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I am trying to build a very thin (0.25 inches) LED board with 4 wire connects. No onboard chip What is the maximum number of LED's I can address? What concept design do you suggest?
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A total PCB of 0.25" thick allows lots of options.
Do you have room to fit a tiny SMD shift register IC on the PCB? The shift register can drive 8 LEDs without needing PWM, and the 3 wires needed to drive it can be bussed over many boards, so you can use the same 3 wires to drive ALL your boards for a large matrixing operation.
Or is there some reason you can't put a tiny SMD chip on the PCB (like the PCB is entirely filled with LEDs etc)?
It was a communications error on my part Chipwizard. I assumed "thin 0.25 inch" was the HEIGHT of the PCB, where it seems you meant the WIDTH was 0.25 inches.
If you have to make such narrow width PCBs full of LEDs (do they go in a tube??), maybe you could make the LEDs surface mount, and TSSOP shift register (or whatever small package you like) on the rear of the PCB. A through-hole double sided board will allow some vias under the IC and a few leadouts each side but it's still going to be tight as the IC requires 3 control wires, 2 power wires and 8 output wires...
Why on earth do you need to make matrixed individually-controllable LEDs in a tiny "stick" form factor??
A lot of design problems are best fixed at the beginning... What are the LEDs displaying, how many LEDs total are required, what is the total overall size of the assembly and and what might be the best way to do that task?
Point well taken. However placing any chip (PIC, LED-Driver, Register) will limit both current and voltage supply to each channel. In the prototype configuration current per channel is about 30mA per these LED specs, however voltage per set of 3 LED+resistor is 12v. With added on-board chip then additional transistors (N-MOS) is required to handle 12V. On a 0.25" wide board as you can see PCB real estate is tight.Maybe in that form factor you coul dput a PIC or other micro on board, and then you only need 2 power wires and one communications wire to control them?