Photoresister

[Howsomever]

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,

[chroot]

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

[davenn]

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,


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 .....

http://www.physicsforums.com/attachment.php?attachmentid=40870&stc=1&d=1321048567

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

[Howsomever]

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.................................

[AdrianN]

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.

[davenn]

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.................................

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


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.

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

[jim hardy]

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

[davenn]

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

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.....

http://www.physicsforums.com/attachment.php?attachmentid=40881&stc=1&d=1321086261

cheers
Dave