I'd like to have an LED strip with two levels of lighting, one triggered by motion sensor (7v because I've tested the meter long smd3528 strip and it operates at low steady level at 7v), and the other 5v to boost up to 12v on a switch (connected to pantry door) for the strip giving me full light.

The reason I'd prefer not to use a dimmer is that I'm using it in my kitchen and when working there don't want to have to wash and dry hands to operate a dimmer. Pantry (like a closet in my kitchen where food and spices stored) door is usually open when I'm cooking so a switch on that for the additional voltage would be convenient.

At other times, not cooking, just need the dim light for getting a glass of water type action.

I thought of splitting the strip, having two smaller ones both at 12v, but kitchen is tight and full of angles and the longer one gives gentler, ambient lighting, less shadows.

I realize this is easy and cheap enough to test for myself but thought I'd ask while waiting for parts to arrive and ask (if successful) if there's a better design for more general applications? Also, would it matter if the 5v supply were added at the other end of the strip rather than joining the 7v line prior to the strip? Do I need that diode or more than that?

(there are two diagrams (I did my best without knowing legit symbology) because I know real world electricity is strange and if wiring both buck converters to the 12v and then rejoining would create a problem, plus if I opened the door during the day when motion sensor was not operating (it's a light sensor too) then I'd be putting 5v through strip and maybe that wouldn't be good for led strip)

Update: Just thought of putting the motion switch before the first buck converter so it controls the whole 12v supply and I wouldn't have to worry about 5v trying to light the strip.

enter image description here


simulate this circuit – Schematic created using CircuitLab


1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) A dual supply dual brightness configuration. (b) A single supply with diode droppers.

It's a bit difficult to understand your unconventional schematics but here are two suggestions. How you incorporate the motion sensor depends on its output switching.

  • The diodes will each drop the voltage by 0.7 V.
  • You will need diodes rated for the LED strip current (at the reduced voltage).
  • The diodes will waste energy as heat all day.

Take your pick.


simulate this circuit

Figure 2. Beware of shorting out earthed DC power-supplies. Note that PSU2 is short-circuited through the earth connections.

  • \$\begingroup\$ Ah, the voltages don't add up like sets of batteries, you're saying? Circuit will use highest voltage available and any additional, lower voltage input is irrelevant? If I put the motion/light sensor before anything else, then diodes won't make heat? I get it (assuming my assumptions on what you told me are correct). \$\endgroup\$
    – Hebekiah
    Aug 17, 2017 at 21:28
  • \$\begingroup\$ You can add voltages but you have to be careful because, unlike batteries, the power supplies could have their common connected to earth or the chassis. "If I put the motion/light sensor before anything else, then diodes won't make heat?" Yes, put it before D4 in (b). \$\endgroup\$
    – Transistor
    Aug 17, 2017 at 21:35
  • \$\begingroup\$ "Except for motion sensor output switching." Do you mean how much it reduces voltage, power loss? And if their common is connected to earth? Right now I have a 3 prong plug on AC side 12vDC power supply (from some old laptop or printer or something). No independently grounded metal case and the LED Strips are mounted to wood. Do I cut off the ground prong on plug? What's going to happen if common goes to ground? I'm quite interested. \$\endgroup\$
    – Hebekiah
    Aug 17, 2017 at 21:41
  • \$\begingroup\$ And I don't have enough cred to upvote your response, changed computers, moved, forgot old login. \$\endgroup\$
    – Hebekiah
    Aug 17, 2017 at 21:50
  • \$\begingroup\$ See the update. Don't cut earth pins off plugs. They're there for a reason. \$\endgroup\$
    – Transistor
    Aug 17, 2017 at 21:52

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