Electronics > Projects, Designs, and Technical Stuff
Surprise photocell circuit.
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schmitt trigger:
For many years now, as my mind ages and my attention span wanes, I have decided to attach automatic photocell on/off circuits to the lights outside the home.
Otherwise I would forget and they would remain on all day long.... |O

One such photocell started behaving erratically, meaning that it really required a lot of sunlight to turn the light off. I replaced it, but instead of trashing it, decided to tear it down.
I've learned a few habits from lurking in this forum.  ;)

The circuit really surprised me! I kind of expected of cheap and dirty electronic circuit, with at least a pair of semiconductor devices.
To my surprise, the system is electro-thermal-mechanic. Not a single silicon device!

The photo, which I have annotated, is self explanatory.

Question: are all street light photocells built the same way?


T3sl4co1l:
Heh, that might explain some lamps turning on and off too (though AFAIK that's also a self-protection mechanism if they don't start properly?).

Tim
james_s:
That's the standard way those small photocells are made, I'm not sure I've ever seen one that was solid state.

They're pretty clever, a CdS cell in series with a heater wound around a bimetal strip. Light drops the resistance of the CdS which increases the current through the resistance wire which heats the bimetal strip which snaps open the contacts and turns off the light. This gives you a built in delay with some hysteresis, it's simple, inexpensive and robust, able to tolerate power surges, current surges from incandescent lamps burning out and inductive loads.
james_s:

--- Quote from: T3sl4co1l on October 17, 2019, 03:14:29 am ---Heh, that might explain some lamps turning on and off too (though AFAIK that's also a self-protection mechanism if they don't start properly?).

Tim

--- End quote ---

If you're referring to HPS lamps cycling, that happens because the arc voltage increases as HPS lamps age and also the arc tube runs hotter as it blackens and absorbs more energy. A hotter arc tube has a higher pressure which also increases the voltage required to sustain a discharge. At some point the voltage needed exceeds what the ballast can supply and the lamp extinguishes, cools off and restrikes, then the cycle repeats. Left unchecked it's not uncommon for the repeated ignition pulses to eventually break down the insulation on the ballast windings or burn out the ignitor.

Some crazy people actually believe that their bodies interfere with streetlights and cause them to extinguish. The phenomenon is not too surprising when you consider how many thousands of streetlights are in a typical city with a percentage of them likely to be cycling with tired lamps at any given time.
mikerj:

--- Quote from: james_s on October 17, 2019, 06:38:15 am ---They're pretty clever, a CdS cell in series with a heater wound around a bimetal strip. Light drops the resistance of the CdS which increases the current through the resistance wire which heats the bimetal strip which snaps open the contacts and turns off the light. This gives you a built in delay with some hysteresis, it's simple, inexpensive and robust, able to tolerate power surges, current surges from incandescent lamps burning out and inductive loads.

--- End quote ---

And also makes the trip point sensitive to ambient temperature...
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