My bathroom lighting consists of recessed down lights and LED pelmet lighting. With everything switched on the light levels are high which is fine but not great for a relaxing bath. I want to be able to dim the lights to create a mood atmosphere. As such I have wired up a single dimmer switch to the supply that feeds all of the lights. The recessed lighting dims fine, but the LED pelmet light comes on pretty much instantly on the dimmed range and lights up the bathroom which ruins the effect.

I would like to retain the single switch so am wanting to install some form of relay which only activates the LED lighting at the top of the dimmed range. Does anyone have any ideas? I was wondering whether it was possible to get some form of voltage sensor which would detect voltages over a certain level, say 220v (at the top of the dimmed range) which activates a relay which in turn activates the LED curcuit.


Don't dim those LEDs! If they come on full bright at a certain dimmer level or flicker at a certain level then their driver isn't designed to be dimmed and could be damaged or worse. One solution could be to get a proper dimmable driver and dim the LEDs with the rest of the lights... assuming the LEDs and driver are separate units.

  • \$\begingroup\$ Thanks for the advice. It looks as though my driver is not designed for dimming as it does infact flicker and humm probably best if I disconnect it from the mains dimmer! \$\endgroup\$ – user3178 Feb 28 '11 at 13:13

A simple solution, if you dimmer uses a potentiometer, would be to get a potentiometer with a switch. Have the varistor part control the main lights and have the switch control the LEDs.

Another more complicated option would be to use a comparator and some optocouplers. You could design a circuit which could use a discrete comparator to see if voltages are same on the dimmer output and on the input voltage and then trigger a transistor or a relay, depending on the LED's power consumption.

It could be easier to program a microcontroler with an integrated comparator to trigger a transistor or a relay.

In both cases, you'd need to provide power supply for the control element of the system, which will take up space and will need to be at a safe location which wouldn't be affected by humidity and water.

  • \$\begingroup\$ Dimmers for mains powered incandescent lights normally use a TRIAC to turn the power on for a fraction of the mains cycle. Using a potentiometer to dim a lamp directly will get very hot. \$\endgroup\$ – uɐɪ Feb 28 '11 at 12:24
  • \$\begingroup\$ @Ian Using a potentiometer directly is a bad idea, of course, but form what I've seen many dimmers have a potentiometer as a control element. I was thinking about replacing that potentiometer with one which has a switch. \$\endgroup\$ – AndrejaKo Feb 28 '11 at 16:40
  • \$\begingroup\$ You make a good point. Some dimmers use a pot to control a 10Vdc signal, but these seem to be fairly rare. However, if that's the system you have then a pot with a switch may work for you. The only caveat would be that the switch operation would likely be inverted and it probably wouldn't be rated for the power and voltage. \$\endgroup\$ – Joe Frazier Mar 1 '11 at 3:58

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