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I want to link a PAR quantum sensor (either Licor or Skye Instruments) to Arduino to log PAR, in order to measure light in a forest. I have an Arduino Uno + data logging shield (with RTC, SD card and ADS1115) and have connected the PAR sensor via A0 to measure differential voltage (A0 –– S+ | S- –– GND | A1 –– GND). Code as follows:

  ads.setGain(GAIN_TWOTHIRDS);
  adc0 = ads.readADC_Differential_0_1();
  Voltage = (adc0 * 6.144)/pow(2, 15);
  Voltage = Voltage*1000; // conversion to millivolts

When I wire the Arduino to itself to measure a differential voltage (A0 –– 3V and A1 –– GND), it measures beautifully.

Furthermore, when I use a voltmeter to test the Voltage across the + - wires from the sensor, I get sensible readings.

Put the two together – nonsense. I get noisy V readings that don't make sense. When I cover the sensor my V is actually going up, not down as it should! I have tried changing the GAIN, but unfortunately this makes no difference to the problem.

I have been looking at PARduino for help/inspiration, but they include an amplifier in their setup, which is expensive and I'm not sure is necessary / would solve my problem.

Very new to Arduino. Any and all advice would be very much appreciated.

  • 1
    Could you connect the sensor to the Arduino and, at the same time, to the voltmeter? Do you still get sensible readings from the voltmeter if you do so? – Edgar Bonet Dec 10 '19 at 8:24
  • Please provide a link to the datasheet of the used sensor. I'm not sure, if you can simply omit the amplifier – chrisl Dec 10 '19 at 8:51
  • Thanks for your comments Edgar and chris. The problem must have been arising from a faulty board or some such. In the end, I was able to wire up the sensor as described above, using a more fleshed-out version of the code. No amplifier necessary, you can just adjust the GAIN. I then ran my Arduino sensor alongside a Skye Instruments SpectroSense 2+ to 'calibrate' it. Plotting the Arduino voltages against the SpectroSense PAR values gave quite a nice fit! Apologies for the long delay in replying – was on fieldwork. – sinderJ May 12 at 4:18

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