photosensor value question

[jcostantino]

I'm repairing a day/night sensing outdoor lamp circuit and need some
advice.The photosensor in the lamp that won't cycle on/off is badly
corroded so I took readings from the other side which still works.

The problem is my meter only goes up to 2 mohm and the total dark value
is more than 2 mohm. The full light side seems to be 10k ohm held up to
a 75 watt bulb so I have no issues measuring that.

I've found a 10k light - 20 mohm dark, 16k light - 500 mohm dark, 5k
light - 20 mohm dark, should I just buy one of each and see wha happens
when I swap them in? My guess is the 10k/20 mohm ought to work but I
may as well hedge my bets for the extra couple bucks.

 

[Homer J Simpson]

I'm repairing a day/night sensing outdoor lamp circuit and need some
advice.The photosensor in the lamp that won't cycle on/off is badly
corroded so I took readings from the other side which still works.

The problem is my meter only goes up to 2 mohm and the total dark value
is more than 2 mohm. The full light side seems to be 10k ohm held up to
a 75 watt bulb so I have no issues measuring that.

I've found a 10k light - 20 mohm dark, 16k light - 500 mohm dark, 5k
light - 20 mohm dark, should I just buy one of each and see wha happens
when I swap them in? My guess is the 10k/20 mohm ought to work but I
may as well hedge my bets for the extra couple bucks.

You could use resistors or a 5 M pot to figure out the settings needed.

But is it worth it? These things are about $10 right?

 

[jcostantino]

Thanks for the idea - unfortunately I can't buy a sensor module by
itself, it's a fairly complex circuit. The lamp uses a photosensor to
turn the light itself on and off and has a second stage that uses an
ultrasonic sensor to detect motion. The lamp is off during the day and
either on at night at half power or on full brightness when it detects
motion. Plus, there are switches to set the intervals for how long it
is on full bright after detecting motion and how long the lamp stays on
after sunset.

I've found an identical replacement lamp on Ebay for about $50 plus
shipping and I'd rather not need to spend in the area of $65 with
shipping to replace the whole thing.

Also, in full sunlight it now shows 1.1k so I have a huge range of
possibilities. Your idea of using the pot should work out very well to
determine the high and low thresholds and I'll order one of those along
with a handful of photosensors.

 

[redbelly]

I'm repairing a day/night sensing outdoor lamp circuit and need some
advice.The photosensor in the lamp that won't cycle on/off is badly
corroded so I took readings from the other side which still works.

Is the one that doesn't work stuck in the on or off position? Also, is
there an adjustment for the on/off threshhold somewhere on your device?
What happens when you adjust it from min to max and back again?

The problem is my meter only goes up to 2 mohm and the total dark value
is more than 2 mohm. The full light side seems to be 10k ohm held up to
a 75 watt bulb so I have no issues measuring that.

It's possible to measure resistances outside your ohmmeter's range.
Get a 1 Mohm resistor, measure it's resistance.
Put your photsensor in parallel with the 1 Mohm, and measure the
resistance again.
Use a little algebra to solve for the unknown resistance:

1/Runk = 1/Rpar - 1/Rref

where
Runk is the unkonwn resistor
Rpar is the measurement of the parallel combination.
Rref is the measurement of the 1 Mohm (or the highest, measureable
resistor that you have lying around)

This would have reasonable accuracy for 20 Mohm resistors, but probably
is rather innaccurate for 500 Mohms, though you'd still have a ballpark
value.

Mark

 

[jcostantino]

Is the one that doesn't work stuck in the on or off position? Also, is
there an adjustment for the on/off threshhold somewhere on your device?
What happens when you adjust it from min to max and back again?

It's stuck on. The only adjustment that I can see is for setting the
motion sensing range and not the day/night trigger which turns the
light on after the photosensor sees that it is dark already.

It's possible to measure resistances outside your ohmmeter's range.
Get a 1 Mohm resistor, measure it's resistance.
Put your photsensor in parallel with the 1 Mohm, and measure the
resistance again.
Use a little algebra to solve for the unknown resistance:

1/Runk = 1/Rpar - 1/Rref

where
Runk is the unkonwn resistor
Rpar is the measurement of the parallel combination.
Rref is the measurement of the 1 Mohm (or the highest, measureable
resistor that you have lying around)

This would have reasonable accuracy for 20 Mohm resistors, but probably
is rather innaccurate for 500 Mohms, though you'd still have a ballpark
value.

Mark

I'm going to give that a shot as well. I came across a meter here that
has a 4m ohm range and I'm ordering a new (better) meter along with the
photocells since I need a better one anyhow.

 

[redbelly]

It's stuck on. The only adjustment that I can see is for setting the
motion sensing range and not the day/night trigger which turns the
light on after the photosensor sees that it is dark already.

It would be good to make sure the photosensor is really the cause of
the problem. You could replace it with a small resistor, say 100 ohms,
and see if that shuts off the light. If not, then the problem is
somewhere else in the circuit.

Mark