Hi All,
I have a hardware question re a sound installation I've been working on.
I have a touch sensor grid, which will have 54 touch sensors, coming from 5 MPR121 [https://www.sparkfun.com/products/9695] capacitive touch sensor chips, going to an Arduino Mega, allowing for serial communication.
I've opted to work off a schematic advised by BlinkM for their Smart LED [datasheet @ http://thingm.com/fileadmin/thingm/downloads/BlinkM_datasheet.pdf] to allow the 5 chips to simultaneously work and communicate. I have attached a screenshot of the schematic from the data sheet.
My hardware knowledge is not great so my question is, would the MPR121 also require the 2.2k resistors? Or as they are different objects/chips, will the require different resistors, and if so, can anyone advise me on which?
Thank you in advance for your thoughts and time. It's much appreciated.
Hi jonnyb
In general, you only need one set of pull-up resistors on an I2C bus (which is the protocol being used in this case).
BUT ... from the link you posted, it looks like the capacitor sensors run on 3.3V while the Arduino I2C bus runs on 5V, and the BlinkMs are shown as running on 5V too.
Some hardware will be required to convert between the different voltage levels. If you search this website, there are a number of threads on the topic. And ready-made level converters are available.
However, there is something else to look at. The datasheet on the Sparkfun website for the sensors says that the device can configured to have have 1 of 4 different I2C addresses. If you connect 5 of them in parallel, you will run out of addresses. There are hardware solutions for this (search for "I2C port expanders") and software ones (search for "Arduino softi2c" or "multiple i2c"). Also, some port expander chips can do level conversion as well.
Regards
Ray
Hey Ray,
Cheers for your response. Yes, you are correct, the sensors are definitely 3.3V. I didn't think of this, so thanks for pointing it out. I made a noise generator last week with a L7805cv voltage regulator so I guess it would that would do the trick? You reckon I'd only need one resistor once that's taken care of then?
My apologies, I meant to type 4, not 5! However, I would prefer the installation to be larger, and I wasn't aware of soft-i2c buses. As I'm trying to avoid using something like the PCF8575 I2C Expander, that's really helpful information.
Thanks a lot Ray!
Jonny.
Hi Jonny
To clarify what I meant about level converters ... as well as a 3.3V power supply for the sensors, you also need to translate the two signal connections between 3.3V and 5V.
An easy way to do this is with a ready made converter.
Adafruit do this one: 4-channel I2C-safe Bi-directional Logic Level Converter [BSS138] : ID 757 : $3.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Cheers
Ray
