Electronics > Projects, Designs, and Technical Stuff
Surprise photocell circuit.
SeanB:
Only manufacturer I am aware that is making them is Matsushita, but the majority of the photocontrols on street lights here are made by Royce Thompson, and have a small integrated circuit inside, encased in clear epoxy, that has the photodiode and all the electronics integrated into it. This does light level sensing, does a 5 minute timed on 30 seconds after power is applied for daytime lamp testing, and also tracks ambient light variations, so the lights come on at a constant light level, integrating even things like tree shadows, car headlights and such, and making the lights stay on all night.
You can program them as well, as in the UK there are many areas where the street lights are only powered from sunset to midnight, turn off till 4AM, and come on again till daylight. They also include a very large 480VAC VDR across the incoming mains supply, and have a very robust capacitive dropper circuit in them, that will survive being run on 400VAC for long periods. The relay they use is around 48VDC, and is rated to switch 16A, as the photocell can be used for either individual lamps ( plugged into the socket on top of the fitting) or to be a group control, switching a whole row of lights, which then just have a shorting cell plugged in, which only has a 16A non replaceable fuse and a 480VAC VDR in it, or just a shorting link.
They do fail, mostly from lightning impulse, or from water ingress, often caused by bird attack, as the common Ibis here loves to congregate on the street lights, and also has an impressive armoured beak. Dave should be familiar with them, they are also common in Australia, and are a pest.
Residential there are smaller versions, typically a phototransistor, capacitive dropper, 48VDC Songle relay rated at 250VAC 10A, and a simple circuit, often using the venerable 555 timer as comparator and hysteresis. They go bang nicely when they fail, or the capacitive dropper goes low value from self heating. I use one to drive general lighting, using a very overkill contactor, 3 phase 60A with all 3 phases in parallel, as that was very cheap at the scrapyard, and came in a box already. Load is around 10A, but lower now with LED lighting, but will never fail, even with the old wiring shorting out like it did this week, having me replacing 60 year old cabling at night.
floobydust:
I've tried to sense outdoor light intensity and it's actually very difficult.
Getting a wide angle view of the sky, not the siding on the neighbor's house, is tough.
CdS is superior for having wide angle, slow response (filters flicker), and spectral response matching the human eye. Silicon phototransistors are closer on the IR end which makes mistakes under cloudy skies.
A better method is sensorless, where you use an "astronomical" timer or light switch.
Knowing date and time and latitude and longitude, and from that you can estimate twilight hours by calculating the sun's position. This is then used to turn on lights.
In reality it is a lookup table in an MCU due to the very complex math of planet's elliptical orbits.
It will be with a few minutes of dusk, assuming it's not too overcast.
james_s:
--- 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 ---
It doesn't, but in practice that isn't really an issue. Photocells for switching outside lights are not precision devices, and the resistance changes pretty dramatically from light to dark so there really are only two states that matter, 'hot' and 'cold'. You generally want the light to come on when it is almost completely dark, and turn off just after dawn when there is enough natural light to see.
Yes they draw some power all the time, this is one reason modern photocells are usually electronic for the larger ones, but the small very low cost type with CdS and bimetal is still quite common. It's one of those things that was developed many decades ago and just kept being made the same way because it was inexpensive to produce and works well. The earliest photocells for street lighting (in the US anyway) used a phototube with vacuum tube electronics housed in the same glass domed plug-in module as our kWh meters. Being bulky and expensive they were usually used to switch groups of lights. The small thermal CdS type enabled inexpensive photocells on the individual luminaires and compared to the 175W-1kW lamps they typically controlled the additional power consumption was minor. I first started seeing electronic photocells in the mid 80s, I think most of the NEMA twist lock type have been that way since then.
Gyro:
--- Quote from: james_s on October 17, 2019, 09:01:45 pm ---
--- 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 ---
It doesn't, but in practice that isn't really an issue. Photocells for switching outside lights are not precision devices, and the resistance changes pretty dramatically from light to dark so there really are only two states that matter, 'hot' and 'cold'. You generally want the light to come on when it is almost completely dark, and turn off just after dawn when there is enough natural light to see.
--- End quote ---
As I said, a few posts up, they are temperature compensated anyway.
schmitt trigger:
Replaced the faulty unit.
Before installing the new one, decided to measure it.
Not surprisingly, the highest consumption is under bright light conditions, see photo.
At dark is only a few milliwatts.
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