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Wow these are small, Photo Light Sensitive Photoresistor

gary350

Well-Known Member
Wow these are small. Does being small make any difference how well photo resistors work or how sensitive they are?

We were gone camping several days arrived home today found these in the mail box. I have not had time to experiment with them yet. All I want to do is build this circuit then put it outside all winter. I want it to be off all day to save battery life. 2 D batteries on a Red LED last 4 months before dead. I hope 2 D batteries last twice as long with this circuit. Sun is low in the sky 11 hours 47 minutes of day light now and getting shorter. Our shortest winter day is about 9 hours of sun light.

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Mickster

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Most Helpful Member
You might want to replace the 100k resistor with a pot.
 

gary350

Well-Known Member
You might want to replace the 100k resistor with a pot.
I'm glad you mentioned that, I was thinking about that. Here in TN winter is overcast and very cloudy photo units we used at work to turn ON all the building lights had to be adjusted of the darkest days to make sure lights were never on all day in overcast weather. Last year we had 6 months of rain Dec to May.
 

rjenkinsgb

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Smaller LDRs often have a higher resistance for a given light level.
If the original circuit was built using a large LDR, you likely need to increase the 100K in proportion

You might want to replace the 100k resistor with a pot.
Good idea, that allows the threshold to be set - but make that a pot in series with a moderate value resistor, eg. 1M pot and 10K resistor.
That avoids the possibility of accidentally going too far with the pot alone and connecting the battery directly cross the transistor base-emitter junction..
 

gary350

Well-Known Member
I tested one of the LDRs, meter reads 500 ohms 12" from 100w incandescent shop light. 50K outside in the dark, no stars, no moon, street light 500 ft away. 1M in total darkness. Continue to stay in total darkness ohms goes up 1M, 2M, 3M, 4M, 5M, 6M, etc.
 

Mickster

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Test the rest and I'll bet you get different results.
Hence the suggestion of using a pot, along with RJ's series resistor, in order to trim the threshold.
 

gary350

Well-Known Member
Test the rest and I'll bet you get different results.
Hence the suggestion of using a pot, along with RJ's series resistor, in order to trim the threshold.
Your right. I tested all 20 LDRs with the 100w light I got an assortment of readings from 430 ohms to 606 ohms. With the 100w light bulb off and a small flashlight 12ft away shinning at the opposite wall making enough light for me to see the meter it reads 7800 ohms to 9460 ohms. In the dark they are all 1M ohm.
 
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gophert

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Your right. I tested all 20 LDRs with the 100w light I got an assortment of readings from 430 ohms to 606 ohms. With the 100w light bulb off and a small flashlight 12ft away shinning at the opposite wall making enough light for me to see the meter it reads 7800 ohms to 9460 ohms.
Poke the LED leads through a piece of black vinyl electrical tape and tape off the back side of the LDR to block stray light and make the LDR more directional. You'll see a significant increase in off-stake (dark) resistance.
 

Mickster

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If that circuit above is to be used for a number of night lights and you want them all to come on pretty much around the same time, you will probably either need to adjust them all individually with a pot, or closely-match the LDR's and stick with a fixed resistor.
Can't find it now, but there was something I read some years ago, which suggested using a 35mm film canister that had an LED in the bottom, and the LDR under test inserted into the inside of the canister cap, with the leads poking outside to take measurements and characterize each LDR.
The LED was off for the dark test, then on for the light test.
LDR's were compared and then selected for the closest matches.
IIRC, it may have been done for use in differential-steering, or light-tracking on a BEAM robotics project.

My 0.02.
 

gophert

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Most Helpful Member
If that circuit above is to be used for a number of night lights and you want them all to come on pretty much around the same time, you will probably either need to adjust them all individually with a pot, or closely-match the LDR's and stick with a fixed resistor.
Can't find it now, but there was something I read some years ago, which suggested using a 35mm film canister that had an LED in the bottom, and the LDR under test inserted into the inside of the canister cap, with the leads poking outside to take measurements and characterize each LDR.
The LED was off for the dark test, then on for the light test.
LDR's were compared and then selected for the closest matches.
IIRC, it may have been done for use in differential-steering, or light-tracking on a BEAM robotics project.

My 0.02.
Matching LDRs or adjusting with pots is a waste of time if he is not placing them all against a uniformly illuminated surface. Any variation 8n background light, surface reflectivity, ... will cause the carefully matched/calibrated sensors to trigger at different times.
 

Mickster

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Matching LDRs or adjusting with pots is a waste of time if he is not placing them all against a uniformly illuminated surface. Any variation 8n background light, surface reflectivity, ... will cause the carefully matched/calibrated sensors to trigger at different times.
"All I want to do is build this circuit then put it outside all winter. I want it to be off all day to save battery life. 2 D batteries on a Red LED last 4 months before dead. I hope 2 D batteries last twice as long with this circuit. Sun is low in the sky 11 hours 47 minutes of day light now and getting shorter. Our shortest winter day is about 9 hours of sun light. "
Adjusting pots is a waste of time?
Suppose his LED is currently on during daytime from 3PM right through to 9AM with a fixed resistor, when a pot adjustment could prevent that?
Are you going to supply his D batteries for him because you think it's a waste of time for him to be adjusting a pot?
He's probably going to make more than one of these to hang around his property, from past posts, so why not let him choose and characterize components, in order to create a base-line project that he can fine-tune for himself?
 

gophert

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"All I want to do is build this circuit then put it outside all winter. I want it to be off all day to save battery life. 2 D batteries on a Red LED last 4 months before dead. I hope 2 D batteries last twice as long with this circuit. Sun is low in the sky 11 hours 47 minutes of day light now and getting shorter. Our shortest winter day is about 9 hours of sun light. "
Adjusting pots is a waste of time?
Suppose his LED is currently on during daytime from 3PM right through to 9AM with a fixed resistor, when a pot adjustment could prevent that?
Are you going to supply his D batteries for him because you think it's a waste of time for him to be adjusting a pot?
He's probably going to make more than one of these to hang around his property, from past posts, so why not let him choose and characterize components, in order to create a base-line project that he can fine-tune for himself?
How are you confusing my opinion that some task is "a waste of time" with a vision that I am somehow not letting a man, more than 300 miles away from me, "choose and characterize components, in order to create a base-line project that he can fine-tune for himself?"

What kind of Jedi mind powers do you think I have?
 

Mickster

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Most Helpful Member
What kind of Jedi mind powers do you think I have?
No Jedi powers, just argumentative-???????????? powers.
Cheerio.

Come on guys, play nicely. - Moderator
 
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gary350

Well-Known Member
He's probably going to make more than one of these to hang around his property, from past posts, so why not let him choose and characterize components, in order to create a base-line project that he can fine-tune for himself?
I had 18 LED lights before I will probably have 18 LED lights again. Many of the houses in the neighbor hood have been broken into. Police notice my lights all over the property they came to the house to ask what type security system I have. LOL. Funny it even fooled the police. Police though I probably have a $15,000. high tech security system. LOL. Real problem is replacing all those batteries when they go dead they don't all go dead on the same day. All the batteries are soldered on. Another problem is being out in the weather LED wires rust & Transistor wires rust and rust gets on the bottom side of the transistors and LED and makes a short circuit. I keep having to clean off rust to keep the circuits working. Next lights I build I am going to incase them in wax for water proffing. I will have to put a round plastic tube over the LDRs to block light from the LED. LDRs are sensitive they detect light going through my finders when I put my fingers over them to block light meter does not show a total darkness reading.
 
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audioguru

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A fake security system? just a red LED that does not even blink on and off?

My daughter bought me a fake security camera in a dome that looks like the cameras in stores. It has a red LED that blinks on and off when it is triggered by a motion detector inside it. It does not have a camera so it was very cheap.

A neighbour has a fake security system that looks like a camera and when a motion detector inside it is triggered then it announces on a speaker, "Beware, you are being video recorded". It has no camera and does not record anything.
 

gary350

Well-Known Member
A fake security system? just a red LED that does not even blink on and off?

My daughter bought me a fake security camera in a dome that looks like the cameras in stores. It has a red LED that blinks on and off when it is triggered by a motion detector inside it. It does not have a camera so it was very cheap.

A neighbour has a fake security system that looks like a camera and when a motion detector inside it is triggered then it announces on a speaker, "Beware, you are being video recorded". It has no camera and does not record anything.
A red light that blinks is 10 times more noticeable after dark than a red light that does not blink. Once I build a good working unit that turns ON after dark then I will combine it with a blinker.
 
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rjenkinsgb

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Add a bit of positive feedback so it switches sharply rather than slowly , plus a slight delay, and let the LDR see the lamp.
You then have an automatic nightime flash unit.
 

gary350

Well-Known Member
This circuit works.

I have another circuit drawing that shows how to make a blinking LED turn ON after dark I will build it next. I assume this circuit will work with LED 1 removed?

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gary350

Well-Known Member
Circuit 1. I have this circuit built and it works GOOD. This combines circuit 2 & 3. Flash rate 1 sec each LED with 100 uf. After lots of testing IN & OUT of the dark I replaced the 1M adjustable resistor R1 with a 47K fixed resistor. R1 & R2 can both be replaced with a 57K resistor if I had one. Photo resistor is so light sensitive it goes from 500 ohms to 1M ohm with a micro small amount of light.

Circuit 2. This is the sun light detector it is extremely sensitive to sun light 1M resistors is changed to 47K.

Circuit 3. This is the LED light blinker that works best. Work well on 3v with 2 D batteries that last 4 months.

Circuit 4. This LED blinker does not work well on 3v too dim to see in the dark. Works on 4.5v batteries last 3 months.

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