Hi,
@ Werk: Sorry, I split apart my first two paragraphs at the last moment; I was considering 1500 W/m2 (i.e. 300 mV) as "full scale" in my "back of envelope" calculation (actually done in my head). I don't know (didn't check) what scaling factor the software actually uses; I think we agree that the minimum gain (at end of track ~= 5k) is at, or just over, 3.00 .
@danner: So it looks as if there is a "zero offset error" on the ADC input of your other Transmitter. Try connecting the "solar" input pin (preferably through a "safety" resistor of say 1 kohms) to various points on the PCB which "should" be earth (zero volts) and see what the software reports (preferably with little or no clipping applied).
I think we should assume that the "precision" calibration aspect of the SP-110 applies to high level (i.e. peak daytime) solar measurements. As Zdenek says, measurements around and below 1 mV are always difficult. Also, there are many other "complications" in making measurements when the sun is at low elevation (close to the horizon). In particular, the "Cosine response" of the (required) vertical-facing sensor gives a much higher proportion of the light from the "sky" compared to the sun itself.
IMHO if you want to improve the reporting of "sunrise/sunset" levels, the Op-Amp needs both gain and offset ("zero") potentiometers. Perhaps you could use the UV channel amplifier (if you don't plan to use it for an analogue UV sensor). The Weatherduino PCB is not an "ideal" starting point but you could try the following:
Swap the 5k and 50k pots (of the UV channel) so that the 5k goes to the output of the Op-Amp (it will still be used to set the gain, but now increasing its resistance will increase the gain). Put an accurate 1k resistor across the wiper-to-active end of the 50k pot (i.e. Op-Amp input to ground). Ensure that the wiper of the 50k is always towards the "open" end, then the gain of the Op-Amp will be adjustable (with the 5k) from about 1 to 6.
Then the 50k (R4) might be used for the zero offset adjustment, but I'm not sure how best to do that yet. It will need the addition of a few high value (Megohm) resistors and a slight modification to the software (subtracting a few W/m2 to accommodate the "sit up" of the output voltage from the Op-Amp). More details if you decide to follow this route.
Cheers, Alan.
(12-10-2017, 10:33)AllyCat Wrote: .. at 1500 w/m2 the SP-110 should give 300 mV output. .... I believe to give "full scale" with the 1.1 volts nominal reference voltage of the Nano needs a gain of 3.667, which I calculate requires 3.75 kohms.
@ Werk: Sorry, I split apart my first two paragraphs at the last moment; I was considering 1500 W/m2 (i.e. 300 mV) as "full scale" in my "back of envelope" calculation (actually done in my head). I don't know (didn't check) what scaling factor the software actually uses; I think we agree that the minimum gain (at end of track ~= 5k) is at, or just over, 3.00 .
@danner: So it looks as if there is a "zero offset error" on the ADC input of your other Transmitter. Try connecting the "solar" input pin (preferably through a "safety" resistor of say 1 kohms) to various points on the PCB which "should" be earth (zero volts) and see what the software reports (preferably with little or no clipping applied).
I think we should assume that the "precision" calibration aspect of the SP-110 applies to high level (i.e. peak daytime) solar measurements. As Zdenek says, measurements around and below 1 mV are always difficult. Also, there are many other "complications" in making measurements when the sun is at low elevation (close to the horizon). In particular, the "Cosine response" of the (required) vertical-facing sensor gives a much higher proportion of the light from the "sky" compared to the sun itself.
IMHO if you want to improve the reporting of "sunrise/sunset" levels, the Op-Amp needs both gain and offset ("zero") potentiometers. Perhaps you could use the UV channel amplifier (if you don't plan to use it for an analogue UV sensor). The Weatherduino PCB is not an "ideal" starting point but you could try the following:
Swap the 5k and 50k pots (of the UV channel) so that the 5k goes to the output of the Op-Amp (it will still be used to set the gain, but now increasing its resistance will increase the gain). Put an accurate 1k resistor across the wiper-to-active end of the 50k pot (i.e. Op-Amp input to ground). Ensure that the wiper of the 50k is always towards the "open" end, then the gain of the Op-Amp will be adjustable (with the 5k) from about 1 to 6.
Then the 50k (R4) might be used for the zero offset adjustment, but I'm not sure how best to do that yet. It will need the addition of a few high value (Megohm) resistors and a slight modification to the software (subtracting a few W/m2 to accommodate the "sit up" of the output voltage from the Op-Amp). More details if you decide to follow this route.
Cheers, Alan.