

I still need to install a few more sensors for the alarm system and I will soon add analog input using the SPI bus.  That worked just as expected on the breadboard.  You can see it connected in the blog post below.

There was some discussion on the forum about the serial connection for the X10 interface.  My CM11a works fine with just Tx, Rx, and GND connected.

The power supply was salvaged from a micro ATX PC case.  The 12V power supply that powers it is only 2A and I had trouble running the RasPi powered from my interface.  It worked fine on the bench with these same power supplies.  I assume that the motion detectors, which are wired to the same 12V supply are drawing too much current. This causes the ATX PS to fail to output adequate 5V current.  For the time being I have removed the power jumper (+1 for configurability) and just power it the normal way.  Once I get a beefier 12V PS, I will try powering the RasPi from the interface again.

Interface Completed

After yet another design change, I finally have the interface complete and ready to test.

Version 7 Finally Comes to Life

          ✓ Serial Port
          ✓ 8 Digital Inputs

          ✓ 4 Relay Outputs

          ✓ 2 SPI Ports

          ✓ 1 I²C Port

          ✓ Fused Power to Pi




Making the fuse involved the tiniest soldering I have ever done.  That is a surface mount poly-fuse soldered to two wires.  Once connectors are added, it is used for the jumper that connects the interface board 5V supply with the Raspberry Pi GPIO 5V pin.

The picture above shows one SPI port connected to an ADC chip on the bread board.  Once everything checks out, it will be time to mount it all.

Interface Nearly Complete

After far too many revisions, the GPIO interface for the Raspberry Pi alarm system is nearly done.  It has:

• 8   Digital Inputs
• 4   Relays
• 1   RS-232 serial port
• 2   SPI bus connectors
• 1   I²C bus connector (eventually)

I still need to to finish wiring the SPI connectors, but the serial port, the digital inputs, and the relays are all working.  I will probably go ahead and add connectors for the I²C bus pins for future use.  I already have an eight port A/D converter for the SPI bus and more GPIO would also be easy to add via SPI.

I added a jumper block near the GPIO connector that allows me to connect (or not) 5V on the interface to 5V on the RasPi.  This would allow me to power the board from the RasPi or to backfeed power to the RasPi from the board.  I plan on making a jumper with a polyfuse inline.  Using that as the jumper will prevent the  RasPi from drawing too much current.