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PlingPlong - A portable synth

This is a small (10x20cm 4x8") portable battery-operated synth with a two octave touch keyboard and four voice polyphonic digital sound.

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I'm using two microcontrollers on the board - a Microchip PIC18F44K22 for the user interface and a Atmel ATMEGA328 for the sound generation.

The PIC18 is using the internal CMTU module for capacitively sensing the 24 touch keys. It is also in charge of scanning the 15 tactile buttons, refreshing the 15 status LEDs and the 7-segment displays, and polling the Rotary Encoder.

Whenever a keypad is touched or the rotary encoder is turned a standard Note On/Off or Control Change MIDI message is sent to the ATMEGA328.

Currently the ATMEGA328 is running a straight copy of the Midi Vampire-I firmware. The ATMEGA328 is also connected to a SD-card connector for future use.

The sound-generating part is contained in a 5x5cm "selfcontained" area to the right on the pcb having pads and via holes for power, midi and SD-card signals. This makes it easy to replace just the synth with something else than the MidiVamp. Design a new 5x5 cm pcb and solder it to the pads on the main board.

Files available at my Github as usual: https://github.com/SmallRoomLabs/PlingPlong

Re: PlingPlong - A portable synth

Reply #1
This is great!

Is the pads for the keyboard just exposed / tinned copper or some other coating?

Re: PlingPlong - A portable synth

Reply #2
The pads are covered with both the green soldermask and white silkscreen so there's no electrical contact with the fingers.  I'm using a capacitive sensing method to detect the presses/touches.

The pads are like small capacitors and when you put a finger on it the capacitance is getting larger which can be detected by the microcontroller.

At start I read and store the capacitance of each pad to have as a base value, then when playing I just make new readings and compare with the base values and if there's a big enough change I consider that as a press.

Re: PlingPlong - A portable synth

Reply #3
Just out of curiosity, do you usually handsolder your SMD parts or use a heat gun / oven?
I consider myself quite good at soldering thru-hole parts but haven't really tried SMD yet.

Re: PlingPlong - A portable synth

Reply #4
I always handsolder with a fine-tipped iron. It's easy and fast enough for me.  If I one day get one of those pneumatic solder paste dispensers I might start using my oven or hotair gun, but until then I only use the gun for desoldering parts when doing rework.

Re: PlingPlong - A portable synth

Reply #5
I tallied up the costs for a PlingPlong unit

PCB $4.70
PIC $2.80
MEGA $2.70
All other parts $1.90 (inclusive rotary encoder and knob!)

The '328 is really expensive compared to the much more powerful Pic18F44k22...

Re: PlingPlong - A portable synth

Reply #6
Cool, I want one. :)

Can I order a kit from you? If not a kit, maybe only the PCB and I'll source the components myself?

Re: PlingPlong - A portable synth

Reply #7
That's a really nice project.
How do you find the time to keep coming up with things like this?
I've been working on my drum-machine for nearly two years now...

Re: PlingPlong - A portable synth

Reply #8
[quote author="project2501x"]Can I order a kit from you? If not a kit, maybe only the PCB and I'll source the components myself?[/quote]
I don't think that this might not be an ideal project for someone that is soldering smd for the first time. With a TQFP32, a TQFP32 and SMD SD-card socket.

Hmm...  Maybe I could do a kit with the hard-to-solder parts already pre-soldered and pre-programmed.  Then there's only standard 0805 caps /resistors and some transistors to solder in addition to the thru-hole parts.  I'll have to do some serious thinking about that.
[hr:][/hr:]
Catmacey: Well, with some experience something like this is rather simple to whip up.  I think I spent three evenings researching touch technology, doing the schematics and the pcb routing.  Then another evening to solder it up and write the basic software for the PIC when I finally got the PCB from the fab.

But now I want to add a step-sequencer to the code and also some kind of bootloaders in both microcontrollers so I can update the firmwares by copying them to a SD-card and just plug it in to the synth.  So to get to a finished & more polished product I will need an additional full weekend I think.

It's not so hard to find time when having no kids in the house. And it also helps that I almost never watch TV. :-)

Re: PlingPlong - A portable synth

Reply #9
[quote author="matseng"]Hmm... Maybe I could do a kit with the hard-to-solder parts already pre-soldered and pre-programmed. Then there's only standard 0805 caps /resistors and some transistors to solder in addition to the thru-hole parts.[/quote]
That sounds like a great idea. If you decide to go down this path, I'll be the first in line! :)

Re: PlingPlong - A portable synth

Reply #10
Since the sensepads are under the soldermask and silkscreen, did you find any info about how much stuff you can have on top and still make it working?
I wonder if it would be possible to sense through plexiglass or something?

Re: PlingPlong - A portable synth

Reply #11
I can detect my finger through 2.55 mm acrylic, but the difference of the values I read with / without the finger there is very small so some tweaking of the charge current and charge time would most likely be beneficial for the quality of the detection.

Thinner plastic like a credit card is of course better...

Re: PlingPlong - A portable synth

Reply #12
very cool, the acrylic can be thin, it's mostly meant to have an nice surface for placing things on..

But i guess one can only detect fingers?, not if something else is placed there?

It seems that this pic have various types of capsense stuff? 28 cap touch channels, but also mTouch support?, CTMU?, or is all this the same?, not possible to do X/Y position sensing?, only points?

Re: PlingPlong - A portable synth

Reply #13
It detects a lot of things.  I placed a stack of old business cards on a pad and the value went down from the base reading of 950 to 915 - easily detectable.  A cr2032 coin cell battery gets the reading down to 880.  A finger 350. A finger while touching my laptop is 170.

Re: PlingPlong - A portable synth

Reply #14
I've registered here just to show interest in this if it ever becomes a kit.  Saw it on DP's FaceBook.