Unknown device on the back of a garage.
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photocell for outside lighting
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Wot Andy said....
Photocell...
Basically it turns lights on when it gets dark...
Then off when it gets light!
e.g.
https://www.toolstation.com/electronic-ip44-photocell-kit/p62528
Oh.. and I forgot to say.. Welcome to the magical electrical forumbulator...
Where you can expect to find a wealth of knowledge and guidance... for all sorts of electrical related questions..
But also hard basic facts when it is quite obvious that someone with insufficient knowledge is trying to wire up a potentially dangerous alteration/addition....
As none of us would like to assist in the creation of any potentially dangerous wiring, that could injure or kill someone before any protective devices can disconnect the source of electrical energy!!
Photocell...
Basically it turns lights on when it gets dark...
Then off when it gets light!
e.g.
https://www.toolstation.com/electronic-ip44-photocell-kit/p62528
Oh.. and I forgot to say.. Welcome to the magical electrical forumbulator...
Where you can expect to find a wealth of knowledge and guidance... for all sorts of electrical related questions..
But also hard basic facts when it is quite obvious that someone with insufficient knowledge is trying to wire up a potentially dangerous alteration/addition....
As none of us would like to assist in the creation of any potentially dangerous wiring, that could injure or kill someone before any protective devices can disconnect the source of electrical energy!!
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Useful tip #103 for electricians. Keep a black bin bag in your van. Perfect to cover such sensor so it thinks it is dark and all the outside lights should come on and you can go and check them.
If you need to replace one, they unplug like a bayonet fitting so no witing needed just unplug the old one, plug in the new one.
If you need to replace one, they unplug like a bayonet fitting so no witing needed just unplug the old one, plug in the new one.
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Useful tip #103a keep the bin bag in a small cardboard box, when the bin bag doesn't work, put the box on it instead. 
(I have had bin bags that are so thin you can almost read through them.)
(I have had bin bags that are so thin you can almost read through them.)
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An old, suitably large glove sometimes works as well.
Or a covid mask (black)
eckcop
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Thanks for the replies. Knowing what it is now I can remove it. The outside lights it connects to don't work and this has been the case for many years. I am more intent on rendering the wall behind it (when the weather gets better) than on fixing / replacing the outside lights. These are further down the to do list.
I have seen these sensors other places, over the course of many years, and never stopped to wonder what they actually do. A shameful lack of curiosity.
I have seen these sensors other places, over the course of many years, and never stopped to wonder what they actually do. A shameful lack of curiosity.
Also you can use a dark coloured fleece jacket to civer the photocell
or since they almost always fail on, keep a dead one spare... or solder a bit wire between SL & L on a spare
I've always used a black wolly hat. Being follically challenged, I have a collection of such hats in the van 
Be very careful when disconnecting it as I would assume that its 3 core flex going into it and what should be the earth cable will more than likely be the switched live !
Looks like it could be a mains siren (not SA), or a daylight sensor (for outside lighting).
eckcop
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Thanks for the warning. The garage has its own consumer unit (which can itself be isolated from the house). As the wiring for the property is old and a bit inconsistent I will check nothing is live before removing the sensor.
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I mulled one to get the connector. Soldered three wires on to it. Wago across L-S/L , one on N. Enables you to test supply, load test etc. I also have a much rarer OEM shorting plug. I’ll post pics later if I can be motivated enough
well look at you with your fancy test adapter
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I didn't bother adding flex, jut put a 240v neon indicator between the linked L/SL and Neutral and refitted the clear plastic dome before labelling it as "P/Cell bypass" on the outside edge - I like mine much better than my colleagues who have OEM shorting plugs because theirs don't prove the supply to the NEMA socket is live
+ Recycling, I saved something plastic from the landfill .... now if I could only actually find uses for all the other broken stuff I've kept just incase.....
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Jealousy is such a cruel mistress
phillip.rounce
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That would be a photocell suppling outside lights possibly
