Computing stuff tied to the physical world

Solar… again – the code

In Software on Jan 22, 2013 at 00:01

With the hardware ready to go, it’s time to take that last step – the software!

Here is a little sketch called slowLogger.ino, now in JeeLib on GitHub:

Screen Shot 2013-01-19 at 16.20.50

It’s pretty general-purpose, really – measure 4 analog signals once a minute, and report them as wireless packet. There are a couple of points to make about this sketch:

  • The DIO1 pin will toggle approximately once every 64 minutes, i.e. one hour low / one hour high. This is used in the solar test setup to charge the supercaps to about 2.7V half of the time. The charge and discharge curves can provide useful info.

  • The analog channel is selected by doing a fake reading, and then waiting 100 ms. This sets the ADC channel to the proper input pin, and then lets the charge on the ADC sample-and-hold capacitor settle a bit. It is needed when the output impedance of the signal is high, and helps set up a more accurate voltage in the ADC.

  • The analog readings are done by summing up each value 32 times. When dividing this value, you get an average, which will be more accurate than a single reading if there is noise in the signal. By not dividing the value, we get the maximum possible amount of information about the exact voltage level. Here, it helps get more stable readings.

As it turns out, the current values reported are almost exactly 10x the voltage in millivolts (the scale ends up as 32767 / 3.3), which is quite convenient for debugging.

Having said all this, you shouldn’t expect miracles. It’s just a way to get minute-by-minute readings, with hopefully not too many fluctuations due to noise or signal spikes.

Tomorrow, some early results!