bypass the need of 100s of resistors

please enlighten me
example :
i have shift register with 8 led connected to it
each led have 100ohm resistor

what happen if i instead of using 8 resistors
i use 2 for the volt input of the shift register

:open_mouth:

What shift register?

What is the spec of the LEDs?

How many LEDs will be on at once?

upgradecomplete:
what happen if i instead of using 8 resistors
i use 2 for the volt input of the shift register

All sorts of unreliable behaviour and possible damage. :astonished:

First and foremost LEDs are current devices not voltage devices. Your suggestion may sort of work but no reliability or life in that circuit. You will have to control the voltage extremely close. You will have fun finding the IC to do what you want. The easiest solution is to get a IC that is designed to drive LEDs with an internal current sources or sinks depending on the part. That will keep your resistor count low and allow the circuit to operate on standard voltages and save you the time of designing a voltage translation circuit.

upgradecomplete:
please enlighten me
example :
i have shift register with 8 led connected to it
each led have 100ohm resistor

what happen if i instead of using 8 resistors
i use 2 for the volt input of the shift register

:open_mouth:

You'll fry the chip as its inputs will be at 5V from the Arduino.

This is not how to do this, you need a driver chip with built-in current sinks for driving LEDs such as: http://www.ti.com/lit/ds/symlink/tlc5917.pdf

Yeah - you either use oodles of resistors (or resistor networks - they make them in both through-hole and SMD, with a few options for how they are wired), or a chip that has constant current drivers designed for driving LEDs.

Or you use WS2812/etc individually addressable LEDs and sidestep the whole problem (actually, these have the constant current drivers built-in - but all you need to care about are the power/ground/in/out pins). Depending on how you want your LEDs arranged, it may be possible to do it all with WS2812 arrays (they make circles, squares and strips), or with the christmas-light shaped WS2811 strips. Note that if you make your own PCBs for these, you need to include decoupling caps along the length of the strip (if you look at strips, you'll notice caps every led/every few LEDs - without these, when you set a LED to be brighter, the voltage can briefly brown out and reset the LED, turning it off).