Need help designing a LED strip

Thread Starter


Joined Apr 21, 2016
Hello all,

I am trying to assemble a serie of LED strips for a Starmap I am building. The problem is I don't know enough about electronics to design the circuit, I can solder and assemble if I have the schematic in front of me.

I am not sure what kind of information is required so I'll try to give as much as I can :

- I need to have 94 LEDs
- 87 3mm LEDs : Working voltage:3.0~4V, Working current: <= 20mA
- 7 5mm LEDs but the specs are unkown
- It can be in different strips
- Would like to use a potentiometer to bring the lights down if it's too bright (maybe two if the 3mm have a different brightness than the 5mm ones, if possible)

- I have a 24V DC 500mA power source but can change if it's not enough
- I don't know where I should put to use the resistors (I have 100, 1K, 4,7K, 10K, 47K, 100K, 1M, can buy more if needed)
- Does it require anything else ? Like a transistor (I have triode BC548 0.1A /30V NPN power transistor TO-92) or so ?

Thanks a lot :)


Joined Nov 30, 2010
The basic procedure is to add up the voltage of some LEDs, subtract that from the supply voltage, and calculate a resistor to use up the remaining voltage when 20 ma is flowing. I would normally allow 10% to 15% of the voltage to be used by the resistor. If you use 20 volts worth of LEDs per string and 200 ohms, you can dial the voltage down from 24V to 20 V to achieve the dimming. If you want to get the best efficiency, you are going to be using a pulse width modulator to drive the LEDs.

Can you make a PWM driver?
Can you adjust the power supply voltage?

Thread Starter


Joined Apr 21, 2016
Sorry for the late reply, winter is back so I'm back on that project too :)

I am not sure to understand everything you wrote, I'm really a newbie in electronics but I can solder the components

Here is the schematic posted by the person who made the Instructable. Is it possible to have a schematic with the resistor and potentiometers values for 8*12 LEDs in series I'll buy what's needed if missing ?

Thanks a lot :)



Joined Aug 7, 2008
The blue,?, 3 V LSDs will have 7 in series with a 150 ohm R, 13 strings in parallel.
The white, ?, 5 mm in one string, possibly same 150 R.
Your test circuit will work for the 5 mm string @ 24 V supply.