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Using a circuit with a photoresistor

  1. Nov 11, 2011 #1
    Hello Wizards,

    I am using a circuit with a photoresistor that sits under the eave and senses ambient light to turn on a light at night to shine on an American Flag. This works fine for a month or two and then quits working. What I find is the photoresistor goes bad. Ambient light in the shop the photoresistor is approx 8K ohms and when I cover it it goes to 2 meg ohms. After the photoresistor goes bad I find at ambient light in the shop it is 2.7k ohms and when I cover it it goes to 3.7k ohms. After going through three of these photoresistors I wonder if moisture is the cause. I will say it works fine night after night than just quits. Thinking moisture or humidity is the culprit I coated my last replacement photoresistor in epoxy. Any thoughts or comments?? The power source for the circuit is a 1000amp hour battery charged by a 75watt solar panel. Outside temperatures at this time of the year are in the 40's.

    Thanks in advance for any responses,
     
  2. jcsd
  3. Nov 11, 2011 #2

    chroot

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    Re: Photoresister

    There are all kinds of different photoresistors! Have you looked at the datasheet for the one you're using, to determine its susceptibility to moisture? Have you checked the voltage range over which it is meant to be used?

    - Warren
     
  4. Nov 11, 2011 #3

    davenn

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    Re: Photoresister


    Ohh my gosh

    I hope you dont just have the photoresistor in series with the lamp and the battery ?

    So for example if the lamp is a 12V spotlight type shining on the flag and it draws 2Amps therefore you have 2A flowing through the poor little photoresistor ??
    They are not designed for that sort of harsh treatment.

    The photoresistor should be used to turn on a transistor which in turn will switch power to the lamp. Please explain/ show the circuit of how you have it wired.

    for something really basic it should be similar to these circuits I have drawn for you .....

    attachment.php?attachmentid=40870&stc=1&d=1321048567.gif

    Now I have shown you two circuits above. The left one is for use where you can use a high current capability transistor. Ensure its ratings is much higher than the current through the lamp.
    The right circuit instead uses a transistor to switch a relay. This time just make sure the relay contacts can handle the current.

    Circuit Explanation

    With a properly adjusted trimpot resistor, the voltage at the base of the transistor in the dark state is less than 0.7 V, and therefore the transistor is in the cut-off state. Since the transistor is cutoff, no current flows from its collector to its emitter, so the LAMP will be off. As the photoresistor’s resistance decreases (as the result of an increase in light intensity), the voltage at the base increases due the voltage divider formed by R1 and the photoresistor. Once the base voltage reaches 0.7 V, the base current starts to flow, and any further decrease in the photoresistor’s resistance causes an increase of base current. This base current increment will be amplified by the current gain of the transistor up to the point that the transistor saturates.


    cheers
    Dave
     

    Attached Files:

  5. Nov 11, 2011 #4
    Re: Photoresister

    Hey thanks for the responses,

    The photoresistor is in series with 100k ohm resistor across 12vdc and off the center if this is a resistor 4.7k ohm driving the first of three transistors. the photoresistor is rated at 80mw and series VT is 900 whatever that is. Also rated from -40C to +75C..........number is VT935G.

    Thanks again for all responses.................................
     
  6. Nov 11, 2011 #5
    Re: Photoresister

    Sounds like long term damage due to accumulated stress. Photoresistors can be easily damaged by voltage spikes. You have a 12V battery, so no spike there, but the battery charger can bring spikes into the system. I would look with a scope at the photoresistor voltage, during battery charge. If you see spikes, connect a ceramic capacitor across the photoresistor.
     
  7. Nov 11, 2011 #6

    davenn

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    Re: Photoresister

    ok cool :) so similar arrangement to my left circuit with an additional 2 transistors


    yup I agree, stress from voltage spikes, moisture or temperature. keep in mind it may not necessiarily be the temp extremes, but rather, the constant hot / cold cycling of day / nite

    a few things for you to ponder anyway :)

    Dave
     
  8. Nov 11, 2011 #7

    jim hardy

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    Re: Photoresister

    just a guess

    daylight is mighty intense and rich in UV

    try covering your photoresistor with a lens from old high quality sunglasses - every junkshop has a bin of scratched up ones.
    it might make a rain shield too...

    the epoxy sounds like a good idea. let us know...

    old jim
     
  9. Nov 12, 2011 #8

    davenn

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    Re: Photoresistor

    Yes agreed :)
    but we are concluding thats its not in direct sunlight or rain, as the OP stated that it was "under the eave" which infers that it was shaded :)

    Still thinking about this voltage spike (variations) caused by the solar panel charging.
    Am thinking it may be worth running the CdS cell via a 10V 3 terminal regulator. The change from around 12V with much higher peaks, to a good steady 10V much be better for it.

    say something like this.....

    attachment.php?attachmentid=40881&stc=1&d=1321086261.gif

    cheers
    Dave
     

    Attached Files:

  10. Nov 28, 2013 #9
    Same type of issue

    Hi,

    I have a similar issue. I use a photoresistor to trigger a chicken coop door. It's now the second time it failed after a couple of weeks, in general after some very moist or foggy days. My circuit is a low voltage circuit, regulated by a LM317. Under normal circumstances there are two small spikes during the day, so in the morning to open the door and during the evening to close it.

    So I wonder if you were able to solve your issue and how you succeeded to do it.

    Regards,

    Carlos
     
  11. Nov 29, 2013 #10

    NascentOxygen

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    Perhaps you could recast your circuit to use a phototransistor in place of the photoresistor?

    The source of a tough photoresistor may be a discarded garden solar lamp, some of these have a photoresistor (it's unnecessary, but they have been designed to rely on it).
     
  12. Nov 29, 2013 #11
    Hi,

    When I first started I wasn't expecting these type of issues with an epoxy resin encapsulated LDR. So my first reaction was to buy the same LDR in an hermetic package. I came across some articles related to photo transistors, so if this hermetic package LDR has the same symptoms I'll certainly replace it by a photo transistor. Unfortunately I don't have a discarded garden solar lamp that I can scavenge, so I'll have to look for a photo transistor. I'm not sure about which one to choose, most of them are optmized for IR and have a daylight shield. So if you can give me any hint on that, I would much appreciate. Of course I could also train my chickens to use IR led glasses :tongue:
     
  13. Nov 29, 2013 #12
    Hello Carlos and the other Wizards..........it has been about a year since I have thought about this issue. First of all as suggested I made some changes......unfortunately more than one. What I did was add a .1 mic cap across the power feed to my circuit. Then I placed a amber pill bottle case over the photo resister. For whatever reason this made it last till two weeks ago. The photo resistor was replaced and I noticed the amber pill bottle case was sun bleached so I replaced it also. Hope it last another year.......I do have some more spare photo resistors. Better but not quite there.........thanks for all input!
     
  14. Nov 29, 2013 #13

    NascentOxygen

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    I don't have one in mind, but direct sunlight can probably penetrate to most phototransistors. Experimentation would be needed. The preamble in this sun-tracking servo article mentions the 3DU33 (though I can't see that Part Nº in the actual schematic). There is a cheap source on eBay of the 3DU33: http://www.ebay.com/itm/10pcs-iron-pack-Si-Phototransistor-3DU33-/261080403764

    Or you could simply regularly replace your LDR :smile: if it were low cost: http://www.ebay.com/itm/20PCS-Photo...hoto-Resistors-Light-Dependen-t-/261043173479
     
  15. Nov 29, 2013 #14

    NascentOxygen

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    Hello Howsomever, welcome back after your 2 years' absence! :smile:

    I think plain glass blocks most of the UV light, so your LDR probably enjoys less UV stressing it. Sounds like you have hit upon a workable solution.

    Just the same, I'll be watching for your next update in Nov 2015 .....
     
  16. Nov 30, 2013 #15
    Time flies when you are having fun........my slip it has been a couple of years since I first posted. I like the plain glass comment NascentOxygen......the amber pill bottle is plastic and may not be the best choice. Great forum by the way and thanks for the great input!

    Just a retired communications technician here with a FCC radio telephone license............
     
  17. Dec 1, 2013 #16
    Thanks guys, for all the replies. I live in Belgium, hot sunny days are not that common over here, we tend to go with cloudy rainy days :cry:

    Since I had ordered a bag of 10 extra LDR's when my first one broke down and was only left with one spare, I can still manage for a couple of months. The hermetical packaged ones are a dead end, the supplier wants 80$ for 5 of them. For that price I can buy new chickens when they get eaten by the fox.

    Nevertheless, I followed some guidelines on this board. I made a small PCB to properly seat the LDR, in parallel I put one big tantalium condensator to slow down the sensitivity of the LDR and one small ceramic one to cut of the spikes.

    I put this in place so I have some time now to look for the phototransistor alternative. I think this is indeed the right solution for my problem. One good reason that everyone should consider is that the LDR is not RoHS, it contains Cadmium and this is a very toxic product for the environment.
    I also believe the LED package style is more reliable in moist environments. They also have darkened ones, these are more fit for IR applications but are still able to distinguish dark from light, maybe this is the right fit for you Howsomever.

    For me, I'm looking to buy a BPV11, it's a three lead phototransistor which gives me a little bit more flexibility, so I can tune it right for the transition between light and dark. I dont' want to shut out my chickens. With the LDR I got it tuned to a 10 minute gap between the chickens and the door closing, I'm convinced I can achieve the same with the phototransistor.
     
  18. Dec 1, 2013 #17

    vk6kro

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    Under eaves your phototransistor would be shielded during the day, but would still be vulnerable to the rising or setting sun.

    This may be why it functions for days or weeks but then fails. When the sun is in the right direction, the lens focuses the sun's rays and melts the phototransistor.

    If this seems possible, maybe you could let the phototransistor look down a tube in a direction where the sun cannot shine.
     
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