Temperature Control with RasPiO duino – or, How to protect your pipes

I’ve recently been working on a new board called the RasPiO^® duino. You might guess from the name that it’s a bit Arduino-like, and it is. I’m currently on iteration 5 and there’s going to be at least one more before production. All iterations have worked, but I keep getting ideas and suggestions from my beta testers (and myself) that I want to include.

But the board is not really what today’s post is about. I want a way of preventing the water inlet pipe freezing in the basement of our house in Poland. In a very harsh winter, the temperature in the basement, where the water inlet is, can get close to zero. That’s bad because if the water in our inlet pipe freezes, it could burst. The central heating water is drained and the system is empty, but there’s a 3 foot pipe sticking up from the ground before it gets to the inlet valve. That pipe and the inlet valve are what I want to protect.

I know there are ‘proper’ solutions for this, but where’s the fun in that? I want to make my own.

Here’s a video about it…

Read Sensors, Control Relays

What I need is a way of safely and gently heating the surrounding area, or the pipe itself, if/when the pipe temperature gets too low. That sounds like a job for a small, autonomous, low-power device that can read temperature sensors and switch relays.

An arduino type device is ideal for this because, if there’s a power cut, it’ll ‘just work’ when the power is restored. You could use a Pi, but there is still a chance of SD card corruption, so it’s a bit less bullet-proof. I haven’t had an SD card corruption for over a year, but I know other people have. Liz mentioned, on the Foundation’s blog last week, an SD card corrupting because someone removed the power lead.

Let’s Use a RasPiO^® duino

So, since I’ve got the RasPiO^® duino to play with, I thought I’d use that and a little relay board, some TMP-36 sensors and a 12V Halogen bulb or two. And, because we all love LEDs, let’s have some of those too, as indicators.

The nice thing about this setup is that you can program the RasPiO^® duino with the Pi, and then detach it, if you want to, for autonomous use.

How Will It Work?

All I really want here is a relay to switch on a lamp to provide a little heat if the pipe temperature gets too close to freezing, and then switch off again when it warms up a couple of degrees. The duino is perfect for that.

Such a system will need some redundancy in case parts of the system fail. There are two relays on the relay board, so I may as well use them both (with a bulb each) in case one fails. I’ll use at least two temperature sensors and have each one controlling a lamp. There are 6 analog channels on the ATMega 328P-PU chip, so we could use up to 6 analog sensors.

Here’s what the whole setup looks like in prototype…

It Needs Smoothing and Anti-Oscillation

Two problems I encountered along the way were…

1. Temperature measurement instability
2. Oscillation – too frequent switching on and off of the heating circuit(s)

I tackled measurement instability on several fronts…

• Stabilised the TMP-36 sensors with a 0.1uF capacitor
• For every measurement, read the ADC 6 times and ignore the first reading for each set.
• Keep the last five results (each one itself an average of 5 readings) in an array to smooth the results and reduce the effect of sudden large swings or outliers
• Only switch on the heating circuit relays in response to the smoothed (moving average) results

Oscillation was dealt with by forcing the heating circuit(s) to stay on for 30 cycles once switched on. The idea here is that, since each cycle is ~1s, the heating lamp (or whatever I end up using) will not be switched on and off rapidly, but will always stay on for a minimum of 30s.

Arduino Code

Here’s the code for the ATMega chip. A lot of the code is duplicated. I expect there’s a ‘tighter’ way of doing it, but I’m no expert at Arduino code (which I’m told is pretty much the same as C++). I had to learn some new stuff to get this far…

//
const int numReadings = 5; // number of readings in moving average array

//TMP36 Pin Variables
int sensorPin0 = 0; //= A0 analog pin 0. TMP36's Vout (sense) pin is connected to A0
int sensorPin1 = 1;

int index = 0;
int thisReading = 0;
int reading = 0;

int readingsT1[numReadings]; // number of results to store in moving average array
int totalT1 = 0;
float averageT1 = 0;
int cyclesT1 = 0;

int readingsT2[numReadings];
int totalT2 = 0;
float averageT2 = 0;
int cyclesT2 = 0;

int LEDpin1 = 9;
int LEDpin2 = 10;
int LEDbrightness;
int LEDbrightness2;
int relaypin1 = 4;
int relaypin2 = 3;

void setup()
{
  Serial.begin(9600); //Start the serial connection with the computer
                      //to view the result open the serial monitor
  pinMode(relaypin1, OUTPUT);
  pinMode(relaypin2, OUTPUT);
  digitalWrite(relaypin1,HIGH);  //HIGH is off for the relays
  digitalWrite(relaypin2,HIGH);
  
  for (int thisReading = 0; thisReading < numReadings; thisReading++)
  {
    readingsT1[thisReading] = 233; // set all values to 233 = 25 degrees C
    readingsT2[thisReading] = 233;
  }
}

void loop()
{ 
         //get digitised voltage reading from temperature sensor
    analogRead(sensorPin0);
    delay(20);
    reading = 0;
    for (int loop = 0; loop < 5; loop++)
    {                                         // read sensor 5 times
       reading += analogRead(sensorPin0);
       delay(20);
    }       
    readingsT1[index] = reading / 5;  // average integer readings + amend array
    totalT1 = 0;
    for (int thisReading = 0; thisReading < numReadings; thisReading++)
       totalT1 = totalT1 + readingsT1[thisReading];
    averageT1 = totalT1 / numReadings;               // float average ADC readings
    averageT1 = averageT1 * 3.29 / 1023;             // convert to decimal voltage
    averageT1 = (averageT1 - 0.5) * 100;             // convert to temp in C
    
    float voltage = reading * 3.29 / 5.0;
    voltage /= 1023.0;

    reading = 1023 - reading;
    LEDbrightness = map(reading, 0, 1023, 0, 255);
    analogWrite(LEDpin1, LEDbrightness);
    
    Serial.print("ID1 ");Serial.print(voltage); Serial.print(" V ");    
    float temperatureC = (voltage - 0.5) * 100; 
    Serial.print(temperatureC); Serial.print(" C.  ");
    Serial.print(averageT1); Serial.print(" C.  Average ADC: ");
    Serial.println(readingsT1[index]);
    
    // if temp <=22 turn on heat and keep it on for 30 seconds/cycles
    if (averageT1 <= 22)
    {
       digitalWrite(relaypin1,LOW);
       cyclesT1 = 30;
    }
    
    // ensure 30 iterations, so heat stays on for 30 cycles (avoids oscillation)
    else if (cyclesT1 > 0)
    {
       cyclesT1 -= 1;
    }
    
    // switch off heat if >22 C AND cyclesT1 = 0
    else 
    {
       digitalWrite(relaypin1,HIGH);
    }    
    
// Now do it all again for sensor 2 and relay 2
   
    analogRead(sensorPin1);
    delay(20);
    reading = 0;
    for (int loop2 = 0; loop2 < 5; loop2++)
    {                                             // read sensor 5 times
       reading += analogRead(sensorPin1);
       delay(20);
    }       

    readingsT2[index] = reading / 5;  // average integer readings + amend array
    totalT2 = 0;
    for (int thisReading = 0; thisReading < numReadings; thisReading++)
       totalT2 = totalT2 + readingsT2[thisReading];
    averageT2 = totalT2 / numReadings;               // float average ADC readings
    averageT2 = averageT2 * 3.29 / 1023;             // convert to decimal voltage
    averageT2 = (averageT2 - 0.5) * 100;             // convert to temp in C

    voltage = reading * 3.29 / 5.0;
    voltage /= 1024.0;

    reading = 1023 - reading;
    LEDbrightness2 = map(reading, 0, 1023, 0, 255);
    analogWrite(LEDpin2, LEDbrightness2);
    
    Serial.print("ID2 ");Serial.print(voltage); Serial.print(" V ");    
    temperatureC = (voltage - 0.5) * 100; 
    Serial.print(temperatureC); Serial.print(" C.  "); 
    Serial.print(averageT2); Serial.print(" C.  Average ADC: ");
    Serial.println(readingsT2[index]);

    // if temp <=22 turn on heat and keep it on for 30 seconds/cycles
    if (averageT2 <= 22)
    {
       digitalWrite(relaypin2,LOW);
       cyclesT2 = 30;
    }
    
    // ensure 30 iterations, so heat stays on for 30 cycles
    else if (cyclesT2 > 0)
    {
       cyclesT2 -= 1;
    }
    
    // switch off heat if >22 C AND cyclesT2 = 0
    else 
    {
       digitalWrite(relaypin2,HIGH);
    } 

    delay(1000); //waiting a second
    
    index = index + 1;
    if (index >= numReadings)
       index = 0;
} 

Python Code Runs on the Pi

There’s also a short Python script which runs on the Pi. This can be used to monitor what’s happening. It was absolutely essential while developing the arduino code, to be able to see what was happening. It reads and displays what the ATMega is writing to the serial port and does further smoothing on the results. There’s a bit of repetitive code that could probably be put into a function.

import serial, subprocess, sys
from time import sleep

def print_there(x, y, text):
    sys.stdout.write("\x1b7\x1b[%d;%df%s\x1b8" % (x, y, text))
    sys.stdout.flush()

correction_factor = 0  # 0 for none
correction_factor2 = 0  # 0 for none

subprocess.Popen("clear", shell=True)
degree = unichr(176)
first_time = True
first_time2 = True

while True:
    sport = serial.Serial("/dev/ttyAMA0", 9600, timeout=1)
    try:                                       # stops program failing if no serial data
        response = sport.readlines(None)[0:2]
        sport.close()
    except:
        sport.close()
        
    if not response:                           # stops program failing if no serial data
        print "no serial data read"
        sleep(0.5)
        continue                               # if no data, skip to top of loop and retry
        
    print_there(1,1,response[0])
    print_there(2,1,response[1])
    if response[0].startswith("ID1"):
        temp = (float(response[0].split()[3]) + correction_factor) #'%.2f' % 
        if first_time:
            temp_list = [temp, temp, temp, temp, temp]
            first_time = False
        del temp_list[0]
        temp_list.append(temp)
                                 # makes a weighted moving average of last 5 readings
        smooth_temp = "%.1f" % round(float((temp_list[0] * 0.1) + (temp_list[1] * 0.15) + (temp_list[2] * 0.2) + (temp_list[3] * 0.25) + (temp_list[4] * 0.3)), 1)

        volts = float(response[0].split()[1])

        output1 = ''.join((" Sensor1: ",str(smooth_temp)," ",degree,"C   ",str(volts)," V"))
        print_there(3,3,output1)

    if response[1].startswith("ID2"):
        temp2 = (float(response[1].split()[3]) + correction_factor2) #'%.2f' % 
        if first_time2:
            temp_list2 = [temp2, temp2, temp2, temp2, temp2]
            first_time2 = False
        del temp_list2[0]
        temp_list2.append(temp2)
                                 # makes a weighted moving average of last 5 readings
        smooth_temp2 = "%.1f" % round(float((temp_list2[0] * 0.1) + (temp_list2[1] * 0.15) + (temp_list2[2] * 0.2) + (temp_list2[3] * 0.25) + (temp_list2[4] * 0.3)), 1)

        volts2 = float(response[1].split()[1])

        output2 = ''.join((" Sensor2: ",str(smooth_temp2)," ",degree,"C   ",str(volts2)," V"))

        print_there(4,3,output2)
    sleep(2)

Use With or Without Pi

The system will work perfectly ‘off-pi’, but the Pi can be attached to give a real-time temperature readout using the serial ports. It could also be used to…

• log or tweet updates/alerts
• monitor remotely via internet
• or even to reprogram the duino

But I may not use a Pi once the duino board is programmed. I don’t have an ethernet port in the basement, and the wifi router is two very thick concrete floors away, on the first floor. We’ll see.

Don’t Use it for Mission-Critical Stuff

I did this for fun. I don’t know if it’s robust enough or even fit-for-purpose. If you have a really important system to control, you’d better not use this unless you know what you’re doing. For me, it was a fun learning exercise.