Which Resistor?

Hi All.

I am putting together a Solar Tracker for my Weather Station Battery Charging System. Having spent many days studying the subject, i have just one question.

Sunshine hours where i live (Central UK) vary from season to season, with obviously more daylight hours in Summer than in Winter. Most of the articles i have read on sizing the R2 resistor for the LDR (I am using 4 NORPS-12's) seem to be aimed at Steady Climates. How can i go about sizing a resistor to cover year round operation, though i will probably put the station on mains power in the winter, and bring the battery indoors.

Can anyone help with good advise please.

You'll probably just have to experiment... Try to get a good "mid range" reading on an "average" day.

Another option would be to use more than one resistor... i.e. a "resistor ladder"... into more than one analog input. Then depending on the readings, choose the "right" analog input for the moment.

Or, with the normal setup you could switch to the optional 1.1V ADC reference whenever the voltage is low. (I have a totally different application where I automatically switch between the 5V and 1.1V references.)

In my experience an LDR will respond well over a wide range of lighting conditions without needing to be fussy about the resistor value. Try 68k.

...R

The light levels don't really change all that much from Summer to Winter, it's just the length of the day that changes. So a single resistor should be fine.

Steve

An LDR is also not the most accurate, best used in a light on vs. light off situation.

There are many light level sensors out there that give you the actual light level in lux. Those are much more suitable for keeping track of light levels.

wvmarle:
An LDR is also not the most accurate, best used in a light on vs. light off situation.

There are many light level sensors out there that give you the actual light level in lux. Those are much more suitable for keeping track of light levels.

For solar tracking all you need is something that can tell if it's brighter to the right or to the left. I reckon LDRs will be fine for that.

...R

It sounds to me that OP is looking for tracking the actual light levels (as they care about total daylight hours and sunshine hours and so), rather than where the sun is. The latter can probably easier be done using a clock, a solar calendar and a stepper or servo. Just point your solar panels to where the sun should be.

Oh I don't know "Solar Tracker for my Weather Station Battery Charging System" sounds like he just wants the solar panel pointing in a reasonably effective direction. Though of course if you want real efficiency you need to change inclination not just direction.

The problem with UK weather it that even in notional Summer it is often rather cloudy so you may find the system hunting madly as it tries to find the brightest patch of sky which keeps moving.

Steve

slipstick:
The problem with UK weather it that even in notional Summer it is often rather cloudy so you may find the system hunting madly as it tries to find the brightest patch of sky which keeps moving.

Just allow it to update its position once every 5 minutes or so.

...R

Hi all.

Many thanks so far for your great advice.

wvmarle:
An LDR is also not the most accurate, best used in a light on vs. light off situation.

There are many light level sensors out there that give you the actual light level in lux. Those are much more suitable for keeping track of light levels.

Hi wvmarie, i chose LDR's initially because i have a few knocking about, i also have a BH1750 Lux Sensor, would this be better ?, what about the Grove v1.2 or the TSL2561 or the LM39 ? too many choices for a novice like me, can you make a recommendation ?

What worries me though, is this, the fact i want a Sun Tracker, mounted on top of my weather station, means it is outdoors, how do you mount / fix these small modules and keep them weather proof but working properly?

Of those I have used the TSL2561 and its successor the TSL2591 and they both give good results (seemingly: I don't have a reference to the absolute lux numbers). The TSL2591 has a higher range.

Generally the best thing to do is to simply try them out. Good chance they all work just fine for this application.

Thanks wvmarie,

What about the weather proofing, can these sensors be used behind glass or clear plastic?

Should be no problem - I've used them behind a milky dome (as used for PIR sensors and so). Exposed to the sun directly the TSL2561 will overload (max 65535 lux); the TSL2591 can do a bit more, over 100k lux, but direct sunlight will still overload it.

When used that way you may need to calibrate: measure with and without the dome. I didn't bother doing that as for now I'm not too interested in the actual values, just the patterns.

Hi wvmarie.

When you say that the TSL2561 / 91 sensor will overload 0ver 65535 lx, does this mean the sensor can be damaged in some way ? i have a BH1750 sensor, that to states a Max. of 65534 lx, will this also get damaged in bright sunlight, i can find no information in the Internet that can advise me, the datasheet does not say do not use over 65535 lx, so i am somewhat confused, Direct sunlight, depending where you look for information can be anything from 32000 > 130000 lx.

I doubt you can damage them with too much light (assuming it's not too hot or so), it's just that the detector is saturated.

The TSL2591 measures up to 88,000 lux.

Hi al.

Many thanks, my digital weather station is now up and running.