Sunday, 16 December 2012

Approaching a working version of Arduino Solar Monitor

Christmas is coming, so I have to get a working version of the Arduino Solar Panel monitor.

My original intent was that it would have the following features:

  1. Measure the collector differential temperature.
  2. Infer the water flow rate from pump speed.
  3. Calculate the instantaneous power being collected.
  4. Calculate hourly and daily average powers.
  5. Log this information to an SD card.
  6. To achieve (5) easily, derive the time from an external Real-Time-Clock (RTC).
Now, features 1-3 are working, phew.   Feature 5 is implemented, but I need to think about what I really want (is daily average power useful?  Or should I just integrate total heat collected in a day?

Features 5 and 6 are proving troublesome, as I think I am starting to get to the limit of a single Arduino board (or more precisely the ATMega 328 controller on the board).

The main problem is that I am running out of RAM, and am going to have to give some serious thought to how to manage it better (just like old times programming a Zilog Z80A.....).

One problem is the number of different interfaces (and hence libraries) that I am having to use to achieve this.  The base software uses:
  1. OneWire.h and DallasTemperature.h to do the temperature monitoring, using a One-Wire bus.
  2. LiquidCrystal.h to drive the LCD display, using parallel data transfer
To add SD card support and a real time clock, I will need:
  1. Wire.h and DS1307RTC.h to access the real time clock from an I2C interface.
  2. SD.h to access the SD card from a SPI interface.
Each library uses a bit of ram , and there is only 2k of ram on the chip, so I am running out of it rapidly.  When I tried to add the RTC code, the board re-booted every few seconds, which I think was an out of memory issue.

So, the de-scoped system is not going to do SD card logging.  As compensation I have added switches to the two spare digital lines to use to provide a simple user interface so you can scroll between instantaneous, hourly and daily data, and maybe even set the clock (but I do worry about running out of RAM again if I get too adventurous!

Given this, I have put the 'Version 1' hardware together, and mounted it in a cheap 2 gang socket pattress box with a blank cover cut out to hold the board:
The toggle switch on the front is for the display back-light - I thought that would be easier than a push-button if you were trying to use buttons for the interface - the new buttons are facing the bottom of the picture on the side of the front panel, so you can't see them on this photo (and I made a mess of cutting the holes for them, so it looks a bit ugly...).

Right, just got to sort out the software now.  Current version is on github.

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