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Photocell Resistance - On or Off

 
 
CDobyns
 
Reply Mon 24 Dec, 2018 03:37 pm
Right up there in one of my regular category of repair projects best characterized as "cracking a walnut with a sledgehammer", I want replace a failed photocell that's part of an ancient electric window candle light my wife inherited (probably indirectly from the Edison family . . .).

These window candles have a very small circuit board in wiring circuit, which is clearly able to measure the ambient amount of light through the semi-opaque candle "stem", and will close the circuit to the bulb when the light detected by the little photocell on the circuit board reaches a certain threshold (and opens the circuit to turn off the bulb in daylight).

I've tested the photocell (and everything else on in the wiring circuit, including of course the bulb), and there's continuity throughout - except across the photocell terminals (and yes, it was tested in light and in complete darkness).
https://drive.google.com/uc?id=1-jg_mWHLtbI_kFC2jlQBcGdKpF7KknM9

With the availability of "everything" on the Internet nowadays, I can actually purchase a replacement photocell for less than a $1.00, and I'm completely capable of removing/soldering in a new component. The one thing I'm hazy on is that photocells trigger desired actions in either the presence of light or absence of light (I want to "trigger" in the absence of light). The specs on most of these photocells state that in light, the resistance is about 5-10KΩ, but when dark the resistance goes up to 200KΩ. I'm right at the "outer limits" of my electronics know-how (the pitfall of being self-taught). How do I know what's "off" and what's "on" relative to these resistance values? And can one (type of) photocell do what I need it to (on at dark, off at light)? It almost seems like you would need two different types of photocells to accomplish opening or closing a circuit in daylight or darkness.

As I said at the very beginning, clearly a case of "sledgehammer vs. walnut", but I thought this would be fun to do, versus just buying a new window candle.
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engineer
 
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Reply Mon 24 Dec, 2018 05:09 pm
@CDobyns,
I think you are overthinking it. The photocell likely is in the triggering circuit for a transistor that is controlling the light. As long as you have the right resistance on the photocell, you are fine. If you want to play, you could replace the photocell with a variable resistor and see what resistance turned on and off the candle.
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