| Electronics > Projects, Designs, and Technical Stuff |
| Surprise photocell circuit. |
| << < (3/5) > >> |
| Gyro:
--- Quote from: schmitt trigger on October 17, 2019, 01:05:32 pm ---That was my thought also. How does this simple mechanism compensate for the wide ambient fluctuations found in streetlight service? Unless it is tripping somehow, with a temperature differential. All in all a very clever device. :-+ --- End quote --- I can confirm that these are used on UK traffic signals (they used to be used on street lamps too, not sure about the newer LED ones). They can be seen as a translucent conical head on one of the signals (or top of the streetlamp). They are actually socketed - a twist and pull action, with a substantial neoprene gasket for easy replacement (once you're on the top of the cherry picker!). In terms of ambient temperature compensation... very simple, there are bimetal strips on both contact actuator arms, only the moving one has the thick film ceramic heating element attached to it. The big advantage of these switches is long thermal inertia. They are resistant to false triggering by passing dark clouds, heavy rain storms etc. The other advantage very high reliability. The design of the microswitch style tensioned contact spring gives a very good snap action and hysteresis. Calibration of the street furniture ones is/was crude - a coating of Aluminium paint on the front of the LDR, scraped away to achieve switching at the correct luminance level. Not great for even power dissipation over the LDR element surface but these are big and conservatively rated. Yes, very simple, reliable and clever. |
| T3sl4co1l:
Incidentally, even though an LDR is made of semiconductor (CdS) -- they are resistive (linear) to an amazingly good degree. It's not surprising that they can be used to control real power (~watts) at mains voltage. About the only shenanigans you can play with them is to put an electric field (100s V) on top of the lacquer coating, making a really shitty MOSFET... :D Pretty good on-off ratio too, considering. I mean, to be fair, sunlight is rather intense, which helps a lot. But even just with LEDs, you can make a very passable UJT* with them! (Their time dependence however, is some kind of wicked awfulness. They aren't practical to use for, say, communication. Weird time constants and diffusion effects in the microseconds to seconds range.) *Unijunction transistor: an identical component, but with the free charges (the semiconductor magically becoming conductive) provided by a forward-biased diode junction, rather than by light. Tim |
| Zero999:
--- Quote from: SiliconWizard on October 17, 2019, 06:24:08 pm ---1.44W is certainly significant. Would be interesting to measure it exactly. Note that this could violate some local regulations. Eg: https://en.wikipedia.org/wiki/EC_Regulation_No._1275/2008 If it really draws that much power in "off" mode, it could not be legally sold in the EU (and probably other parts of the world). --- End quote --- CdS cells are alsobanned under RoSH. I imagine it would be fairly easy to design a circuit using a TRIAC, which will use a tiny fraction of the power. |
| jimmc:
I had a similar one, the photocell was covered in paint and the light level to operate was set by scraping areas of the paint off. Jim |
| Gyro:
--- Quote from: Zero999 on October 17, 2019, 07:29:07 pm ---CdS cells are alsobanned under RoSH. --- End quote --- Are you sure about that? New stock is available in quantity from all the major distributors. I know the magic word Cadmium is there but I wonder. :-\ |
| Navigation |
| Message Index |
| Next page |
| Previous page |