Saturday, 29 September 2012

Customizing the Logging Shield

In hindsight, two mistakes: not using stacking headers; having the meter stick up too far. Both prevent putting the shield in the middle of a stack where it could be really useful.

It is now over a year later and the RTC has lost about 8 minutes, well within the "minute a month" they advertised for the accuracy of quartz watches back when they were a novelty.

 I started out by assembling the data logging shield per instructions and doing a bunch of experimenting with programming for real time acquisition and logging. The only way to see what's going on on the card is to either watch the LEDs or have it report to the serial monitor on the attached computer, which really limits its ability to function without the computer. I also found myself regularly attaching my multimeter to various pins to see what was going on. A little time and money made the shield more self contained.

A Voltage Display

A DIP switch was added to the prototype area, with all the pins on one side tied to the input of a little voltmeter module. The other side pins connected to digital pins 9 (switch 1) and 3 (switch 2), A0 through A5 (switch 3 to 8). The voltmeter module was hot glued to the SD card housing, with a small piece of sheet plastic in-between.

This gives me switch selectable monitoring of each of the analog inputs and two of the PWM capable output pins. Be sure to set only on of the switches to be on at a time.

Interrupt Push Button

A push button was installed at the bottom right to allow momentary connection of pin 2 to +5 volts to generate an interrupt event.

LEDs for Status

The two LEDs that came with the kit were connected to pins 2 (red) and 3 (green) for additional status indication.

Friday, 28 September 2012

Installing Instrumentation for Dragon III

Circuit board mounted off to the side
of the engine compartment
Terminal block to bring all
the connections up to the bulkhead
I had some trouble with the instrumentation on the engine early in the season, and never had a tachometer, so I decided I would try to monitor them electronically as a challenge. Here's what the actual installation looked like.

The experience of hookup and testing proved interesting. I wired everything up and it worked, giving believable output for the tach, which changed as the engine speed changed in very believable ways, so I felt good. Then I disconnected the tach circuit and the Arduino kept registering the same data anyway! WTF!

Moving away from the engine reduced the indicated RPM and towards the engine increased it. With the input wire on the Arduino unterminated, there was enough electrical noise that it was picking it out of the air like an antenna, with voltages high enough to generate an interrupt. Don't let anything float - pull it high or low!

An additional wire to get the signal from the negative side of the coil.
The coil and wiring create enormous electrical noise!

Boat Instruments on the Way

The instruments on Dragon III are aging, like the boat which will be 33 this year. The senders seem pretty reliable, but some of the gauges are a little sticky and the connections a little dubious. Besides that, there never was a tachometer there, just an opportunity to guess at engine speed from the sound. Today I did a little investigating in the line of getting the temperature gauge to read again. Along the way I cleared out some spider webs and figured out several things.

The water temperature and oil pressure senders are just variable resistors, changing with temperature or pressure. They are each wired as the bottom leg of a voltage divider, grounding through the engine. Both result in around 2 to 3 volt signals that could go directly to an A/D converter.

The temperature sender is about 100 ohms at ambient and drops its resistance with increasing temperature.

The pressure sender is about 89 ohms at zero pressure and... I need to check my notes.

The negative side of the coil is easily available to provide about a 200 volt AC signal that could drive a tachometer. With help from I came up with this circuit to produce countable pulses.

I hooked up a breadboarded version this afternoon and could see a rapidly flashing LED when the engine was idling. Visible flashing probably means under about 25 Hz. The Atomic 4 will fire on all four cylinders once every two revolutions (4 stroke) so 2 firings per rev. 25/2*60 = 750 rpm or less, which is certainly in the ball park. So I soldered it up on a little chunk of perfboard for some further tests.

In the mean time, I started from the Arduino demo code for interrupts and wrote a tach sketch to count rising pulses on pin 2 and convert them to RPM once a second, with a window for some damping. I tested the code with a push button (hence the debounce check) and by counting from pin 13 and both produce sensible results.

int pin = 13;
volatile int state = LOW;
volatile long last = 0;
volatile long kount[] = {0,0,0,0,0,0};
volatile long per = 1000;
volatile long sper = 0;

void setup()
  pinMode(pin, OUTPUT);
  attachInterrupt(0, count, RISING);

void loop()
  long rpm = 0;
  digitalWrite(pin, 1);
  digitalWrite(pin, 0);
  if(millis()-sper >= per){
    sper = millis();
    rpm = kount[0]*30+kount[1]*15+kount[2]*8+kount[3]*4+kount[4]*2+kount[5];
    rpm = rpm * 2 / 4;  // two rotations per power stroke for each of four cylinders

void count()
  // 4 cyl engine at 12000 rpm = 4*12000/60/2 = 400 Hz or 2.5 ms
  if(millis()-last >= 2){  // cancel HF noise by waiting
    state = !state;
    last = millis();