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I have a set of LED lights that I like that only have a battery pack option.

To make it work, I cut the battery pack off and wired it to a 5V power supply. Only half the lights turn on at a time. If I reverse the polarity, the other half come on.

How I would fix this?

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  • \$\begingroup\$ Question is not clear. Show us the schematic of what you have. And explain clearly what you want it to do.... and also what sort of components you have access to (or the skill to use)... like a 555 timer etc \$\endgroup\$
    – Indraneel
    Nov 16 '20 at 9:21
  • \$\begingroup\$ How many cells were in the battery pack? It could be that 5 V is not enough (or too much) for it to operate properly. \$\endgroup\$ Nov 16 '20 at 10:13
  • \$\begingroup\$ It was 3AA. So 4.5v. I checked with the manufacturer and the LEDs on the wire are rated for 5v as well. \$\endgroup\$
    – anthonyv
    Nov 16 '20 at 18:43
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The simplest option would be to open up the battery pack you cut off, and find the circuit inside that drives the LEDs. Re-connect that to the string of fairy lights, and power it from your power supply.

Given that the LEDs are connected with different polarities, the circuit must be converting the DC from the batteries to AC.

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  • \$\begingroup\$ As mentioned above, part of the problem with that is that the light controller doesn’t really work the way I want it to. I want to control it with an independent timer (which is driving other equipment etc)... the timer that comes with the battery box is a fixed time, etc. I just want them to turn on and stay on whilst power is supplied. \$\endgroup\$
    – anthonyv
    Nov 16 '20 at 9:03
  • \$\begingroup\$ Is there any way of getting the leds on the same polarity? Like is it the way the wires are joined together that I could change, or is it something about each led? \$\endgroup\$
    – anthonyv
    Nov 16 '20 at 9:11
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    \$\begingroup\$ @anthonyv Short of re-soldering half the LEDs (as suggested by syntax), there's nothing you can do about the polarity. If you simply want to light all the LEDs, see the answer from Neil_UK. \$\endgroup\$
    – Simon B
    Nov 16 '20 at 10:17
  • \$\begingroup\$ @anthony if you wanted to control just the power then a relay or mosfet/transistor would be used between the power and the original circuit. \$\endgroup\$
    – Passerby
    Nov 16 '20 at 21:16
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The 'battery pack' you cut off isn't just a battery pack, it's also a controller that can produce partial string lighting effects. The lights are connected in two reverse paralleled groups for that reason.

To light all of them, you'll need to reverse the polarity quickly enough to give the visual effect of all LEDs staying on. A few hundred Hz is fast enough for most people, but kHz should be good enough for everybody.

If you don't want to build something from discretes, then a H-bridge motor controller, or maybe even a class D audio amplifier module could be pressed into service.

As you're wiring it to a power supply, you could use an AC power supply of either the right voltage, or one of higher voltage with a series resistor to drop the voltage. This will mean that the alternate strings of LEDs light only at 50 or 60 Hz, depending on your location. This is slow enough that almost everybody will notice a perceptible flicker, but as they're a decoration rather than 'lighting', maybe you'll get away with it. It's by far the simplest solution.

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  • \$\begingroup\$ Thanks for the input. What would be the easiest option to wire inline? Also, as mentioned below, of what I don’t understand at the moment is that when I wire the power just to the battery pack the same polarity issue exists (that didn’t with the battery). I was thinking that I could just wired the power directly into the pack, but it doesn’t seem to work either. Any thoughts? \$\endgroup\$
    – anthonyv
    Nov 16 '20 at 7:28
  • \$\begingroup\$ @Anthony it should work by powering the pack from a charger. Unless it uses a special wiring of the batteries. How many batteries does it use? Are the batteries wired in series or parallel or both? As in is there more than just two wires running from the batteries to the pcb in the pack? \$\endgroup\$
    – Passerby
    Nov 16 '20 at 7:46
  • \$\begingroup\$ No there aren’t more than 2 wires, but I did try and run 5v to the board rather than the 4.5v that the batteries would have supplied... hoping that I didn’t fry the board. With the options that avoid needing to use the board, what’s the easiest/cheapest to get up and running, is there an off the shelf component for doing the desired polarity switching that’s easy to wire in? \$\endgroup\$
    – anthonyv
    Nov 16 '20 at 8:00
  • \$\begingroup\$ Unfortuantely, by the laws of consumer electronics economics, the cheapest way to drive that string of LEDs is with a mass-produced custom LED string driver, like the one you've just cut off. \$\endgroup\$
    – Neil_UK
    Nov 16 '20 at 8:27
  • \$\begingroup\$ Well part of the problem with that is that the light controller doesn’t really work the way I want it to. I want to control it with an independent timer (which is driving other equipment etc)... so even if it costs slightly more, I’m ok with that. It’s more having the ability to install the component(s) required and that they don’t cost an arm and a leg. \$\endgroup\$
    – anthonyv
    Nov 16 '20 at 9:00

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