Need help with multiplexer capelet

Rinkel123

Hi everyone,

I’m pretty new to Bela (I do have some experience with Arduinos and similar stuff), and I’m really enjoying it so far! I’ve been working on a project that uses a bunch of pots and switches. Up until now they were all connected to an Arduino Mega, which sent serial data to a Max/MSP patch handling real-time audio processing controlled by those pots and switches. That setup worked great, but ideally I’d like the whole project to be as portable as possible (so, no laptop involved). Bela seemed like the obvious next step.

The issue I’m running into is the following: I have 13 pots and 26 switches that all need to connect to the Bela. I’m using the multiplexer capelet, and some smaller tests combining pots and switches worked fine. After that I wired everything up to the multiplexer capelet as follows: for the pots, the outer pins go to 3.3V and ground, and the wiper goes to the capelet inputs (I’m using the breakout boards soldered onto a PRT-12070 solderable breadboard). For the 26 switches, I connected one pin to ground and the other pin to the capelet inputs. I also wired all leftover unused multiplexer pins to ground to avoid any floating inputs. I then ran the “multiplexer-spectrum” example from the Bela IDE and saw that the pots worked perfectly; changing their values clearly affected the sound. The switches, however, did nothing.

I figured that since all the switches were tied to ground, no voltage could be measured and therefore no proper readings were happening. So I rewired all the switches to 3.3V instead and ran the example again. This time the exact opposite happened: pressing the switches clearly changed the sound, but the pots stopped working entirely.

So that’s where things stand now. I’m not sure whether this is a wiring issue in how I set everything up, or if there’s a deeper problem. Since both the pots and the switches work individually, I feel like I’m missing something to get them to work together the way I need. Any help would be greatly appreciated!

giuliomoro

The question you should ask is: what voltage do you expect to read on the capelet pin when the switch is not pressed?

If you think about it, you will quickly come to the realization that you don't know! And if you don't, how can the machine know?

You'll need to drive a voltage (0V or 3.3V) on the pin while the switch is not pressed and the other voltage (3.3V or 0V, respectively) when the switch is pressed. This is customarily achieved via pull up or pull down resistors. These are large-ish resistors that connect the pin to the voltage you want to read when the switch is not pressed.for instance:

3v3
 |
R 10k
 |
 --------- Va
 |
 \ S1
 |
GND

When the switch S1 is open (not pressed), the voltage at Va will be 3.3V; when you press the switch and close the circuit, it connects Va to GND and the voltage at Va is therefore 0V.

R1 at 10k is large enough that there will not be excessive current flowing to ground through it when the switch is closed and is small enough that the resistor divider it forms with the input resistance of Va won't drop the voltage very far from 3v3. A wide range of values would work there, e.g.: 1k to 100k.

The above is the pullup configuration. Swap 3v3 and GND in the diagram for the pulldown configuration.

If you wonder why does it work without an external resistor when connecting directly to the digital pins, the answer is that they have a pulldown resistor built in, so there's no need for the external resistor.

Rinkel123

It took me a while to get around to rewiring everything, but now everything works perfectly! Thanks for the quick reply, and I really appreciate the extensive explanation!