Power On At Pre-Set AC Voltage

Thread Starter


Joined Feb 8, 2021

I have one white and one yellow LED 120V input dimmable ribbon/rope light that is powered by the same dimmer switch (some distance away). I don't want the white light to come on until the dimmer is set to around 1/2 dimmer level (so white is only on when a lot of brightness is required). The yellow light would be on all the time.

I'm wondering if there is a device/switch that a novice person could make (can't find premade), that would be reasonably small, and that doesn't allow power to go through until a certain voltage is reached (for example, maybe it turns on at 40 Volts AC).

The space in which this has to fit is fairly limited, the ribbon is in a narrow slot in a soffit for indirect lighting and I can't easily run new wires or a separate switch (which I understand would be a lot easier solution if I could run new wires)...

In my initial research, I've seen posts about Comparators, Zener diodes, and Diacs, but they are over my head. I have some rudimentary understanding of electronics, a bit more about household wiring, and have built a lot of funky projects, but that is about it.

Is there a direction that you think I should be looking/researching (Comparator, Zener diode, Diac, or something else?) assuming up to 120V AC input, but set to "cut-in" at around 40 Volts? Any help, tips, information would be greatly appreciated.



Joined Sep 24, 2015
Welcome to AAC.

So you want the yellow light on at 100% power at start-up. When more light is required you want white lights to come on at 100% power. Is that correct? If so - why not just use two switches? Or a three way pull chain switch? (click on the underlined link)

Assume the yellow light produces 650 lumens (a measure of light output) and the white light produces 800 lumens. In that instance you can turn on the yellow. If that's not bright enough but having both lights on is too much, then turn the white on. It's 250 lumens brighter. And only if that's not enough, switch them both on.

They have pull chain switches that will turn one circuit on with the first pull; the other circuit on with the second pull; and both circuits on with the third pull. The fourth pull turns them all off.
Take a look at insteon and the UPB stuff. We might be able to find or make something that could give a digital output at some level. I'm initially thinking of something that could look at the voltage and create a digital output >40VAC (or whwtever). Insteon/UPB communicate through the power line. Those two devices should be able to be linked by a third unit to turn on based on the digital signal.

One of them augments the automation signal with a RF Zigbee,

Not going to get into what 40VAC really is.

So, what I'm suggesting is a UPB on/off module and a UPB digital input module.
And some undetermined device that can create a digital signal at some AC voltage. It depends on lots of things.

it might be easier to detect current: https://www.workaci.com/sites/default/files/product_cutsheet/Miniature Adjustable Current Switches.pdf

You can increase sensitivity by wrapping more turns of a single wire.
Last edited:
Recom makes these hockey puck power supplies which have wired leads. https://www.workaci.com/sites/defau...eet/Miniature Adjustable Current Switches.pdf

They could, in theory, power a current sensor.

In theory, the relay could switch a 120V coil remotely located or

in theory, the current sensor could activate a digital input module and control of the light be done with an insteon module.

Typical issues:

Houses wire power to the light (L,N and Gnd) and one has a switch loop to the switch. Automation devices require a Neutral in the switchbox which typically isn't there.

In theory, the automation system would monitor the digital input and turn on the LED strip.


Joined Jan 18, 2008
There are also voltage controlled relays. If the output of the yellow dimmer shows a change in voltage as the percentage of brightness is increased, that might be an easy way to control the white LED's. I have used such a relay to cut out a starting capacitor on my rotary 3-phase converter.