I fitted a PIR sensor at my parents' to turn the bathroom light on and off automatically last week but the lights are very dim. On inspection, there is only half of the potential difference across the bulbs and I have no idea why. I'm fairly electrically literate but I am at my limit here so hopefully one of you folks can give me some pointers.

Previously, when the lights were operated by a simple switch, the live was switched and the neutrals were connected in the loft above.

According to the wiring instructions for the PIR sensor, the live and neutrals should be a constant supply to the sensor, with a third feed that becomes live when the circuit closes (when someone enters the room). I have drawn a circuit diagram to illustrate how the circuit looked on the old setup (which worked the lights at full brightness/full 230V potential difference [I live in the UK]), and now, at 115V.


Before on the right, now on the left. Note that the bulbs are wired in parallel both before and after (I drew them wrong). This to me would be the first thing that looks amiss, however this was the case before and they worked just fine. The 3 feeds coming into the junction box are go off to different parts of the house's lighting main, unrelated to these lights.

I grabbed a voltmeter and took some readings across some noteworthy points in the circuit:

At the PIR sensor, the potential difference between the live and neutral terminals is 230V, as expected.

When the PIR sensor is closed (on), the potential difference between the output (A) terminal and neutral is 110V, not expected, should be 230V.

This issue is happening before we even get to the bulbs (which are 11.4V LEDs on transformers, I expect there are also bridge rectifiers built into the transformer modules), so the issue must be with my wiring, but I can't see anything untoward.

Any and all suggestions gladly received! TIA

  • if Connecting A wire to L gets full brightness , then PIR current rating is a problem and some switch inside must be getting hot with 120V drop with light current. Or remove 1 bulb in each and if 110V rises that also confirms bad switch Nov 24, 2019 at 9:15
  • In both cases - old and new, the primary windings of transformers are connected in series; so the voltage applied to each of them really should be 230/2 = 115. Another reason can be an existing (internal) diode in series (half-way rectifier).
    – Circuit fantasist
    Nov 24, 2019 at 9:15
  • Sorry, I have not read that the voltage considered is at the PIR output (point A). Only my second remark about the diode remains in effect.
    – Circuit fantasist
    Nov 24, 2019 at 9:25
  • Is it possible that you are running the PIR unit with the wrong AC orientation? If the PIR unit has a small internal transformer you might be passing the hot AC wire though that transformer then passing it through your bulb transformer. That might give you about 1/2 the voltage into your bulb transformer.
    – Nedd
    Nov 24, 2019 at 10:53
  • 1
    You corrected the error in the "before" drawing to show that the lights are connected in parallel on the secondary side of the transformers, but were the primaries always connected in series like that? If those are 230V transformers, the primaries should be connected in parallel, too. This would explain why they're only getting half the expected voltage.
    – Dave Tweed
    Nov 24, 2019 at 16:37

2 Answers 2


You have your transformers in series they should be in parallel, sorry I could not keep reading and all the comments. With the transformers in series the output voltage will be 1/2 . Connect the transformers in parallel and your voltage will double. I don’t think your before drawing was accurate. I have seen this done many times by new DIY folks trying to add lights.

  • Many thanks Ed I will give this a try when I am over my parents' later in the week. I could have missed the fact these were in parallel before, stranger things have happened for sure.
    – Jacob King
    Nov 25, 2019 at 16:59
  • Worked a treat, thanks!
    – Jacob King
    Nov 28, 2019 at 10:54

To work out the problem I would recommend you temporarily go back to your exact "Before" wiring arrangement (including the manual switch) and verify that everything still works normally.

Then with that same wiring, connect the PIR power wires to the two input AC lines. Then test the PIR unit using a standard 220VAC bulb connected between the "A" line and the opposing AC wire you believe is correct. If that testing works remove the 220VAC bulb and connect up your transformers to the same wire points.

If you leave the manual switch in the circuit you would then have a backup method to light the small bulbs in case the PIR unit ever goes bad.

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