[posted] MiniTouch : low cost T3.2 synth for children workshops

E

emmanuel63

Well-known member
Hello,

Here is a simple project I am happy to share. I have designed this compact and low cost synth especially for children workshops. It is easy to build and play. Children from 10 are able to build their own instrument (under the supervision of an adult). The wooden box is laser cut. This synth is basic but musical and fun to play thanks to big capacitive pads and a few pots to tweak the sound.

Here are the main functionalities :
- Teensy 3.2
- audio output : builtin DAC
- 1 band limited oscillator - amplitude envelope ADSR - LP filter - filter envelope - LFO - reverb - portamento (glide) - transposition
- polyphony : 4 voices
- 15 capacitive touch pads
- MIDI
- 15 presets
- stand alone small app for presets programming
- line/headphone output
- battery operated
- builtin amp and loudspeaker
- cost : around 30 €

There are 2 PCB. The first one is for the keyboard, and the second for the T3.2, pots, and mini-amplifier. The capacitive pads use FastTouch library. I have used inter-digit electrodes to avoid ground return path issues. Touch detection is very fast a reliable with a rather simple code. I have used the builtin DAC. Sound quality is OK even on big amps. The mini class D amplifier requires short connections and good grounding. I have used a standard TV 10W speaker.
The code is very commun :
- touch pads calibration and detection routines
- synth section
- eeprom management
- MIDI management
Sharing all the code doesn't make sense since it works only with my design. But I would be happy to share parts of it or explain some aspects if you are interested.

I noticed the use of band limited oscillator is very CPU intensive. I had to limit the polyphony (4 voices) and overclock my T3.2. But it sounds so much better that it is worth it.
Thank you for your feedback on this little project.

Emmanuel Presselin



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clap.jpg

Great little device! I'm not a kid, but I want one!!! It sounds like you did this to contribute to kid workshops. Are you going to release a build document, pcb design file, or application code? It would be a great little project for my nephew. No worries either way just wondering. Congrats on the project!
 
Hi

Nice project.

Have a bit more info on the inter-digit electrodes pcb? Special things to be aware of?
Have you tried making a capacitive slider?
 
Hello Jay,
I am thinking of packing this project with a "how-to" guide. But it takes a lot of time ! I think my code also needs to be reviewed by a qualified coder. But if many people ask, I would be happy to collaborate and give the PCBs and everything.
Emmanuel
 
Hi Alain,

My electrodes consist of 2 tracks. The first one is connected to a Teensy pin, the second to the ground (see the joined doc). Doing this way makes possible to operate the synth with batteries. I have used FastTouch library to manage the capacitive readings. It works very well with consistency.
You can also use MPR121. This circuit works very well with battery operated devices and don't need special electrodes design. But for this project, I really try to keep the price as low as possible. MPR121 is a very efficient chip (if you dig a little the code and skip automatic calibration).

I didn't investigate a lot on capacitive slider. I know it's possible with special PCB but I didn't try.
Emmanuel

View attachment PCB_PCB_MiniTouch - sensors V3_2021-08-05.pdf
 
Great project Emmanuel!

I think my code also needs to be reviewed by a qualified coder
Click to expand...
I'm sure on this forum there are lot of qualified coders that will be happy to review your code!

Paul
 
Hi Paul,
Thank you for your feedback.

So if there is anybody that would be OK to help me to review my code, I would appreciate a lot.
I can comment my code to make it as clear as possible and give explanations.

Emmanuel
 
Hello,

I have added a drum set to play beats.
I have also made a little android app to program sounds from your smartphone.
 
emmanuel63 said:
Hi Alain,

My electrodes consist of 2 tracks. The first one is connected to a Teensy pin, the second to the ground (see the joined doc). Doing this way makes possible to operate the synth with batteries. I have used FastTouch library to manage the capacitive readings. It works very well with consistency.
You can also use MPR121. This circuit works very well with battery operated devices and don't need special electrodes design. But for this project, I really try to keep the price as low as possible. MPR121 is a very efficient chip (if you dig a little the code and skip automatic calibration).

I didn't investigate a lot on capacitive slider. I know it's possible with special PCB but I didn't try.
Emmanuel

View attachment 25511
Click to expand...

Thanks.

Did you notice a better or worse result depending on the size of the electrodes?
Have you designed the electrode layout you're self?
 
emmanuel63 said:
Hello,

Here is a simple project I am happy to share. I have designed this compact and low cost synth especially for children workshops. It is easy to build and play. Children from 10 are able to build their own instrument (under the supervision of an adult). The wooden box is laser cut. This synth is basic but musical and fun to play thanks to big capacitive pads and a few pots to tweak the sound.
Click to expand...

Great project for kids of all ages!
 
Hi AlainD

Bigger electrodes tend to give a stronger signal, but it is nor really important. I have 1 cm electrodes to change octave and it works.
Yes I have designed my electrodes. But of course I have not invented interdigit electrodes ! You can make your own design with any PCB software.
Emmanuel
 
Hi Adrianfreed,

Are you the author of the brilliant fastTouch library ?
If so, here is a big thank you. Your library is absolutely great. I have used it for this little project.
 
Hi,

I have updated my synth a little. Minor bugs correction, stability and box design. I have also added a drum kit.
I have re-draw the MIDI interface to tweak and save the presets.
A lot of MiniTouch have been made now during various workshops. Children love it and some people use it on stage.

Here are now pictures.

EmmanuelMiniTouchBR.jpg
pannel.png
 
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