Inside a UK compliant dusk switch.

It varies depending on the ambient intensity. In bright light it is a very fast transition and at low illumination levels it can take tens of seconds.

Big Clive

What's the time delay on this? A 20k resistor and 100uF capacitor has a time factor of around 2 seconds, I imagine it would be more than that...

Gordo

Yeah, I was puzzling about that too. Does it refer to the relay?

Big Clive

"Electronic rotary load switching" (?)

This is kind of old school isn't it? Cool. Indeed I wouldn't have blinked if it had a cadmium sulfide photocell inside. :) I wonder if the mystery diode is to protect against backfeeding into the 555?

Michael Thompson

I did ponder if it was to nudge the voltage down, but the 14/15V seems a bit high as it is. I've taken a look at another version and the zener is marked 12V so maybe it's just a tolerance issue.

Big Clive

Yes. I actually got a connection wrong. It's between live and neutral, so it must be to protect the circuitry from inductive spikes from the loads.

Big Clive

They still make the old-fashioned ones, too. LDR and bi-metal. If you use the one you disassembled within 3 meters of a 5KW AM tower, it won't last a week. Take it from the horse's mouth. I went through two "new" types without suspecting. The store I got them at was sorry and gave me the last one because it was NOS. Turned out it was the old type. I figured it out, and had them order two more as spares. BTW, the sensor was used to turn on the tower lights at dusk, and off at sunrise. Photocell had to be facing true North, also. Great presentation!! Just not good around radio transmitters.

Chuck Kirchner

I found that a really interesting circuit- maybe because it was kind of retro- back to before the days of a blob or IC that did the lot. Keep 'em coming Big Clive, This sort of stuff is fascinating.

Mike Hughes

What was the MOV used for? Clamping spikes?

Mike Bird

The connector looks the same as the ones used in the US for streetlights. It looks like a NEMA 6-30, but it is not. I have found one proper connector at a surplus store, I don't know what it is. The delay cap is also necessary in a lightning storm to keep the light on.

Paul Malloy

That socket looks like the USA style 30A 120V twist lock. https://www.amazon.com/Legrand-Seymour-L630PCCV3-Industrial-Strength-Suitability/dp/B000BPNMKY/ref=sr_1_6?crid=2KMZ6FJGFH2JU&keywords=30a+twist+lock+plug&qid=1583500376&sprefix=30A+twi%2Caps%2C153&sr=8-6

Charles Bruckner

I might almost like to mod the unit and put a good old wound transformer in there. It might generate the same amount of heat but will deliver a better power factor. There is only the space issue.

Frank

The mystery diode isn't there just to drop the voltage a bit given it's a 15v rail and a nominal 12v relay? Seems unlikely as the coil shouldn't be that sensitive and one diode wouldn't really drop it enough, but all I can think of... Maybe just belt and braces protection to stop turn off transients hitting the 555 given they're industrial?

Charleso

Sweetness!!! 😉

Zane Revai