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I want to build a circuit, that is a Raspberry Pi Zero W, powering 8 Adafruit Alphanumeric displays (or even more).

The max, theoretical current draw from an Adafruit Alphanumeric is 280mA. Therefore, 8 should be a maximum of 2280 mA. The Adafruit Alphanumeric display runs on 3.3v power.

The Raspberry Pi Zero W, should in theory is meant to be able to provide 50mA, but independent testing proves that it can potentially run up to 800mA source.

I have successfully run 2 of these devices off a pi, but when I add a third, I can start to see some power drain.

Therefore I am trying to figure out how to power the i2c displays, while still controlling them with the Raspberry Pi Zero W.

I purchased myself a 5v wall wart, with a 3A current rating, which in theory should be enough to power all the things I have. The people I purchased the device off sold me a 5v to 3.3v step down module, which was used in a personal project by the salesman to achieve something similar.

When I wire up a breadboard like so:

enter image description here

  • The wall wart is wired straight to the bottom breadboard rails.
  • The step down is wired up so that 3.3v power is supplied to the top rails.
  • The i2c device is powered off and grounded to the 3.3v rail.
  • The i2c device comms cables are connected to the pi.
  • The 5v power rail is used to power the rasberry pi via the gpio pins.

The rasberry pi boots up, but is not able to find the i2c device.

If I run a ground cable from the Raspberry Pi Zero W to the 3.3v ground, the i2C becomes visible, but there is a voltage drop on the rail to the i2c device. Here is a diagram:

enter image description here

I am not 100% sure what I'm missing here, I am making the assumption that the grounds isn't properly shared in the first diagram and is not able to communicate correctly with the Raspberry pi. I'm basing this off the fact that in the second diagram it's able to detect it.

Christos Hrousis
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2 Answers2

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Your question is unclear, and so is the wiring. One can only guess from the colours what is supposed to be what.

All Gnd pins on the Pi are connected (and to the ground plane). The same is not true of the breadboard, but it is unclear what you expect to be connected. If you expect the power rails on both sides of the breadboard to be connected YOU will have to connect them.

You appear to be trying to power the Pi through connections to the µUSB port pins!

I would STRONGLY advise you NOT to do this. If you want to power the Pi this way connect to pins 2 (+5V) and 6 (Gnd), although you would be better to power through the µUSB port and route the 5V TO the breadboard.

I don't understand why you are using a 3.3V regulator. The Pi 3.3V pins should be able to supply more than enough current. If you are using a regulator decoupling capacitors are MANDATORY.

PS Whatever you do DO NOT attempt to supply power to the Pi 3.3V pins. You will destroy the Pi!

Milliways
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  • Thanks for the tip on the 3.3v power, I have improved my questions a lot more to explain what I'm trying to accomplish, is there any chance you can help out a little further here? – Christos Hrousis Feb 07 '19 at 09:12
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When I first ran this test I was using a raspberry pi 3 B+, but when I rewired it to match the first diagram in the question, it actually worked (without having to bridge the negatives).

My theory is that either:

  • The 3 B+ was drawing too much current and for whatever reason was buggering things up
  • I had a faulty ground wire somewhere, or poorly grounded connection somewhere that allowed things to operate but not correctly allow comms.

I think the second issue was the one that was more likely.

Christos Hrousis
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