solar light detection (day/night) problems

[Dan K]

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

 

[Martin Griffith]

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

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

martin

 

[Dan K]

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

martin

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

 

[Martin Griffith]

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

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


martin

 

[Rich Grise]

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 ....
Next I tried a photo transistor. Simple enough... light hits

....

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

 

[PeterD]

...

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

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

 

[Dan K]

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

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

 

[Rich Grise]

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.

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