multiplexer with teensy 3.6 - 5V or 3,3V ?

[emmanuel63]

Hello,

I have designed a midi controller with 144 pads. I use twelve CD74HC4067 multiplexers to drive my pads, plus pots and switches.
Result is fine with fast response.

My pads are pressure sensitive to retrieve velocity parameter. Each of them are wired with a resistor to form a regular tension divider. I have connected the 3,3V teensy output pin to every tension divider.

Do you think I could use the 5V out pin ?
This would give me a bigger range of values, without exceeding 3,3V max input of analog pins.

I have enclosed diagrams to make it clear. Diagram shows the wiring for 1 pad connected on the first mix channel.

Thanks
Emmanuel

 

[oddson]

https://www.sparkfun.com/datasheets/Sensors/Pressure/fsrguide.pdf

Final pages show interface circuits.

If you go with a higher upper voltage make sure you limit the output or use a 5v tolerant Teensy like T3.5

Your option 2 is hazardous if R2 is not sufficiently large.

 

[emmanuel63]

Thank you for your answer. It makes sense.
Thanks a lot also for the link.

 

[oddson]

Some/most of the circuits have an op amp post divider as the impedance is very high when little or no pressure is applied.

Given that you can use 3.3 volt as VCC on a rail-to-rail op amp and it will limit the voltage for you and provide a more stable voltage for the pin to read when lower pressures are present.

At least I think so... not an EE and I've only had limited experience with FSRs.

(Edit - somehow I didn't really register the multiplexer part in this. I think it may limit the output voltage to it's Vcc... if so that would protect the T3.6 pins but you would want more than my 'I think' before you try it.)

(Edit 2 - given it's analog and specs give maximum on resistance it may not limit to Vcc on outputs... did I mention I'm not an engineer )