Can I drive 4 shift registers in parallel?

Other Hardware LEDs and Multiplexing
1

Hi all,

I’m building a little mood light project and so far I’ve got 2 common anode RGB LED’s and a switch which changes the light “moods”. Each LED has it’s own program so in total I’m using 6 pin outputs and a ground plus 1 pin for the switch.

I want to extend this so that I have 4 mood lights (1 for each corner of the room) all connected to the arduino as the central controller. Each mood light will consist of 4 RGB LED’s - 2 will take the arduino pin outs and I’m thinking 2 wired in serial to boost the lumens a bit.

I’ve got a car battery power supply with a car socket / USB adapter (which I’m not using yet) and I’ve ordered an LM2596 voltage regulator which I plan to put before the car socket to regulate the 12v.

I’m pretty sure that I could just get 6 transistors and amplify the existing signals to the mood light modules but this would require 7 core cable to each mood light.

I’m wondering if I can limit the wiring (and the number of transistors) by putting a shift register in each mood light and then sending the data in parallel to each one? I’m thinking here of running the latch, clock and data pins from the arduino into a transistor and then connecting the shift register pins to that output. This way I would only need 5 core cable (12V, Gnd, Latch, Clock, Data) - is this possible?

If it is, how could I add those 2 extra serial RGB’s without overloading the shift register pins?

Is there a simpler way of achieving the same effect?

Many thanks,

Ben

2

I've got 2 common anode RGB LED's ............ and I'm thinking 2 wired in serial to boost the lumens a bit.

You can not wire LEDs with common anode or common cathode in series.

You need to wire the shift registers in series and run softPWM code.

http://www.elcojacobs.com/shiftpwm/

3

Grumpy_Mike:
You can not wire LEDs with common anode or common cathode in series.

You need to wire the shift registers in series and run softPWM code.

http://www.elcojacobs.com/shiftpwm/

Roger that. So I need to put 6 transistors in each mood light then to get that shift register pin signal to my parallel RGB's with enough current or could I get away with driving 2 LED's from each pin if I current limit each one to 15ma? Or is there a simpler way?

Thanks again.

4

could I get away with driving 2 LED's from each pin if I current limit each one to 15ma?

Well no because that exceeds the power for conventional shift registers.

However, if you used a TPIC6B595 power shift register you can sink 150mA from each output. You need common anode RGB LEDs for this, but it is probably what you need.

5

You sir, are the man.

I checked out the data sheet for that TPIC6B595 and it looks like it's exactly what I need - I'm going with some of those common anode piranha LED's.

Thank you so much for finding the time to answer my question.

Best,

Ben