Tuesday, 11 February 2014

Sleepy Wireless Temperature Sender

It would be nice to know the temperature in the greenhouses at Made in the Shade without having to be there. The obvious first solution step is a commercial remote monitor, good enough to read 300 ft away, which just manages to display temperatures from 3 different greenhouses on an LCD display if the base station is by the back window of the house. It doesn't do much good if you are off the property, and doesn't have an obvious way to hijack the readings from the base station. To get them off the property, we'll have to reacquire the temperature signals, preferably with a battery powered wireless gadget. The obvious solution is some kind of Arduino / XBee combination, except an UNO with an XBee attached consumes about 100 mA and would burn through an AA battery pack in a day or less.

Low Power Means Putting Things to Sleep!

Plugged in a new XBee on the FTDI cable and couldn't remember the right terminal settings (9600, Raw, Echo,  then +++ with no return should get an OK.) I used ATID2711, ATMY0, ATDL1, ATBD6 to match the one at the sailboat end of the IOM controller so the TX can listen to either one at 57600 baud. Don't forget the ATWR to write it all out! Upgraded the IOM TX program to echo any serial it gets back on the XBee to the console.

Now to get a trinket talking to the XBee... I couldn't get software serial to work reliably on the trinket at 57600, so backed all the XBees down to 19200. I created SleepyTrinket based on the code from Nathan Seidle at  https://github.com/sparkfun/H2OhNo/tree/master/firmware/WatchDogTest. That code uses pins 3 and 4 for the software serial, but I switched it to 0 and 1, then used pin 2 for the XBee sleep line, which leaves 3 (A3) and 4 (A2) for something analog useful. I cut the current draw on the trinket by 3 mA by breaking off the green power LED. The XBee sleeps when pin 9 is set high if sleep mode is enabled (ATSM1 for hibernate or ATSM2 for doze, plus ATWR).

I first powered the Trinket and the XBee directly on the 3V line from a 3.7 V 100 mAh LiPo, hoping to bypass any regulator losse, but that worried me a little so I switched to the regulated battery input. Either way, with everything awake it used about 60 mA and with everything asleep that fell to about 40 uA! That's about 1 mAh per day while sleeping and 1 mAh per minute while awake.

How quickly can I wake everything up, spit out a serial burst and go back to sleep? It works with delays in the code of 1 s, 500 ms, 100 ms, so 50 ms is probably fast enough with a total time around 200 ms per burst. To split the power consumption between awake and asleep would mean reporting about 300 times a day or about once every 5 seconds.

When I enabled the ADC and started reading a TMP 36 the sleep load came up to about 270 uA, or about 6 mAh per day, which would still get months on a 1300 mAh battery. Still, enabling and disabling the ADC each cycle brings the sleep load back down to about 60 uA. Better stability for the TMP 36 comes from a 2 M resistor and a 104 ceramic cap bridging the signal pin to ground. Based on these measurements, the little 500 mAh LiPo in the background should be good for more than 6 months between recharges. The next step is a little tidier packaging and something at the house end that will listen, repeat the data out to the world, and maybe send an SMS if alarm conditions come up.

A little more testing shows that the accuracy of the temperature measurements from this combo are a little dodgy, so time to track down where the error comes from. (Yes I know that the TMP36 spec says +-2C, but I want better ;-) )

Use with Adafruit Trinket 3V - remove green power LED - compile as trinket 8MHz

about 6 mA awake and 25 uA asleep, 10 mA if the LED is turned on for just the trinket

sleep current goes up to about 1 mA if connected to FTDI cable

Rick Sellens adapted from:

 Spark Fun Electronics
 Nathan Seidle

 This code is public domain but you buy me a beer if you use this and we meet someday (Beerware license).
#define TRX 1      // Trinket RX pin
#define TTX 0      // Trinket TX pin
#define XBS 2      // XBee sleep pin
#define BLNK 1     // Blinking LED pin
#define TMP 2      // Analog pin number for TMP 36

#define WT 50

#include <avr/sleep.h> //Needed for sleep_mode

#include <SoftwareSerial.h>

SoftwareSerial mySerial(1, 0); // RX, TX

volatile int watchdog_counter;

//This runs each time the watch dog wakes us up from sleep
ISR(WDT_vect) {

void setup()
  digitalWrite(XBS,LOW);  // start with the XBee awake
  mySerial.begin(19200);    // flaky at 57600, but seems OK at 9600, 19200, 38400
  mySerial.println("\n\nRWS Sleeper");
  watchdog_counter = 0;

  //Power down various bits of hardware to lower power usage  
  set_sleep_mode(SLEEP_MODE_PWR_DOWN); //Power down everything, wake up from WDT
  ADCSRA |= (1<<ADEN);    //Enable ADC, costs ~230uA
  ADCSRA &= ~(1<<ADEN);   //Disable ADC, saves ~230uA
  setup_watchdog(6); //Wake up after 1 sec (6)

void loop() {
  static int rdg = 0;
  static int n = 0;
  digitalWrite(XBS,HIGH);  // sleep the XBee
  sleep_mode();            // sleep the trinket

  if(watchdog_counter > 0){
    // trinket awake, so take a reading
    ADCSRA |= (1<<ADEN);    //Enable ADC, costs ~230uA
    rdg += analogRead(TMP);
    ADCSRA &= ~(1<<ADEN);   //Disable ADC, saves ~230uA

    if(watchdog_counter > 9){         // wake the XBee every 10 counts
      digitalWrite(BLNK,HIGH);        // LED on  
      digitalWrite(XBS,LOW);          // wake the XBee
      mySerial.print("Awakened... ");
      digitalWrite(BLNK,LOW);        // LED off
      long int itemp = rdg;          // temperature in tenths of a degree C
      itemp = (itemp * 3300 / 1024) / n - 500;
      mySerial.println(" tenths of a degree C.     ZZZ...");
      // rest the accumulator and counters
      watchdog_counter = 0;
      rdg = 0;
      n = 0;

// 0=16ms, 1=32ms, 2=64ms, 3=128ms, 4=250ms, 5=500ms
// 6=1sec, 7=2sec, 8=4sec, 9=8sec
// From: http://interface.khm.de/index.php/lab/experiments/sleep_watchdog_battery/
void setup_watchdog(int timerPrescaler) {
  if (timerPrescaler > 9 ) timerPrescaler = 9; //Correct incoming amount if need be
  byte bb = timerPrescaler & 7; 
  if (timerPrescaler > 7) bb |= (1<<5);        //Set the special 5th bit if necessary

  //This order of commands is important and cannot be combined
  MCUSR &= ~(1<<WDRF);                   //Clear the watch dog reset
  WDTCR |= (1<<WDCE) | (1<<WDE);         //Set WD_change enable, set WD enable
  WDTCR = bb;                            //Set new watchdog timeout value
  WDTCR |= _BV(WDIE);                    //Set the interrupt enable, this will keep unit from resetting after each int