Voltage levels on analog inputs on Bela

bvk

Hi,

I have two FSRs connected to the analog ins of the Bela with the Bela powered over USB. I am using a simple 10k resistor and some breadboard connections with the Bela's ground and 5V connections for this (similar to the Bela tutorials online (). However, the voltage levels as depicted using the oscilloscope in the Bela IDE are changing almost daily and in seemingly unpredictable ways. We have tested with a dedicated power supply and exchanging FSRs.

Any advice on how to make the voltage levels on the analog ins more stable?

giuliomoro

I recommend you use 3.3V instead of 5V, it is going to be much quieter and reliable.
Can you show a diagram of the actual wiring that you are using?

bvk

Ok, I will test with the 3.3V soon. More quiet and reliable sound great. I assume it will not fix changing reference levels though?

I connected the FSR as in this diagram: https://learn.bela.io/tutorials/pure-data/connecting/analog-input/

giuliomoro

bvk I assume it will not fix changing reference levels though?

well .... I don't understand exactly what you mean by "changing reference levels" and how that could be "almost daily". With 5V as your voltage reference, you are at the mercy of your 5V power rail, which can show severe fluctuations depending on a variety of external factors. By using 3.3V you remove those external factors and you can verify whether the issue persist and if needed explain your issue in more detail.

bvk

Hi Giulio,
Thanks for the reply.

Attached two screenshots with the analog in "reference levels" from two FSRs connected to the Bela as depicted here. Screenshots are from the IDE oscilloscope. As you can see from the PC time, the "reference levels" (meaning no force applied to the FSR) changed within the span of 10min.

I will order a 3.3V adapter but delivery time is several weeks (Canadian winter season;-)). I read here that "Powering Bela Mini from a battery with less than 4.5V will cause the analog inputs to stop working". I do not have a Bela Mini but doesn't the Beaglebone and normal Bela require 5V?

Thanks for any input!

giuliomoro

What's the resistance of your fsr when it's disconnected from the circuit when nothing is pressing on it and when it's fully pressed?

Can you show a picture of your wiring?

5V is what they need for power in order for everything to work. The ADC range is 4.096V and in order to work the ADC needs at least 4.5V on the 5V line.

bvk

Hi Giulio,

After some testing, I see that the resistance of the FSRs changed from 2M ohm to 30k ohm, not sure why. It might be the quality or shelf life of the FSRs then? I think safe to conclude the issue is not on the Bela side but on the FSRs, thanks for the support in any case.

On another note, I do not understand when you say "5V is what they need for power" in your previous reply when you also said "I recommend you use 3.3V instead of 5V" higher up. Can I power my normal (not-mini) Bela with 3.3V as a more stable power supply?

Thanks!

giuliomoro

bvk On another note, I do not understand when you say "5V is what they need for power" in your previous reply when you also said "I recommend you use 3.3V instead of 5V" higher up. Can I power my normal (not-mini) Bela with 3.3V as a more stable power supply?

The ADC needs at least 4.5V to work properly. Its full scale input is 4.096V. For most applications, using 3.3V as the reference voltage for the sensors you connect to them is the best choice. It gives you decent range (about 81%) and noise. Even lower noise and full-range is achieved by connecting to pin 10 of the analog in header. That is a 4.096V buffered voltage source from the ADC. Make sure you don't inject any noise into it or the performance of the ADC may be degraded.