I have a string of LED fairy lights (5v) that when I connect to power, only half of them light up. If I reverse the polarity, the other half light up. It’s my understanding that I need a h-bridge to rapidly switch back and forward polarities. I’m waiting to ensure that there is no chance that I will see flickering (hence need something in the kHz range?).

I’m wanting information on the type of h-bridge I should be looking for given the intended usage, and how I would wire it in? I looked around and see many options, but non stood out as being the obvious one to use.

  • 1
    \$\begingroup\$ To have an answerable question, you'd need to provide specifications of the lights. Not a purchase link, actual specifications. At the very least show how you've driven them in your experiment. Also worth noting that people generally can't quite see mains frequency flicker unless it interacts with something else; eg, rapid movement, or something at the same nominal frequency without an actual phase locking, eg, some video displays. \$\endgroup\$ Commented Nov 16, 2020 at 18:54
  • \$\begingroup\$ Note that an H-bridge is a circuit you make yourself out of MOSFETs - usually not a single part you buy. \$\endgroup\$ Commented Nov 16, 2020 at 19:33
  • \$\begingroup\$ Were these lights intended to be powered from 5 Volts AC? \$\endgroup\$
    – rdtsc
    Commented Nov 16, 2020 at 19:46
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    \$\begingroup\$ @user253751 on the contrary, for low to moderate power situations an H bridge typically is a purchased part, specifically an IC. The unstated current requirement is why this question is currently unanswerable and off topic. \$\endgroup\$ Commented Nov 16, 2020 at 20:02
  • \$\begingroup\$ May be to parts connected by mistake. Check if you can split it. If it pss \$\endgroup\$
    – user263983
    Commented Nov 16, 2020 at 21:56

1 Answer 1


An h-bridge provides the means to switch power, but not the control. With typical fairly light string, a simple small signal transistor based h-bridge would work as not alot of current is needed. Something like http://members.tripod.com/robomaniac_2001/id206.htm

Then you need a way to drive the h-bridge. A microcontroller or timer ic circuit like the 555 would work. There will be considerations for the led string resistor that you need to confirm based on the current draw.

  • \$\begingroup\$ Is it just a case of wiring the v5 power to the 555 timer, then to the h-bridge and onto the lights? Also will it switch fast enough that one wouldn’t notice a flickering? \$\endgroup\$
    – anthonyv
    Commented Nov 17, 2020 at 3:07
  • \$\begingroup\$ No. The 555 needs several passive resistors and capacitors to set it as a oscillator. Plenty of calculators online. And it should be enough to set to multiple kilohertz without issue. But it all requires some trial and error. It's not a single ic solution like the original board is. \$\endgroup\$
    – Passerby
    Commented Nov 17, 2020 at 4:04

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