Two level LED strip on different voltage with separate switches

Question

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.

^{simulate this circuit – Schematic created using CircuitLab}

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.

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^{simulate this circuit}

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