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Multiple LED Backpacks
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Multiple LED Backpacks

by ForToffee on Wed Aug 28, 2013 10:02 am

Hi there,

Firstly, my electronics experience is somewhat rusty so please bear with me; I last picked up a soldering iron in anger some 15+ years ago when I did electronics at school.

I'm having a stab at recreating the Delorean Time Circuit project with my Pi. As a result I need to wire together a few LED Backpacks to achieve the effect. I have a couple of (probably very newbie) questions as a result

1) In the excellent article about the backpacks and a Pi it suggests using a logic level converter (http://learn.adafruit.com/matrix-7-segm ... rything-up) if running at 5v. Do I need to just wire the Pi SDA/SCL to A1, B1 and the multiple backpacks into A2, B2? Or do I need to use one channel pair per backpack?

2) If I'm powering the backpacks via the 3.3v rail does this do away with the need for a logic level converter

3) If I power the backpacks via an external 3v power supply (2xAA) will this cause any problems? I have a feeling that I'm going to run out of juice if I power the whole lot via the Pi.

For reference I have 3x 7-Segments on a backpack, they're common anode and with some funky looking bit bashing code I have successfully driven them for my needs at 3.3v. Now I want to add a further 2 more backpacks.

Many thanks
ForToffee
 
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Re: Multiple LED Backpacks

by adafruit_support_mike on Wed Aug 28, 2013 12:20 pm

ForToffee wrote:1) In the excellent article about the backpacks and a Pi it suggests using a logic level converter (http://learn.adafruit.com/matrix-7-segm ... rything-up) if running at 5v. Do I need to just wire the Pi SDA/SCL to A1, B1 and the multiple backpacks into A2, B2? Or do I need to use one channel pair per backpack?

The backpacks use the I2C communications protocol, so you can run all of them from the same SDA and SCK lines. You'll only need one level shifter to protect those two lines.

Every I2C device has an address, and every I2C command starts with the address of the device the controller wants to talk to. We've designed the backpacks so it's easy to give each one its own address: http://learn.adafruit.com/adafruit-led- ... 2c-address

Once each backpack has a unique address, adding them to your I2C bus is just a matter of connecting the wires.

ForToffee wrote:2) If I'm powering the backpacks via the 3.3v rail does this do away with the need for a logic level converter

Yes, but we don't recommend it. The chip that controls the display is made to work from a 5v supply, with about half a volt of tolerance either way (4.5v from three AA batteries should work). Outside the recommended voltage limits, there are no guarantees on how the chip will perform.

ForToffee wrote:3) If I power the backpacks via an external 3v power supply (2xAA) will this cause any problems? I have a feeling that I'm going to run out of juice if I power the whole lot via the Pi.

See above about using 3xAA rather than 2x, but using external power is a good idea. The RasPi's GPIO pins are made for communication, not power.

It's okay to use multiple power sources in a circuit, but they all need a common reference point. You get that by connecting all the GND lines together.
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adafruit_support_mike
 
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Re: Multiple LED Backpacks

by ForToffee on Sun Sep 01, 2013 6:22 pm

Many thanks for the reply Mike, very thorough.

Partly driven by not wanting to order more bits for this "little" project and partly by "coz I can" I wired all three backpacks (with 3x 4 digit displays) to the 3v3 rail (pin 1 on the GPIO header) to see if it would work...... and it did!. Granted the refresh rate is visible when changing all 12 digits on one backpack but for my purposes that's fine.

So just in case anyone else is planning on doing anything similar; it's feasible but your mileage may vary!
ForToffee
 
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Joined: Wed Aug 28, 2013 9:42 am

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