The title says it all. The little thing that looks out the window needs replacing, and maybe it will work after I replace it. This is on a 1971 FP 6660. I guess I mainly need to what it is called.

The title says it all. The little thing that looks out the window needs replacing, and maybe it will work after I replace it. This is on a 1971 FP 6660. I guess I mainly need to what it is called.

That little piece that "sees" outside the window is the photocell (sensor) itself. Is yours a thin rectangular piece of ceramic material or is it round enclosed within a metal ring with glass upon it? (they were made both ways in 1971, the metal-glass sealed ones I believe were an option...).

That little piece that "sees" outside the window is the photocell (sensor) itself. Is yours a thin rectangular piece of ceramic material or is it round enclosed within a metal ring with glass upon it? (they were made both ways in 1971, the metal-glass sealed ones I believe were an option...).

It is a round piece of ceramic. But it wasn't in glass.

It is a round piece of ceramic. But it wasn't in glass.

A CdS based photoresistor. They degrade over time in the way they loose conductivity, so need more and more light for the same response, so the photocell turns ON sooner and sooner till it becomes a dayburner.

If it is big (aka 2x2 cm or so; the size typical when directly driving the relay), you won't find it working anymore.
If it is small (5mm diameter or so; when driving some electronic amplifier circuit before the relay), you may still find them on ebay.

Do you think these might work?

Do you think these might work?

Depends how the photocell is constructed.
If it is just a relay coil (or heater, in case of a thermal type) in series with the photoresistor connected to the mains, then not.
But if there is a low voltage (24V or so) electronic evaluating the photoresistor and then driving the relay or triac, then yes.
The main sign of difference is mainly the original CdS photoresistor size. These were quite expensive materials, so were used only the size needed for the task, so with direct relay drive they were about 20mm or more (an inch,...) in diameter, in the electronic photocells they were tiny 5mm spots.
So if the original was the large sensor, it was most likely needed to be that big, so these 50V rated ones wont suffice.

Take a detailed photo of your photocell internals...


Maybe an array of these (few in series or parallel or combined, depends on the overall sensitivity you need; to make sure the power dissipation never exceeds the 200mW rating).
So if the relay has 18kOhm or higher resistance, a single would be enough.
If the relay has 18..12kOhm, two would work. Be careful they both get equal light exposure, to make sure the power dissipation really splits.

This is one that I found that ships in US. Here is the eBay item number. Keep in mind this is a smaller CdS cell than the one that is already in the photocontrol by probably less than half the size. I'd be curious to know how this works out as I have considered doing this myself in the past.

254880199139

Keep in mind this is a smaller CdS cell than the one that is already in the photocontrol by probably less than half the size.

With the series (so it will trip at higher light levels) or parallel (lower light levels) combination it will spread the power to multiple cells.
Good ballpark is to maintain the same total surface area as the original. Just keep some margin for uneven power distribution (happens when unevenly illuminated)
2s2p combination will keep the sensitivity original, while allowing to drive a 2VA (rated at full mains) coil/heater relay.

Other option is to use some simple electronic circuit (e.g. a triac, diac and some resistors and capacitors) to evaluate tye CDS and then drive the relay. Then using the small, readily available 5mm CdS (or even a way more stable silicon photodiode/transistor) could be good enough.
You may use e.g. [https://www.electroschematics.com/light-operated-light-switch/]this[/url] to operate the original relay instead of the big LDR directly.