photocell question

[mistercris]

So I took apart a photocell switch (turns on exterior lights automatically when it gets dark) as it wasn't working, figured it might be the photocell that's the problem, internet search says you test them by measuring resistance across it and shining light on it.
In darkness - high resistance, with light, lower resistance.
On mine, with light - resistance in the megaohms. W/out I get null (Infinite resistance?). Is it broken? Or is that what it's supposed to read?

[iainwhite]

Probably varies considerably, but this one changes from approx 200k Ohms Dark to 10k Light
https://www.digikey.com/product-detail/en/luna-optoelectronics/PDV-P8001/PDV-P8001-ND/480602

I suspect yours has failed

[Brumby]

I'd like to see a photo or two before commenting...

[mistercris]

With the cell flashlight on it, it's reading 11.5 M ohm. Covered it shows 0.L
Picture shows a dark line that looks like a crack but the surface is solid (no cracks or imperfections top or bottom) so can't tell if it's actually damaged or if it's some sort of stain.

[Audioguru]

Cheap photocells like that are rarely used anymore because they got sunburned like yours probably did. Even a solar garden light costs only one dollar and it does not use a photocell anymore. You might still be able to buy a photocell that might last a couple of years.

[cdev]

You can use an LED "in solar cell mode" as a light detector. The light is converted into a tiny amount of electricity.  There are actually several ways to detect light using LEDs, exploiting different properties of semiconductors. Some semiconductors are very sensitive to light.

https://www.sparkfun.com/news/2161


You can also use two LEDs face to face as a bidirectional optoisolator

https://en.wikipedia.org/wiki/Opto-isolator

One interesting application uses clear plastic tubing with two infrared LEDs in it, this has the effect of acting as a bend sensor.

[tecman]

Actually "cheap" CDS cells like you have are still widely used for low cost devices like outdoor light control.  Since they are variable resistance devices they are easy to apply to such devices.  They do age and suffer from UV degradation, as it appears your unit has.  You can replace it, but the general size of the device does dictate the power handling (like any resistor), so try to match the approximate size of the original unit

paul

[Zero999]

The good thing about CdS cells, is they have a similar spectral response, to the human eye.

Photocells are not used in garden lights any more, because it's cheaper to use the solar panel as a light sensor, as well as charge the battery.

The cell won't be dissipating much power, so the power rating won't be critical.

[mistercris]

I'm not going to repair the unit. I think it was a poor set up - no way to shut the light off at all as it had been wired directly to the mains so it was always on as soon as it got dark. Or at least that's how it was when it worked. Waste of energy. I'm thinking to wire up a motion sensor instead.
Was just wondering if the thing was usable for other things - I'm tearing things apart just to learn stuff. So this thread was good. Didn't know anything about these things so now I know a bit.
Thanks!

[james_s]

A lot of old style photocells require the CdS cell to carry a significant current. They resistive wire wrapped around a bi-metal switch and when light falls on the CdS the resistance drops and this allows enough current to flow through the resistor around the bimetal switch that it heats up. When it heats up, the strip bends and opens the contacts. This also inherently provides a time delay and hysteresis.

I would think most are electronic now using a phototransistor but I still see a lot of the old CdS types around.

[Zero999]

A lot of old style photocells require the CdS cell to carry a significant current. They resistive wire wrapped around a bi-metal switch and when light falls on the CdS the resistance drops and this allows enough current to flow through the resistor around the bimetal switch that it heats up. When it heats up, the strip bends and opens the contacts. This also inherently provides a time delay and hysteresis.

I would think most are electronic now using a phototransistor but I still see a lot of the old CdS types around.

Are you sure about the bimetal switch and resistive wire?

I would have thought a reed switch, inside a coil of magnet wire, forming a relay, would have made more sense.

[Gyro]

Yes, they were (probably still are) used as the light level sensors for road traffic controllers and probably other industrial applications. Some of them can switch significant mains power. The thermal mass gives a nice slow response (with snap contact action) which is helpful for ignoring dark clouds etc. You can still see them, they're the pointed cones that you see on some signal heads (one per junction). [Edit:Also seen on non-led streetlamps].

The calibration method is interestingly crude (on the ones we used to buy anyway). Aluminium paint on the front of the cell, then scratched off until the right sensitivity is reached. I always thought that it would cause uneven dissipation, but they were extremely reliable in practice.

[james_s]

A lot of old style photocells require the CdS cell to carry a significant current. They resistive wire wrapped around a bi-metal switch and when light falls on the CdS the resistance drops and this allows enough current to flow through the resistor around the bimetal switch that it heats up. When it heats up, the strip bends and opens the contacts. This also inherently provides a time delay and hysteresis.

I would think most are electronic now using a phototransistor but I still see a lot of the old CdS types around.

Are you sure about the bimetal switch and resistive wire?

I would have thought a reed switch, inside a coil of magnet wire, forming a relay, would have made more sense.

Yes I'm sure. A reed switch would not offer any time delay, nor are most reed switches capable of switching high currents. Most photocells are designed to control at least 300W at 120-240V which is higher than reed switches can typically tolerate. Take one apart and have a look, you'll often find an exposed bimetal switch with some fiberglass cloth wrapped in nichrome wire around it. At least in the US, I don't know if UK photocells used the same design. The only UK photocells I have are for streetlights and are both solid state.

[Gyro]

Take one apart and have a look, you'll often find an exposed bimetal switch with some fiberglass cloth wrapped in nichrome wire around it. At least in the US, I don't know if UK photocells used the same design.

Actually some of the UK ones are a bit more high-tech, Omron [Edit:or maybe Diamond H] have a little ceramic plate heater, with  thick film track, riveted to the bimetal strip at one end. They probably give more predictable thermal coupling - or more likely, the need to work at 240V.

The CDS cells they use, about 1 inch diameter, have a pretty impressive resistance range and, of course, mains voltage rating.

[Zero999]

Yes, they were (probably still are) used as the light level sensors for road traffic controllers and probably other industrial applications. Some of them can switch significant mains power. The thermal mass gives a nice slow response (with snap contact action) which is helpful for ignoring dark clouds etc. You can still see them, they're the pointed cones that you see on some signal heads (one per junction). [Edit:Also seen on non-led streetlamps].

The calibration method is interestingly crude (on the ones we used to buy anyway). Aluminium paint on the front of the cell, then scratched off until the right sensitivity is reached. I always thought that it would cause uneven dissipation, but they were extremely reliable in practice.

That looks like the one at my parents' house but I'm pretty sure their one uses a relay or TRIAC for the switching, not a bimetal strip.

[james_s]

I think most of the modern photocells are electronic now, especially since ROHS regulations severely limited the application of CdS cells due to their toxic Cadmium content. The old style bimetal ones were very common though, both the large commercial ones used for streetlights that plug into a twist lock NEMA socket and small residential controls built into post lamps and flood lights.

Curiously UK photocells use the same NEMA socket as US photocells however I think that's the only thing that style plug is used for there whereas it has a number of applications in the US. The photocells are styled differently too, ours are typically blue or gray cylinders with a window on the side while the ones I've seen used in the UK and other parts of Europe have transparent or translucent housings with a conical top. I have no idea why they've stayed divided like this, both types are functionally identical and interchangeable, the US style is available in 240V as well which is used for some streetlighting.

[cdev]

You can use LEDs as "photodiodes" and they have the advantage of being widely available!

Note that their response is quite fast which is kind of interesting.