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solar light detection (day/night) problems

I just want a detector to tell if its day or night. Easy, right?

I started out with a photo resistor (CdS Photoconductive Photocell I believe) in a voltage divider feeding a comparator. Worked fine on the bench. Worked fine outside the first day. Stopped working the second day (failed to detect night) and when I brought it in and tested it, the photo resistor was measuring a whole new range of resistances. For example it originally measured 1M ohm dark and 100k ohm light...after a day in the sun it reads 100k ohm dark and 10k ohm light. I believe this is due to saturation, but see no mention of this in a data sheet.

Next I tried a photo transistor. Simple enough... light hits transistor, transistor turns on. This worked fine for a few months, however a couple days ago it failed to detect dark. I swapped out the detector and brought the faulty one in to test. The transistor was on all the time...acting like it was in bright sunlight, even when it was in total darkness. This was after it had been in the dark for at least an hour. A day later, the transistor was working normally again. I believe this is also a saturation problem, and the device seems to recover after a while, but I'm looking for a solution that works all the time, not just most of the time. The data sheets don't seem to mention this problem, or if they do I'm not understanding what they are saying. Could someone help? I can switch sensors if thats the problem.

Thanks

Dan

Reply to
Dan K
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What voltage rail are you using for the phototransistor? Are you exceeding it's maximum Vce or current?

martin

Reply to
Martin Griffith

Just Vcc = +5v with a collector resistor of 33k and the emitter is grounded. Can't possibly be violating anything (unless I'm missing something). Spec says max collector current is 25 ma, so I'm about 150 times less than that. Max Vce is way more than 5v (around 30v).

Dan

Reply to
Dan K

Cad sulphide has high photoconductive gain but really really horrible hysteresis--you can get 5:1 resistance variations based solely on previous history. You can use it reliably for this job---a billion night lights can't be wrong, and they all use CdS. You just have to characterize the hysteresis over its range and design around it. The bright/dark ratio of CdS is huge, so 5X is actually not a serious problem for a coarse application like that.

The phototransistor should work--there are no such memory effects there. If it misbehaved and then recovered, it's a temperature or contamination issue, or a flaky part. I hate phototransistors because they're slow and needlessly noisy and unstable, but they work fine for night lights too.

(You might mount a small 120V coil relay on a $2 night light and use that!)

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Since I'm located down in a hilly "bowl", photocells looking for day/night changes are erratic at best.

So I cheat... I have a collimated tube with photodiode looking at the streetlight across the way. It's high enough up in the air to be a reliable indicator of day/night. (I look for an AC signal ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
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Reply to
Jim Thompson

I'm using a Vishay Tept5600 ambient light sensor, and it does what it says on the box.

martin

Reply to
Martin Griffith

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Were your sensors pointed right into direct sunlight?

They should be pointed downward, behind a shroud. You see them all the time on automatic parking lot lights and stuff.

Good Luck! Rich

Reply to
Rich Grise

and/or pointed towards the north (in the northern hemisphere). They should never see direct sunlight.

Reply to
PeterD

Hey, thanks you guys...thats probably it. The sensor is pointed straight up. So it will be in direct sunlight for a number of hours each day. I'll try turning it around and use a shroud.

Dan

Reply to
Dan K

And it should be on the north side of the building, so the shroud and enclosure don't heat up in the sun - overtemp will also damage the sensor.

Good Luck! Rich

Reply to
Rich Grise

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