Sensing output from a solar panel

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stocky6409

Senior Member
I'll post a schematic once I have adjust various sections to keep some members happy :)

I have a 5v 40mA solar cell charging 3 x AA nicads via a Schottky diode with the output via an LDO powering an 08M & Xbee module.

I'm using the 08m to enable/disable the xbee "sleep" pin so that i can sleep the xbee for periods longer than the maximum internal "cyclic" sleep period of 268sec.

I want to have the xbee send updates every (approx) 15 minutes during daylight and hourly during darkness.

I could use an LDR to sense the light levels BUT that would mean i have to expose another sensor in what is an environmentally harsh location. I thought if i could sense the output from the solar cell that would be a better solution.

Any suggestions? A simple transistor with its base fed via a suitable resistor would seem logical - or maybe a small fet - pulling a pin low on the picaxe? Then that can be acted on in code to select one of two timing loops (day/night)

Stocky
 
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stocky6409

Senior Member
Thanks Manuka - yes green leds are handy for that - i've used them before for solar tracking applications :)


Just bb'd that up and it works well - just have to decide if i really want to drill another hole in the case........
 
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moxhamj

New Member
Even though the internal resistance and other picaxe factors mean 10k is recommended for driving analog input into a picaxe, I suspect if you are just deteting 'light' or 'dark' you could get away with much higher resistances. I reckon 100k or even 1meg but it would be a quick protoboard experiment.

Just for starters, how about two 100k resistors in series direct from the solar panel and feed the midpoint into a picaxe. Then try 1meg. The load on the solar panel ought to be negligable yet you should get enough of a reading to detect light or dark. I agree it would be good to avoid another hole in the case.
 
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stocky6409

Senior Member
G'day Dr_A

DOH! I'll give that a bash - always try and over complicate things don't I! [facepalm]

with correct resistor selection i should be able to set the "trip" point without to much hassle

Stocky
 
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stocky6409

Senior Member
Just for everyones benefit - Manuka's green LED idea works rather well!

Using a 14M I have a status LED on output pin5 which can also be an ADC input when reconfigured.
In my program I just convert that pin to an input, take the ADC reading them revert it to an output for the status LED again.
The led in this app only needs to blink once every 1-5 seconds so its available as a light sensor more often that its needed as a LED!

:)

I will probably still go with Dr_A's voltage divider for the final app as it saves one more hole in the box but the LED idea is something I had forgotten from a long time ago.

Stocky
 
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mikie_121

Member
With no load on the solar panel you will get maximum output voltage (or 95% of it) in low light/shade. So consider this if you are relying on the voltage reading from the panel.
 
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