# Two lights, two switches... one problem!

I'm looking to set up an AC 240-volt lighting circuit consisting of two lights (let's call them L1 and L2), a standard switch (SW1) and a PIR.

The goal is to have SW1 turn on only L1, but if the PIR activates, to have it turn on both L1 + L2.

In my attempt so far, I've effectively created a standard 2-way circuit, but that has the unintended behavior that either the PIR or SW1 turn on both L1 and L2.

Is this possible using only these components, or is there some other component required to make this work?

Thanks!

Context: SW1 is an internal switch, and the PIR is outside. L1 are lights in the soffit of a building, and L2 is a security floodlight.

Table 1. (For OP to edit.)

SW1   PIR  |  Lamp1   Lamp2
-----------+----------------
Off   Off  |
On    Off  |
Off   On   |
On    On   |

• What is the output of the PIR? What happens when you turn SW1 off (does the PIR still turn both on)? Feb 14, 2019 at 23:30
• @TomCarpenter the PIR gives power to L2 when motion is detected. When SW1 is off, the PIR should still turn both on. Feb 14, 2019 at 23:32
• Is the PIR output a relay? Perhaps a diagram would help. "gives power to" is not really useful in terms of figuring how to wire things up. Feb 14, 2019 at 23:34
• Actually, to make things super clear, it would be good to list what you want in each of the possible switch cases (off/off, on/off, off/on, on/on) as I don't think we understood you first time round. Feb 14, 2019 at 23:36
• @Richard: We often use truth tables to summarize requirements like this. I've added a table for you to edit. Feb 14, 2019 at 23:44

simulate this circuit – Schematic created using CircuitLab

This requires that SW1 is a SPDT switch, which are commonly available for commercial light switches. In the UK these are called "1 gang 2 way".

When SW1 is turned on, it will turn on L1 only.

When the PIR is turned on, it will turn on either both L1 and L2 (if SW1 is off), or just L2 (if SW1 is on).

SW1   PIR  |  Lamp1   Lamp2
-----------+----------------
Off   Off  |   Off    Off
On    Off  |   On     Off
Off   On   |   On     On
On    On   |   On     On


This means that both L1 and L2 will be illuminated by the PIR regardless of SW1. And only L1 will be controlled by SW1.

As pointed out in the comments, this circuit should only be used as shown above, with a common live. If you house has more than one live phase, make sure both lamps are on the same phase and fuse/breaker.

• Very elegant, Tom. Note to OP: only do this if both lamps are on the same fuse circuit. You don't want two different live supplies on the one switch. Feb 14, 2019 at 23:57
• I don't think "most commercial light switches" are SPDT. An SPDT switch is called a "three-way" switch where I live and they are much more expensive than a standard SPST. Feb 15, 2019 at 0:03
• @ElliotAlderson fair enough. All the ones I've seen in the UK are. I'll edit anyway Feb 15, 2019 at 7:56
• @TomCarpenter Thank you for the super quick reply! I am, however, struggling to source an SPDT switch in the UK. Main electrical contracts are Screwfix and TLC Direct, and neither seem to carry 3 way switches. Do you have any suggestions? Feb 15, 2019 at 9:13
• @Richard They seem to be called "1 gang 2 way" switch in the UK. Should be commonly available as they're nothing special. You need those to control a single light using multiple switches, as done in many a normal house, anyway. Feb 15, 2019 at 9:34