The Plan

I am working on finishing my basement and planning on installing LED strip lights inside a lowered crown facing up.

Now where it gets complicated. I'd like the lights to have a dim setting to be controlled by a timer, on at sundown and off at midnight (easy too), but I would like the lights to come on at full brightness when the overhead lights are flipped on for the specific room. 5 zones in total, bathroom, kitchen, family room, hall and gym.

No Software

I don't want any software solutions with smart switches / dimmers, etc. (Aside from the timer switch) and no wireless anything.

Idea #1

I was originally thinking the easy way would be to have two strips, one at dim on the timer and the other controlled by the power for the light switch, but I don't like this idea because of the additional cost and I'd need 6 power supplies.

Better Idea (so I think)

I was thinking last night and thought about having two power supplies, one set to dim and one to full brightness. Then have 5 double throw relays, that are switched to low when it's not energized and then switch to full power when it gets a 120v signal from the overhead switch for the specific room. What I'm not sure about this idea is if the power supply will keep the LEDs in the different zones at constant brightness when the load increases/decreases. I'm thinking I just need a constant output voltage power supply, but I've never tried this myself.
(Not thrilled with having to run 14/2 for each overhead back to the utility room, but I can live with it. Easier than dropping the voltage and sending a 12v signal back)

Will this idea work or is there a better way to achieve my goal.

Additional info

  • No drywall in the basement yet (done with framing, plumbing and running electrical now), I can run anything anywhere.
  • Total is about 200' of LEDs
  • Planning on having all LED controls in one spot, utility room
  • Power for LEDs will distributed with thermostat wire.
  • White LEDs, no color.
  • Each room will have additional lights, mainly 6" recessed
  • Locale, US
  • Mostly dealing with low voltage issue here, but I need to pass inspection NEC 2020 rules.
  • thermostat wire will not power 200' of LEDs. Simplest would be to have a dim strip and a bright strip, independent of each other. Or just put a power resistor inline with the strip to make it dim, then use a relay to short out said resistor, thus driving the strip at full power.
    – dandavis
    May 3, 2021 at 17:18
  • Thanks. Interesting thought about the power resistor, I'll think about that. I plan on having several home runs of the thermostat wire to each zone and even breaking up larger zones to prevent voltage drop over long runs of LEDs
    – Gary Bak
    May 3, 2021 at 18:02
  • I really doubt if using a power resistor is code compliant, even though the code doesn't really care that much about low voltage circuits. Besides that, I think this is a bad idea, very one-off. Those resistors do get hot, depending upon the load. like Dan said, maybe two strips...one dim on the timer and the other on the switch? May 3, 2021 at 21:14
  • The idea of using a power resistor reflects unfamiliarity with LED technology. LEDs are semiconductors, they are not ohmic. And LED strips are massively paralleled, so limiting current does not work. May 3, 2021 at 21:49

2 Answers 2


Be warned that your understanding of this tech is limited and you'll need to skill up more to pull off a successful project.

The power section

OK, the way I'd do that is by having an always-on 12V (or 24V, better for the voltage drop) power supply in the basement.

I will remain silent on the question of that having a battery back-up, but, if you do have a battery back-up, that favors 12V since TVs, Internet modems and routers are readily available for 12V, not so much for 24V. 12V USB chargers are sold in more places than eggs.

The next step out of the 24V supply will be a timer with PWM dimmer, set to do that "sundown-midnight" thing that you want.

So now we have 2 power sources: Negative always-24V, and "negative sometimes" PWM 24V. LED systems use positive common.

I'd watch voltage drop like a hawk. If I found it advantageous to carry it some distance to remote distribution hubs, I'd use fat aluminum feeder for that, because it's cheap, using MAC Block Connectors for the Al-Cu splices.

The switching section

I would use 3-wire cable (e.g. 12/3*) to carry both of those, with positive common, in 3-wire cable to the switch for each LED segment. I would use these colors:

  • Black = always-negative
  • White with black marking = sometimes-negative PWM
  • Red = positive common

The switch is a 3-way switch. The "negative sometimes PWM" goes to one traveler screw, "negative always" goes to the other. The "3-way common" goes onward to the LED negative. Positive common passes through to the LED without going to the switch.

The 3-way switch either connects the LED power to "sometime PWM" or "always".

Now if you actually want "3-way" i.e. multiple switches controlling the light, that is simplicity itself: the first control position (the one power comes into) gets a "4-way" switch instead of the normal 3-way.

Complying with Code

You need to comply with Code's low-voltage wiring rules. The safest way to do that is use wiring methods legal for 120V. 90% of Code is about physical execution of work - correct wiring methods, correct terminations, etc. You are not allowed to rinky-dink this thing together in a barf of wires all over the place, like a typical workbench electronics project. You need to comply with NEC 110.12, "neat and workmanlike".

Power supplies and controllers will need to be physically installed correctly and may need to be UL approved. You may get in trouble with the AHJ for using non-approved luminaires, NEC 110.2.

The #1 area of concern is complying with the building codes regarding required lighting in any given room. There must be a light switch in the normal/expected location(s). The light switch must operate a light (a switched 120V receptacle is an acceptable substitute). You will need to satisfy the AHJ that this is good enough.

There's another reason to use 120V rated cables and wiring methods: Resale value. Later when you are selling this house, you don't want the home inspector going "WTH" and warning the buyer that the lighting circuits are bizarre and unsupportable. Also, if this experiment fails, absolutely no one will be surprised. If you make it easy to "fall back" to 120V arrangements and wiring methods, these problems become trivial, and you can proceed with confidence that you always have a plan-B.

The additional cost of wire and proper boxes is trivial compared to the advantage of avoiding difficulties with the mortgage lender and future home buyer.

* Romex for easy fallback to AC mains if stuff doesn't work. #12 not #14 for voltage drop...

...and because everywhere Code requires a 15A circuit, it accepts a 20A circuit. At the scale I do wiring, the cost diff per foot in #14 vs #12 is not large enough to justify tying up capital in a completely redundant set of wire reels, as well as having to drag those around.


Thoughts about LED Tape

I'll assume you're considering one of the many varieties of LED tape offered in the online markets. There are a few things one should know about these when considering a project scope as large as yours is.

  • voltage drop in the strip: as in all conductors, voltage drop happens. This causes the LEDs to glow less brightly at points further along the strip (more distant from point of power connection). Get a sample of the product, power it up, and note the difference yourself. Depending on your tolerance for relative brightness/dimness along the length, you might find that you need to connect the strip to power as often as every 5 feet. Note that it works well to use a continuous tape, tapping into the power conductors without cutting the tape at each feed point.
  • efficiency: largely because of the resistor-based design of the product, it produces a fair amount of heat relative to light. They're fine as an accent light or as a "night light" but I wouldn't use this stuff for ambient lighting.
  • dimming: although you can dim the LEDs by reducing the supply voltage (using a series resistor, for example), it'll be superior to dim with PWM (pulse width modulation). PWM dimming modules can be found advertised as "dimmer" or "brush motor speed controller" in the same marketplaces as the LED strips.
  • longevity: over time things will change. Dim spots will develop. Color might shift. A foot-long section might completely go out. Have a plan for how you'll deal with it, whether that is "ignore" or "buy extra material and store it as replacement parts."

Thoughts about Wiring

18 gauge thermostat wire, or even 20-22 gauge wire, is a nice size for attaching to the LED strip. It's not heavy enough to serve all the strip in one room though. You'll need to make multiple home runs of this wire, or else step up to larger wire in some way.

Detecting When Room Light is On

Since all LED controls will be in the utility room, put the detection logic there too. An easy way of accomplishing this is to wire a bank of ordinary AC outlets in the utility room so that one outlet is powered from the AC lighting circuit in each room. You can plug in a power supply to each outlet, such as a USB phone charger, to get a low voltage DC signal corresponding to state of the AC-powered lighting in each room.

Tying it Together

Big power supply -> PWM dimmer -> LED strip. Also connect USB power supply -> relay. Adjust the PWM dimmer to the dimness level you prefer; wire the relay to bypass the PWM dimmer for full brightness during lights-on conditions. Insert the timer module at the appropriate place (should the timer affect the dim mode only, or full-bright mode too?). Add fuses according to wire gauge and short-circuit current capability of the system (ampacity and location of fuses will be dependent on your wiring layout).

Bench Testing

This is an experimental system. Don't assume you'll get everything right on the first try. Before the drywall goes up, and even before installing the wiring to the rooms, get samples of the LED strip, dimmer, power supplies, etc. Lay the tape on the floor around the room, wire it up, and make sure everything operates as you wish it to before committing to the full installation.

Thoughts about Fire Safety

Yes, fire. It turns out that one can kindle a mighty fine fire with only 12 V DC and small-gauge wire. Fusing and/or low-capacity power supplies will be important to ensure that, should a short circuit occur in the system, power would be cut before a small wire can overheat and ignite its insulation and surroundings.


Yes.. conduit. If it's metal conduit it'll help mitigate fire risk, and whether it is metal or plastic it'll provide a path for the inevitable future in which changes to the system are wanted!

  • You can get voltage or resistance controlled PWM dimming supplies. Or a voltage/resistance PWM dimmer. Just swap between two controller values (one of which will probably be 100%) using a small SPDT relay. A little more power draw but might be cheaper in parts.
    – jaskij
    May 4, 2021 at 0:05
  • Re: fire, I'm not sure about US, but in Poland I've seen 24 VDC systems appearing, which might do a lot for reducing any fire hazard by simply halving the current required. I've been planning a LED room lightning system recently and seeing 8A appearing in my spreadsheet for something I'd leave running for a long time and unattended was scary.
    – jaskij
    May 4, 2021 at 0:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.