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9v rgb led mod

I am able to power on my LED unit here with a 9V battery. I'm using a recharble lithium 9V as well.

I can't figure out why I can't get blue, purple, and white. The only colors I get are red, orange, yellow, and green.

The LED panel is normally used for some lights in a car, 12V DC, but I know that 9V can power everything at a lower brightness.

Any ideas what is going on? I can't change power sources because the mod goes into a collectible figure that only has room for a 9V sized battery.

The LED panel is this.

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  • \$\begingroup\$ Looks like It is WS2815 set or similar. 12V should be. \$\endgroup\$
    – user263983
    Apr 17, 2021 at 21:10
  • \$\begingroup\$ @user263983 I think its using plain 5050 rgb not smart leds. \$\endgroup\$
    – Passerby
    Apr 17, 2021 at 23:08
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    \$\begingroup\$ I know that 9V can power everything at a lower brightness How, exactly, do you know that? \$\endgroup\$ Apr 18, 2021 at 5:31
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    \$\begingroup\$ "I know that 9V can power everything at a lower brightness." That would be true for filament light bulbs, but LEDs don't work the same way. They need a minimum voltage before they produce any light at all. If the device was designed for automotive use, the minimum supply voltage would be 12V (and the typical voltage more like 14V to 14.5V) so as the answers describe, 9V simply isn't enough \$\endgroup\$
    – alephzero
    Apr 18, 2021 at 13:11
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    \$\begingroup\$ @Destructo 9V may work for your single color LED because the LEDs may be organized differently from the LED lamp that does not work with 9V. You can't expect two different LED lights to behave identically if they are not identical. \$\endgroup\$
    – Justme
    Apr 19, 2021 at 15:50

3 Answers 3

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It is a bit hard to see from the picture, but since each LED has six pins, and knowing that it is meant for 12V supply, it is extremely likely that the LEDs might be arranged so that there are three parallel chains of three LEDs in series.

The blue LED requires approximately slightly more than 3V voltage drop per LED, and since there is 3 in series, the three blue LEDs require slightly more than about 9V to work. A 12V supply with small series resistance and transistor to drive them would work.

So basically, the LED board is meant to operate with 12V supply, and it can't work with a 9V supply.

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    \$\begingroup\$ More like 3.4V drop. \$\endgroup\$
    – Passerby
    Apr 17, 2021 at 22:54
  • \$\begingroup\$ @Passerby A 3.4eV photon would be at the edge of our visible spectrum. More likely it would be approx 2.8eV for visibility sake, and the additional ohmic drop means that a very low current is driving the LEDs. \$\endgroup\$
    – Aron
    Apr 20, 2021 at 1:50
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Blue LEDs require a higher voltage than red and green. If the panels are designed to work on 12 V, there may not be enough voltage to light the blue LEDs when run on 9 V.

The blue LEDs must work for the panels to produce white and violet light.

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    \$\begingroup\$ What about trying an A23 battery? These little guys produce 12v. I'm sure the usage time is significantly shorter than a 9v. \$\endgroup\$
    – Destructo
    Apr 18, 2021 at 2:08
  • \$\begingroup\$ A 55 mAh battery vs a 900 mAh battery. At a 180mAh draw (100% full white). You will have 15 minutes run time, more like 9 minutes. \$\endgroup\$
    – Passerby
    Apr 18, 2021 at 6:21
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    \$\begingroup\$ Bought a 12v A23 and it worked! Everything lights up now! Shame there's not a bigger 12v the size of a 9v. \$\endgroup\$
    – Destructo
    Apr 18, 2021 at 10:30
  • \$\begingroup\$ @Destructo: A pair of AA or AAA batteries in series with a 9V would do it. (Their mAh capacity will very likely be higher than the 9V). You can easily get NiMH rechargeable AAs, but rechargeable 9V batteries are rarer so you might just go for 8x or 9x AAA batteries to get the voltage you want in a rechargeable format. (I've had chargers with 9V ports, years ago when 9V batteries were used by more things.) Or I guess a few AA batteries with a boost converter could work, if you want to include a DC-DC boost regulator IC in your homebrew. \$\endgroup\$ Apr 19, 2021 at 4:05
  • \$\begingroup\$ Unfortunately the space is limited inside the figure, basically the size of a 9v battery. Anything larger has to go outside the figure which wouldn't work for the intended hiding all the wires and having the figure be portable. Thank you though! \$\endgroup\$
    – Destructo
    Apr 19, 2021 at 5:48
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Without knowing the schematic im going to just guess these are standard 5050 leds put in a 3 led series setup. For purple you need Red and Blue 100% on. For white you need Red Green and Blue 100% on. Since your source voltage is much lower than the typical 3.2 to 3.6V forward voltage for a blue led diode time 3 ( 9.6 to 10.8) , thats an issue. You need to account for the voltage drop across any resistor and transistor used to turn them on at a minimum. You are lucky green works as some green led diodes are also 3.2 to 3.4V at 20mA. Red is typically 1.8 to 2.2V so they are much more forgiving.

Since it sounds like you are modding some device internally, you may need a small step up or boost module. This is likely taking up to 180 mA for full white plus the Bluetooth modules power so let's say 500 mA at peak. There are a ton of boost modules the size of a dime or quarter online for cheap. Like https://www.pololu.com/product/2117 (no affiliation or experience with this one). Some questionable boost modules on eBay and the like for a buck (pun intended).

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  • \$\begingroup\$ Thanks for the info. I'm not sure how I would imement this either. This particular DIY doesn't attach to anything specific. It just rests inside of a figure that has a translucent chest. So the light will shine through the plastic. \$\endgroup\$
    – Destructo
    Apr 18, 2021 at 10:32
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    \$\begingroup\$ So you just wire it in line with the power wire. Battery wire step up module wire led panel \$\endgroup\$
    – Passerby
    Apr 18, 2021 at 10:50

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