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Alright, so I bought my son a battery operated Jeep for Christmas that we're trying to make look like his dad's. It came with a number of lighting components like his dad's, but hilariously the driving lights on the back bar are just plastic housing sans lights. I popped them open and there's space for 7 lights in each housing (so 14 total), but given some quick math I'm thinking of omitting the center light and just doing the 6 that are in ring (12 total). For simplicity sake I'd prefer to keep it to 12 volts so we can just use an 8 AA battery holder rather than two 9v (I have a perfect spot I can mount a flat 8 AA battery holder for easy access. I'm working with white 3v 20mA diodes. Have I totally miscalculated that I could do four parallel series of 3 each, needing 4 resistors (I think 150 ohm each??)? I should also mention I'm going to mount a switch on the dashboard so my son can flip them on and off as he wants, but it's illuminated. The switch supports 12 or 24v. So, if I'm adding an illuminated switch into the equation do I need to go all the way to 24v? If so should I just wire all 7 LEDs?

Usually math helps me make decisions but in this case it's twisting me around a bit. Any help would be much appreciated.

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  • \$\begingroup\$ 12v, 3 series and 4 parallel with 150 ohms is a reasonable solution. \$\endgroup\$ Commented Nov 22, 2021 at 2:14
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    \$\begingroup\$ Your calculations are perfect for a 12VDC solution. Your battery pack will last 2x longer if you go with a 330 ohm resistor. Slightly dimmer but your son will be just as happy. \$\endgroup\$ Commented Nov 22, 2021 at 2:14
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    \$\begingroup\$ Choosing narrow beam <30,000 mcd LEDs @ 20 mA in < = 20 deg are more important than the choice of R. But it means you can use less current. 30,000 mcd at 20 mA at arms length is hard to look at and possibly harmful closer so 5~10 mA is adequate for these types in 4500' to 5000'K \$\endgroup\$ Commented Nov 22, 2021 at 3:39

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Your general understanding is correct. You can run 3 leds in series with a single resistor, off 12V. The 7th Led you can wire it with its own resistor. Not as efficient but would still work.

I'm assuming this is a rideable power wheels thing? You could tie into the existing battery instead of adding separate ones. Most power wheels run off a 12V SLA battery. Or use a USB C power delivery capable power bank and a "pd trigger module" for 12V out (or just use 5V and all the leds in parallel) as it may be a better battery solution for you.

The illuminated toggle switch would be like wiring another led or neon lamp in parallel. So you don't need to change anything for the 12V idea.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Yep, power wheels type thing. I had considered tapping into the included battery but don't want to void the warranty or anything. It already has a TON of LEDs on it so I want him to be able to use it without the battery running out in 10 minutes. It could be because it's late...but when you say 5v and all the LEDs in parallel...First, I don't think I'd be able to pull that off because there's a good 8" between the two lights, so at best I could do is two sets of 7? Which I think then I'd need at least 6v? And resistors for each diode (never done anything this complex and finding mixed info)? \$\endgroup\$
    – KC Reeley
    Commented Nov 22, 2021 at 3:48
  • \$\begingroup\$ @kc electronically it would be 14 sets of 1 led + Resistor each. 5V is enough. (5 V - 3 V) / 0.02 = 100 Ohms but 150 may be better. If you look at my schematic above, the dotted boxes around the leds are really just for your reference, there is no electronic difference. All the strings are in parallel. \$\endgroup\$
    – Passerby
    Commented Nov 22, 2021 at 4:00
  • \$\begingroup\$ @kc with 5V though you would need a different illuminated toggle switch though. \$\endgroup\$
    – Passerby
    Commented Nov 22, 2021 at 4:01
  • \$\begingroup\$ So now I know I'm just tired but wouldn't it be two sets of 7 parallel leds? I'm having trouble wrapping my head around how to make them parallel when the "clusters" are so distant from one another? That aside, 6v cutting it too close to have the illuminated switch? Although, now that I'm thinking about it even more, all the illuminated toggle switches I've seen are 12v minimum. Simplest and most logical path forward given that? \$\endgroup\$
    – KC Reeley
    Commented Nov 22, 2021 at 5:07

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