Inside a UK compliant dusk sensor switch. (with schematic)

This unit is clearly made down to a price, but it still manages to be a good logical design.
In a sense it almost looks a bit old fashioned, but if it works then it makes sense to keep using the same circuitry. It's very similar circuitry to the cheap eBay boxy looking dusk sensors, but with proper cable connection facility as opposed to the slightly shady individual cores on the Chinese ones.
These units are also designed for easy maintenance. If the circuitry fails or the relay welds shut with a high current load or fault, then you can just twist the old unit off and twist another on. There are also bypass versions that bridge the live and load connections for testing, and there are more specialist versions with smart circuitry that measures the length of the night and then switches off at a surprisingly accurate mid-darkness time based on the previous night's duration.

I've installed this to test it and also to test some LED festoon for storm and time resilience. It was easy to mount, but did require four core cable (don't use earth as a circuit conductor, even if sleeved).

Products like this make you realise that the humble 555 timer chip is not going away soon despite being one of the earliest integrated circuits. Even the name "photocell" is a hint to the heritage of these units.

There's an error on my schematic. The MOV transient suppressor is across live and neutral and not across the contacts. I guess it's probably to protect the circuitry, especially the dropper cap.

One thing that might have been an interesting addition is an NTC inrush limiting component to take the switching stress off the contacts. I'm not sure if the extra heat it generated would be an issue though. Now that I've just typed that I've realised that the 470 ohm resistor will be acting as a heater. I've just calculated that it will have about 24V across it and dissipate about 1W at 50mA. That's possibly to keep the unit slightly warm for reducing condensation and stop snow from sitting on the sensor. That's clever. It also explains the 2W standby power. Just over a watt for the heater and about 0.75W for the zener. (15V * 50mA).

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