what hardware/AD converter for 20 capacitive sensors with raspberry pi

Thread Starter

TillFly

Joined Oct 26, 2016
57
Hey there,
I need to measure 20 capacitive sensors. so far I just have the experience to handle a few over an arduino.
now I need to do it with a rapsberry pi to control a software.

what ADCs would you recommend me? I dont need a big resolution, just need to detect two states - no-touch or touch, if that makes sense.

I imagine its also important to know how to structure and package all the wires with microcontrollers and the pi. can you recommend me any guide how to wrap things up thightly for clean installation? it shall hang on a wall in the end.
do I also have to care about isolating the cables, to protect from interference?

I hope questions are not to mixed up,
Focus is on ADCs first :)

Kind Regards,
T
 

nsaspook

Joined Aug 27, 2009
7,181
If you mean raw capacitive touch PCB trace sensors I would use a external controller with a ADC module designed for capacitive touch applications instead of just a regular ADC. While you can use a regular ADC the software to handle many sensors in a reliable manner can be complex.

You could use a couple of dedicated touch chips like this
https://www.seeedstudio.com/Grove-12-Key-Capacitive-I2C-Touch-Sensor-V2-MPR121.html?utm_source=blog&utm_medium=blog

This might be a steep learning curve if your experience is arduino. :( You can build a controller that can handle 20 sensors and more but it requires ACTUAL programming and circuit design.
http://ww1.microchip.com/downloads/en/Appnotes/TB3198_CVD_90003198A.pdf
http://ww1.microchip.com/downloads/en/Appnotes/TB3146-Analog-to-Digital-Converter-with-Computation-Technical-Brief-90003146B.pdf

A 43 channel channel controller. https://www.microchip.com/wwwproducts/en/PIC18F57K42#additional-features
 
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Reloadron

Joined Jan 15, 2015
5,341
what ADCs would you recommend me? I dont need a big resolution, just need to detect two states - no-touch or touch, if that makes sense.
If you are just detecting two states wouldn't you only need a DI (Digital Input) since it would be a yes / no touch / no touch?

If for some reason you actually want an analog input A/D I just did some experimenting with a ADS1115 from TI. While it only has four channels (two if you want differential inputs) the nice part is it communicates over I2C so you can stack them up to four which while only 16 channels my point is I would look for something which has I2C capabilities. Here is an example of an I2C 8 channel 12 bit from Linear Technology. You could use 3 of those. While the Rasberry Pi also has pins to configure as GPIO. I guess since you have a true (touch) or false (no touch) I don't understand the need for any A/D conversion?

Edit:

If you mean raw capacitive touch sensors I would use a external controller with a ADC module designed for capacitive touch applications instead of just a regular ADC.
OK so your read was analog outputs to read? I was confused with this:
what ADCs would you recommend me? I dont need a big resolution, just need to detect two states - no-touch or touch, if that makes sense.


Ron
 

DickCappels

Joined Aug 21, 2008
6,304
If you are using an Arduino with an AVR controller (ATMEGA to be specific) look into Microchip's QTouch technology. It might even be supported by some Arduino code somewhere in the Universe.

- Atmel® QTouch® library support
̶ Capacitive touch buttons, sliders and wheels
̶ QTouch and QMatrix® acquisition
̶ Up to 64 sense channels
 

danadak

Joined Mar 10, 2018
4,057
Another approach to use PSOC.

Here is a basic Capsense project. Note it used only a small amount of overall resources onchip,
see right hand window. Single chip solution. Drag and drop and config. Also routable onchip and out to
GPIO.

In PSOC a "component" is an onchip resource. Attached a list of onchip components for PSOC 5LP family.
Includes LUTs, basic logic, SAR ADC (another onboard ADC used for the Capsense), Digital Filter, OpAmps, Vref, PGA,
Comparators, DACs, Mixers, PWM, Counters, Timers, COM (SPI, UART, I2C, SMBUS...), LCD, Display

1580651043712.png

I only show 10 buttons, and a slider, just add more buttons in the wizard to get to the limits of the GPIO on the board.

Simple $ 10 board to do this (for ~ 32 GPIO which can be CapSense) - https://www.cypress.com/documentation/development-kitsboards/cy8ckit-059-psoc-5lp-prototyping-kit-onboard-programmer-and

IDE, PSOC Creator, and Compiler free.

Core is ARM, M0, M0+, M3, or M4 /M0+ dual core families.
 

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