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I have two separate lights in a room. And two separate switches (actually one switch and one motion sensor). I want one switch to turn both lights on, and one switch to turn just one of the lights on.

As a truth table:

Switch 1   Switch 2   Light A   Light B
0          0          0         0
1          0          1         1
0          1          1         0
1          1          1         1

My trouble is: Since Switch 1 has to turn on both lights, the "inputs" of both lights have to be connected. So if Switch 2 the sends power to Light A, Light B will also turn on since they are connected.

Is there an easy way to do this?

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  • Where are you on this planet, and how many wires can you run between the various boxes involved?
    – ThreePhaseEel
    Commented Mar 31, 2019 at 12:11
  • Also, is the motion sensor Switch 1 or Switch 2 in your truthtable?
    – ThreePhaseEel
    Commented Mar 31, 2019 at 12:12
  • I can run as many wires as needed. Switch 2 is the motion sensor.
    – Lars Ebert-Harlaar
    Commented Mar 31, 2019 at 13:29
  • Switch 2 needs to be a single-pole double-throw, called a "3-way" switch in the USA.
    – A. I. Breveleri
    Commented Mar 31, 2019 at 13:32
  • 1
    The power is coming from the first switch (double pole, turning on both lights). From there a cable with 5 wires is running to the motion sensor and lights. If I do it like this, I could use the 5 wires for ground, neutral, phase and the two switchable wires from the double pole switch running phase. No new wires needed, right?
    – Lars Ebert-Harlaar
    Commented Mar 31, 2019 at 13:51

1 Answer 1

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This isn't as hard as it seems at first

From your truth table, Light A is an OR of Switch 1 and Switch 2 (aka the motion sensor), while Light B is controlled by Switch 1 alone.

Since power is coming in at Switch 1, we can simply run a 5-core cable between the two switches as you have already done, then standard 3-core (twin&earth) to each light from the second switch location. At the first switch, the incoming live is connected to both poles of the switch as well as to the live wire in the 5-core cable going onwards, while the two switched wires go to the two poles of the switch. All the neutrals connect to each other, and all the grounds connect to each other as well, of course.

At the second switch aka the motion sensor, the motion sensor's line side connection goes to the incoming live, while the motion sensor's load side connection connects to both the live feed to Light A and to one of the switched wires from the first switch. The other switched wire from the first switch then is hooked to the live feed to Light B, while the neutrals connect to each other and the grounds connect to each other as usual.

If you are feeding Light B from Light A, by the way, you'll need a four-core cable to carry the two switched lives from the second switch to Light A, of course.

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