Hi, I'm Andy!

Biosensing, Brain-computer interfaces, CS + HCI at Carnegie Mellon

MPR121 Touch Sensing Across Barriers

Published March 13, 2021

Hello! Today I received the MPR121, a nifty little 12-channel touch sensor:

Glamour shot of the MPR121 touch sensor

Glamour shot of the MPR121 touch sensor

The chip is pretty simple, uses I2C to measure each pin's capacitance using the charge then discharge trick I used here. It gives a touch/no touch output, or the raw sensor values per each pin (ranges from 0-400ish). Also has a built-in filtered output that provides smoother data if you want it.

The setup is easy and not very interesting since the chip is so single-purpose. I'm much more interested in showing you how well the sensor detects touch through a barrier.

Testing materials

I used what I had near me, and those were: saran wrap, masking tape, and two kinds of Scotch tape. Here's the table.

Table of sensor outputs for the MPR121

As mentioned before, the sensor values range from 0-400. On the bare surface I was able to get down to 48, which is a big SNR! None of the others even came close to that, only dipping 10% of the max value for the Saran wrap. This was probably the thinnest barrier I tried, and even that was too much. Despite the low SNR, the sensor still detected my touches consistently when separated by most of the materials I tried. Works great! :)

The capacititance didn't drift that much across my trials (~15 minutes), so I'm not so worried about needing to recalibrate very frequently. These types of biosensors are usually pretty finicky in that regard, so I'm glad. I also didn't try fabric or any porous material because I needed this to be waterproof, but it'd be cool if YOU (dear reader) did, and then told me about it!

That's all for now! Cya!

MPR121 wrapped in saran wrap during testing

MPR121 wrapped in saran wrap during testing