I have an outdoor security floodlight (with motion sensor and photocell) that I'd like to be able to dim.

The user manual says to not connect it to a dimmer, but doesn't explain why.

What is the reason for this?

Is it a fire hazard, or will it simply prevent the motion sensor from working if I dim the light too low?

  • Dimmers do weird things with the power running through them. With a regular incandescent lamp (or LED lamps designed to tolerate dimmers) the current is "averaged" so that the choppiness of the incoming juice is smoothed. But anything containing electronics (and which isn't designed to be dimmer-tolerant) will be, at the very least, confused by the chopped-up incoming power, and may actually be damaged by it.
    – Hot Licks
    Commented Nov 28, 2020 at 19:20
  • 1
    Can you replace the bulbs, or fixture, with ones that have lower lumen levels?
    – Alaska Man
    Commented Nov 28, 2020 at 19:23
  • Can you move the photocell/motion sensor somewhere else in the circuit? How do you want the dimmer's manual switching to interact with the photocell/motion sensor turning the light ON/OFF? Commented Nov 28, 2020 at 19:25
  • Just replace the 100w bulb with a 60w bulb (old money) or a 1000 lumen bulb with a 500 lumen bulb (new currency). Skip the dimmer entirely.
    – FreeMan
    Commented Nov 28, 2020 at 19:38
  • 1
    you can simply get a smart bulb that can be dimmed itself.
    – dandavis
    Commented Nov 28, 2020 at 20:33

4 Answers 4


Generally, schemes where you want to control a light 2 ways at once, do not work. The two methods end up in contention/conflict with each other.

The motion sensor and photocell units need 24x7 power. They do not work on reduced power, partial power or the wacky waveform you get out of a triac dimmer, which is made only for lights. Since you are using a single light+motion sensor+photocell unit, you can't separate out and dim only the light, unless the internal arrangement of the unit allows you to do so, which is pretty rare these days.

Modern motion sensor+photocell+LED units generally have a single power supply on board that makes 12V, both for the motion sensor and the light. (12V motion sensors are ~$10, 120V motion sensors are ~$20 due to the need for an internal power conversion and a higher safety rating switch). If you could access the internals of the unit, it's certainly easy to get a PWM dimmer for ~12V LEDs, that could go between the sensor and the LED. The cheapies have a physical knob that will stay where you put it.

Honestly, your best option - though it'll be a little complex - is to do exactly that with discrete components. The photocell is generally part of the motion sensor, and so you don't need 2 components there.

So you have a 12V power supply... then a 12V motion sensor... then a 12V PWM dimmer... then 12V LED lights of your choice, which are readily available.

It'd be possible to do that same thing in 120V, however choosing a dimmer would be a challenge. Most dimmers these days do not like being downline of a switch, and will do undesirable things like reset themselves to full brightness etc.


The light only takes one power feed and all the attached electronics, I.E. Sensors, require full voltage.

Your product would have to be compatible with a low voltage dimmer system (purple and gray wires if present)

  • "The light only takes one power feed" Can you explain that, is someone suggesting more than one power feed? Answers are better, and more likely to get up votes, if there is comprehensive explanation for the advise being given.
    – Alaska Man
    Commented Nov 28, 2020 at 19:46
  • Don't most DC circuits, such as the electronics that control the motion sensor logic, require 5 volts to operate? Why would the manufacturer require all 120V to flow through?
    – Phillip
    Commented Nov 28, 2020 at 20:04
  • @Phillip - They may, but they derive that lower voltage from the incoming 120V AC. If you "chop" the 120V input with a dimmer, the small/simple power supply that powers the motion detector electronics probably won't work.
    – SteveSh
    Commented Nov 28, 2020 at 20:13

The motion sensor and photocell require a certain input voltage to operate. If that voltage is lowered by a dimmer switch, the switching won't take place and your light won't turn on. I have security lights that have an input range of 95 to 125 volts so I have installed dimmers and can dim the lights a small amount but if I dim them too much, nothing works. I've also installed low wattage bulbs to increase the dimming effect. You might want to check your voltage ratings and experiment with a cheap dimmer.


Is this a solution?:

  1. Power the photocell with straight power from electrical source
  2. Connect photocell to dimmer as you would for lights ( Now the power is in the dimmer)
  • Power will be sent at dusk and power will be cut off at dawn at the dimmer instead of lights
  1. The dimmer will send the voltage received from photocell as desired to your lights as you adjust it
  • 1
    you answered with how, but the original question is why
    – Ack
    Commented Jan 27, 2021 at 5:17

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