Wiring a motion sensor to garden lights

I currently have a series of 5 lights in my garden which are on the same switch.
4 of these lights have their own built in motion sensor, but one (which is an older light attached to the house which i want to keep) does not have a sensor.

This means that when the switch is on the older light is on permanently whilst the other 4 lights come on/ go off according to motion, which is not ideal.

Now my question is whether it is possible to connect a motion sensor to the side of my house that will turn on the single light without affecting the other lights, and how this should be wired? Bearing in mind that I would like them all connected to the same switch.

Currently all lights are in series with the power cable first going to the older light and then from here to the other 4 lights in turn.

They're not in series. Or else none of them would ever work.

Why not change the older light fitting? It'll be easier.

I would have preferred help or useful feedback because you didn't really help much, sorry mate. I really don't wish to change the light fitting, easier or not. Rather I wish to add a sensor to it (imagine it like all the other 4 lights but rather than having a built in sensor I wish to add one to it) - of course if it's possible

Perhaps i don't know electrical jargon so well but what i mean by series is that the mains cable goes to this first light (without sensor) and then on to the next light (with sensor) and then the next and the next terminating at the fourth one. And all work nicely.

But my question, whether it is easier or not was if it was possible to put a sensor to this 'odd' light also and how to do it.

Your existing setup is wired as a radial circuit. If you remove the light fitting that doesn't have a motion sensor and fit a stand-alone PIR sensor in its place, you can use the L, N and E available there as the feed. Unless this is the last fitting in the circuit you will have to double up with the terminals inside the PIR and have both incoming and outgoing L/N/E sharing their respective terminals.

The old light fitting will then be fed from the load side of the PIR, the exact wiring method will vary between different manufacturers products but it should be pretty clear from the instructions. They will either provide a completely separate set of load (output) terminals for your L/N/E connections, or simply a switched live terminal and then N/E sharing the same terminals as the supply.

Thanks for this - it makes sense, thank you. If I run in to problems I will let you know, very helpful thanks.

Of course, if you really are in Estonia, bear in mind local regulations when doing this work.

Yes I am in Estonia, Tallinn.

Estonia is very bureacratic in many things but not when it comes to home electrics. It is nowhere near as strict or regulated as the UK. When a house is first built it requires a project by the contractor/electrician who installs it (which is a simple schematic) and then a letter of certification that all is as the project, and that is it pretty much it. And believe me when my house was built (2 years previous) the builder's electrician (who was Russian) didn't get many things right, which had to be changed - he even put the consumer unit door on upside down!?

There is still one cable in my house that i found that is connected (not live but there are some ohms? running throught the circuit - someone else checked it for me) somewhere, but for what and where we couldn't find it.