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I have a 12 V LED strip that works at 10 W. I want it to run at multiple current configurations with two relays and without the help of a microcontroller. I am attaching a diagram of my intended setup.

I need 3 options.

  • option A - only relay 1 is on and the LED strip must run at 3 W
  • option B - only relay 2 is on and the LED strip must run at 7 W
  • option C - both relays are on and the LED strip must run at combined 10 W

Basically I need something in between the relay output (or input) that splits the current accordingly. Even if I achieve options A and B through some way, I have absolutely no idea how option C can be achieved.

Power supply is a 12 V SMPS. Relays are standard sugar cubes. I have logic level MOSFETs with me, would they be useful or what is the best solution to this problem? To split the current from my power supply and then to combine them if I want?

enter image description here

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  • \$\begingroup\$ What will you be using to control the relays? I'd suggest adding some PWM circuit, where you can change the duty-cycle depending on the input signals. Without any relays at all. \$\endgroup\$
    – Klas-Kenny
    Commented Jan 3, 2022 at 8:20
  • \$\begingroup\$ Wait, is this a single color (white) led strip or is it dumb rgb or smart rgb? \$\endgroup\$
    – Passerby
    Commented Jan 3, 2022 at 8:57
  • \$\begingroup\$ @Klas-Kenny a cheapo 433MHz RF keyfob module. PWM will be done only if this thing won't work out. \$\endgroup\$
    – Kokachi
    Commented Jan 3, 2022 at 16:51
  • \$\begingroup\$ @Passerby single color 2835 led strip 60 leds per meter. \$\endgroup\$
    – Kokachi
    Commented Jan 3, 2022 at 16:51

2 Answers 2

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Assuming your LED strip contains only resistors and LEDs you can do something like this (if it has an active circuit, then all bets are off):

schematic

simulate this circuit – Schematic created using CircuitLab

U1 acts as an oscillator and gate driver with a 70% duty cycle so when RLY1 is closed the strip gets 70% of full power through that contact.

U2 acts as an inverter and gate driver so it has a 30% duty cycle so when RLY2 is closed the strip gets 30% of full power through that contact.

When both relays are closed the strip gets almost 100% of full power.

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  • \$\begingroup\$ Yes you are correct my led strip is the simple type with just resistors. Thanks for the circuit, I think your idea will work. \$\endgroup\$
    – Kokachi
    Commented Jan 3, 2022 at 17:12
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With the limitations you put in place, the simplest option is to split your led strip into 2 sections. One thats 3W and the other that's 7W. When both are powered, you get the combined 10W usage. This would mean cutting the strip into sections you can wire independently. Maybe every 3rd segment or every 3rd foot of led strip. You would end up with gaps in the light if only the 3W sections are on or only the 7W sections are on. You will need to do alot of extra soldering and wire management.

Without the use of a microcontroller or modulating circuit (555 timer) or solid state design, you are very limited. With a microcontroller, this is trivial.

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  • \$\begingroup\$ That's a cool idea, but unfortunately I don't like having only certain sections of my strip lighting up. Guess I would go with the microcontroller option since my expectations were simply too much with this circuit. \$\endgroup\$
    – Kokachi
    Commented Jan 3, 2022 at 16:54
  • \$\begingroup\$ Is it possible for me to use 2 different power supplies for the same led strip if I scrap option B altogether? 1 power supply runs at 3w while the other at 10w. Just don't want the bigger power supply to back feed the smaller one and make it blow up. \$\endgroup\$
    – Kokachi
    Commented Jan 3, 2022 at 16:59
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    \$\begingroup\$ @kokachi if you alternate between supplies, you still wouldn't be able to do it simply. A given object will draw the power it needs, and the supply needs a way to either supply it, or protect itself. Your led strip of a fixed size with fixed resistors draws X current. Only way to change it is to change the voltage or modulate it. A typical 3W constant voltage supply does not have a current limiting function, it will brown out, shutdown or blow a fuse. So then your options. If you use a constant current device, it may regulate the voltage too low for the leds to turn on. \$\endgroup\$
    – Passerby
    Commented Jan 3, 2022 at 20:44
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    \$\begingroup\$ @kokachi so the only basic way to reduce a given strip to a specific current/wattage at a fixed voltage is through a modulating driver circuit. Adding multiple brightnesses makes it a bit more complex. Spheros answer shows one possibility. A microcontroller is another. There may be some dedicated ICs that do 1/3 2/3 3/3 off (and blinking) which is common in flashlights. Or as you mentioned, a cheap rf led strip controller. \$\endgroup\$
    – Passerby
    Commented Jan 3, 2022 at 20:49
  • \$\begingroup\$ I understand. I'll go with the microcontroller and mosfet pwm method. Thank you \$\endgroup\$
    – Kokachi
    Commented Jan 4, 2022 at 17:09

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