Designing Flexible Sensors in KiCad with Flex PCB

Trill Flex brings flexible touch sensing to your maker projects. Trill Flex comes with a one-axis multitouch sensor printed on flexible PCB, but we designed this sensor to be detachable so you can create entirely custom sensors perfectly suited to your own projects. This tutorial shows you how!
In this tutorial we will create a custom 30-button flexible touchpad. You will learn how to set up a KiCad project and include the Trill design files, draw the pads, add a ground plane, and get your design ready to be printed. (Every purchase of Trill Flex comes with a unique discount code for $20 off flex PCB printing with our friends at OSHPark.com.)
Want a text version of this tutorial? Visit learn.bela.io/flex-design
Trill Flex is available now on the Bela shop: shop.bela.io/collections/tril...
What you’ll learn in this tutorial:
Intro to Flex PCBs 0:00
Set up: Download and install KiCad 2:00
1. Download the KiCad Trill Flex libraries 2:44
2. Load the trill-flex symbols and footprints 3:27
3. Set up a new KiCad project 5:22
4. Add the Trill Flex symbol to the schematic 5:42
5. Add the Trill Flex footprint to the PCB 6:30
6. Draw the pads 9:12
7. Connect the nets 14:10
8. Make the connections 15:46
9. Draw an outline 18:34
10. Run the design rules and electrical check 19:23
11. Generate the file for manufacturing at OSH Park 20:24
12. Get your sensor ready to use! 21:47
Learn about Trill: bela.io/trill
Buy Trill Flex: shop.bela.io/collections/tril...
Music by Vula Viel (vulaviel.com).

[@MakingSoundMachines]

Really nice tutorial! Thank you!

[@irfanrajput8236]

Thank You, Great Tutorial!

[@andrephilipp3525]

OMG! What a class!

[@Bianchi77]

Nice video, thanks :)

[@robinjansen51]

13:04 Why are you adding a ground plane? And why is the ground plane above the traces?

[@billyhart3299]

Can the flex PCB go direct to a regular FR4 board or does KiCAd force you to use a connector?

[@BelaPlatform]

Hi Billy, if you would like to create a PCB which is a combination of FR4 and flex substrate have a look at Rigid-Flex PCBs which combine the two in one design. There is more information here: www.pcbway.com/blog/News/What_is_Flex_PCB____An_Overview_of_Flex_and_Rigid_Flex_PCB.html

[@jaydenstrudwick4329]

That was an awesome tutorial! Have you investigated placing the Flex Board behind a material (e.g. plastic), would this affect the reading?

[@BelaPlatform]

Thanks, happy you enjoyed the tutorial! Regarding sensing through other materials, it is definitely possible to sense through thin layers of vinyl, acrylic or other non-conductive materials. Experimentation with different settings is key here. See here for some more information on materials which work and how to adjust the settings learn.bela.io/products/trill/integrating-trill-into-your-projects/#covering-trill-with-another-material

[@jaydenstrudwick4329]

@@BelaPlatform Thanks for the reply! When wet, damp or raining would this affect the readability?

[@BelaPlatform]

@@jaydenstrudwick4329 You're welcome! Moisture in general will definitely affect the reading you get as it conducts electricity. If the top of the sensor was wet then you might end up with sporadic readings as there will be many activation points. The plus side of this is that you can use water as an extension of the capacitive pad. This means that with our Trill Craft sensor you can attach the pad connection to a glass of water or something similar and detect when the water is touched.

[@thilinarajapaksha1711]

Awesome tutorial. I just wanted customized interdigitated electrodes using flex PCB for research purposes. Is this recommended for that?

[@BelaPlatform]

Thanks, happy to hear you enjoyed it! You can definitely use Trill Flex for these kind of experiments. You have 30 channels of capacitive touch at your disposal so you could even draw up a custom flex PCB with 15 variations of the interdigitated electrode design (presuming each one uses two channels). Just make sure the design ends in the ribbon cable like in the tutorial. You can then read from the Trill Flex board with a microcontroller of your choice or an embedded system like our Bela. Let me know if you have any other questions.

[@thilinarajapaksha1711]

@@BelaPlatform Glad to hear that. Thanks for quick reply

[@josue_garcia]

How can I associate the ground plane to pin 1 and pin 32 at the same time?

[@BelaPlatform]

Hi Josue, these two pins are already connected together on the symbol and footprint of the connector so it is only necessary to connect to one of them and there will automatically be an electrical connection between the two. Make sure to route your connections to both though.

[@ziqijia5203]

Hi, Bela, It's a great video! Do you have an idea that how small of the sensor unit can be? Can I have smaller overall size with same amount of sensor units?

[@BelaPlatform]

Hi Ziqi, thanks! The sensor units can be quite small, well less than the size of a fingertip. Smaller sensors won't produce quite as much signal and they won't sense through overlay materials as well, but they definitely work. For example, if you look at our designs for Trill Hex you'll see it uses a lot of very small interlocking diamond-shaped pads.

[@ziqijia5203]

@@BelaPlatform Thank you for the prompt reply! I already made the order.(the one shipp to Gsinesville, FL) I will let you know if I have further questions!

[@Mennenth]

Would it be possible to create multiple strips/areas per unit? Ie, a 4x4 grid (16 channels), then 2 strips each 7 channels long. All on the same board? Thanks!

[@BelaPlatform]

Sure, all the pads are independent. You can group them as you see fit as long as you interpret the readings appropriately on the host. You can use the CentroidDetection library for that.

[@ziqijia5203]

Hi, I was trying to follow the tutorial to customize the sensor but got stuck at 10:53. The square I drew is not filled. I tried to enable the "show filled zone", tried different net designation, tried to connect the pad with vias. But non of the ways above make the copper pad fill. Do you have and idea about this?

[@rockndancenroll]

Did you solve it ? You have to edit the pad and assign to it the net of the trace and via that connects to it. If for example the trace is called "signal1" you have to assign the specific copper area to signal 1 through the settings.

[@ziqijia5203]

@@rockndancenroll Thanks for your reply. The pattern is still not filled even if I assigned net. I couldn't get any support on this issue from neither the Kicad forum nor the Bela. I think it is because the unconventional design patterns. So I switched to Altium designer. The pattern fills in that software

[@Lion_McLionhead]

Easier to import a bitmap than use that kicad polygon editor to draw copper shapes.

[@Willpowerstudios]

Hi, when drawing the shape with the 'Add a Filled Zone' tool, it has no fill. it's only an outline. because of this, I cannot round the squares. Do you know why I have no fill ? How can I draw filled squares ?

[@chair_audio]

To (re)- fill zones, press "b"

[@Willpowerstudios]

@@chair_audio hi, thank you. it's not working. it has no effect... a window pops up called 'Fill All Zones', a time bar fills up but no changes... I wonder why...

[@mattdorsey2244]

In your next video will you pronounce kiwi?

[@marinkojukic6498]

are these sensors touch and pressure responsive?

[@oromoiluig]

they measure capacitance only. With the right geometry of pads you can get touch size which can work as a proxy for touch pressure in certain contexts.