Solar logging

Logging enabled with Raspberry PI and Arduino

After calibrating the power supply to a voltage of 4.86v I was able to use a simple voltage divider with two equal impedance resistors to measure the voltage from my two panels.

This basic line graph plotted with Excel shows the data starting at 4AM and ramping up exponentially. It smooths off at around 9.30am where the sun starts to change its angle.

The code

After including the pyserial module with easy_install I wrote this code to parse the output of my Arduino. It smooths out n samples before logging to a file.
Usage: python volt_read <file.txt> <samples>

from datetime import datetime  
import sys

class writer:  
        def __init__(self,filename):
                self.format='%Y-%m-%d %H:%M:%S'
                self.filename=filename
        def get_stamp(self):
                return datetime.now().strftime(self.format)
        def write_stamp(self, voltage):
                val = self.get_stamp() + "\t" + str(voltage) +'\n'
                print val
                open(self.filename, 'a').write(val)

def setup():  
        ser = serial.Serial('/dev/ttyUSB0', 9600)
        return ser

if __name__ == '__main__':  
        if len(sys.argv)<2:
                print "need argument of <file path> <samples>"
                quit()
        ser=setup()
        writer1=writer(sys.argv[1])
        readings =0
        total=0.0
        samples=int(sys.argv[2])

        while 1 :
                val = ser.readline()
                val =val.rstrip()
                if val.startswith('Voltage'):
                        print "Reset"
                else:
                        print '\''+val+'\''
                        valIndex = val.find('-')
                        floatVal= float(val[(valIndex)+2:])
                        readings=readings+1
                        total =total+floatVal
                        if readings == samples:
                                voltage = total /readings
                                writer1.write_stamp(voltage)
                                #print float(voltage)
                                readings=0
                                total=0

Example raw data

This is read from the text file output by the python program.

2015-03-13-06:24:53     3.50966666667  
2015-03-13-06:25:53     3.42983333333  
2015-03-13-06:26:53     3.39016666667  
2015-03-13-06:27:53     3.4355  
2015-03-13-06:28:53     3.5545  
2015-03-13-06:29:53     3.71  

Taking it further

  • Include current sensor or shunt resistor to measure power generated - this is much more interesting than voltage because the panel will generate around full voltage even in dim lighting.
  • Automatically calibrate the power source to give accurate readings whatever is powering the device.
  • Log additional data such as temperature of the panel and consider how this affects the output
  • Put the panel/measuring setup in the garden in various places.

Alex Ellis

Read more posts by this author.

United Kingdom http://alexellis.io/

Subscribe to alex ellis' blog

Get the latest posts delivered right to your inbox.

or subscribe via RSS with Feedly!