Wednesday, May 18, 2011

In electronics we trust

Having measured the LDR resistance from dusk to night I also wanted to find out the dawn values. I managed to actually hear the alarm on my cell phone and the alarm clock and got up quarter to five today. It was still dark. Turned on the Fluke 8842A which was already hooked up to the breadboarded LDR circuit outdoors.

I was gazing at the DMM. I was puzzled. Instead of giving me a stable reading, the numbers on the readout kept creeping upwards. I thought it would settle after a few seconds--after whatever capacitance had fully charged/discharged--but it didn't.The reading started at around 350mV and just kept climbing by a couple of millivolts per second. What the ...

After some 10 minutes--when the sky visibly began to light up--only then did I realize that photons from the sun had already began to trickle in even before I woke up. That would explain the gradual increase in voltage. The LDR circuit was actually doing its job and doing it well. It was picking up the minute traces of sunlight at that fuzzy divide between night and dawn.

Lesson: Trust your instruments more than your oh so fallible and insufficiently sensitive senses.

Lesson #2: Wake up even earlier to do the measurements!

I've created a Google spreadsheet which includes graphs of the morning and evening LDR values. I will be updating the spreadsheet with new data as they come in. There are two "sheets" in there labeled "dusk to night" and "predawn to sunrise." The sheet tabs are at the bottom left of the page. Click to move from one sheet to the other.

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