Build Project: Dewster's D-Lev Digital Theremin

Posted: 12/2/2018 7:35:53 PM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

Very nice!  I assume U4 is a 3.3V regulator.

Yes.  This is the same circuit that was used on the AFE boards.

Is the extra line of thru-holes above the display board connector to transfer connectivity from one side to the other?

Also yes.  I thought I could get away with a single-side board, but when I decided to use the rear-mount straight header instead of a front right-angle header, I had to put the pads on the front.  One of the downsides of these homemade boards is that you can only have opposite-layer pad connections for parts where you can't access the component side for soldering.  This requires extra vias and it eats up space around the components.  Fortunately the machined-pin DIP sockets allow enough space for top-soldering.  The other type of sockets are much more difficult to solder.

Posted: 12/6/2018 12:17:31 AM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

I ended up making a main board to support the FPGA module and LCD display and at the same time take care of all of the interconnects (except to the encoder boards on the sides - these will still be jumpers).  For a while I was just going to do what Dewster did with the Dupont interconnects for all FPGA connections, but the EEprom and toslink transmitter needed a board anyway, so I just went for it.  I seriously doubt that this board will become operational without at least some cut lines and blue-wire mods, but hopefully there isn't a major problem with flipped orientations or something like that.  I was confused quite a few times in the layout process.  

This board is 4" x 6" and I was a little worried about the layer registration using the faster toner-transfer process, so I had to make it using transparencies and photopolymer resist film.  Toner transfer works well for small 2-layer boards, but despite  my best efforts in pre-drying the transfer paper base, I still get unpredictable shrinkage and mis-registration over larger areas when heat bonding to the copper-clad material.

This is a picture of the exposed board and the artwork used to make it:

And here is the etched and drilled board:

This main board will hold the LCD display on standoffs and will itself be suspended on brackets by the two encoder boards which mount to the acrylic panel.  This way the entire assembly with encoders may be removed from the panel and bench tested.

The back of the board has only two active components other than the FPGA module.  It's all just interconnects.  The header banks in the four corners  connect to the headers on the encoder boards.  If I didn't screw anything up, the order should match.  If not I'll have crossed wires, but that's still not a problem.

And this is what the board stack looks like:

Next up:  Making the tuner enclosure and main board mounts.

Posted: 12/6/2018 2:11:04 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Lookin' sharp Roger!

"... blue-wire mods..."

Where I worked it was "white-wire" and I never gave it any thought - until a new guy said it was "blue-wire" where he came from.

Posted: 12/6/2018 2:39:53 PM
DOMINIK

From: germany, kiel

Joined: 5/10/2007

Well done! Roger, though i rather won't label my boards i am interested in your technique. It looks like silkscreen. You are the one for climbing steep hills for sure … but silkscreening single boards … ??

Posted: 12/6/2018 7:00:32 PM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

Where I worked it was "white-wire" and I never gave it any thought - until a new guy said it was "blue-wire" where he came from. -Dewster

I think it depends on what your official corporate repair wire color happened to be.  We had a large spool of blue wire-wrap wire, which everyone knows makes far better repairs than the white type .

Well done! Roger, though i rather won't label my boards i am interested in your technique. It looks like silkscreen. You are the one for climbing steep hills for sure … but silkscreening single boards … ?? - Dominik

The labeling is done by the same toner-transfer etch-resist process that I use for etching smaller boards when not using photopolymer.  I have an old stock of toner-transfer paper similar to the type sold at pcbfx.com, which is really just a laser-printable paper that has a water soluble coating.  You print the silkscreen layer from your board layout software (positive/mirrored for the top silk) onto the special paper with a laser (not inkjet!) printer.  The paper is then aligned with the board and run through a hot laminator, and the toner from the print melts onto and adheres to the board. The board and paper are submerged in water for a couple minutes, and the paper floats off, leaving the black toner print on the board.

The only difference between printing a graphics layer this way and printing traces to act as etching resist is that an extra step is included where a thin plastic foil is run through the heat laminator to bond to the surface of the traces, providing a more effective seal against etchant than the toner alone can provide.  My results in the past were always unreliable until I bought a decent thermal pouch laminator and started using the foil.  The traces are more ragged under the microscope when compared to those done with the photo process, but they are still very good and the process is extremely fast.

There is also a white foil for bonding to the board labels if you want your screening to look white.  I find that the black on G10 substrate is nearly as readable.  All of the foils are quite cheap, and the paper is about 1USD per sheet.  YouTube has lots of videos showing the process.

But since you brought it up - yes, I have done single board and panel artwork silk screening (genuine) at home in past years, and I'm afraid I'm going to have to do it again for the artwork logo for the Moog Melodia front panel.  I still have my screens, but I need to get some fresh photo film (it's different from pc board film).  I'm hoping that the recent experience with photo boards will make it less painful and messy.

Feel free to send me an email if I can be of more help.

Posted: 12/7/2018 3:44:55 AM
DOMINIK

From: germany, kiel

Joined: 5/10/2007

Thanks! I wasn't aware that toner transfer is working that good.

Posted: 12/8/2018 11:04:10 PM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

Doing a little work on the final assembly of the main enclosure...

I bent some strips of 1/8" x 3/8" steel to support an internal acrylic top that would provide additional mounting surface for the toslink audio converter and USB blaster, and whatever else might come along.  I didn't want to have these extras mounted to the wood cabinet or the removable acrylic front panel.  Now by removing only the front panel nuts the entire electronics assembly can be separated from the outer acrylic shell to allow access to all sides of the circuit boards.

This whole assembly then mounts inside the outer shell with only the encoder nuts holding it in place:

It is getting dangerously close to showtime, and with that the reality of trying to make this thing work.  Nothing has been pretested, and the laws of probability say that there will be at least some wiring errors on the boards.  These are the last beauty shots that I will have before programming the FPGA and the inevitable cutting and modifying begins.

Wait.. I do have to make a tuner enclosure.  This will help me procrastinate a bit more.  

Posted: 12/9/2018 4:48:38 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Wow Roger, super nice!  Nothing says "real equipment" like internal metal chassis stuff going on.

You probably won't need the USB Blaster in there much, but it's good that it has a spot in case you do.

I'm still not sure what I'll do with the SPDIF box (in or out of the enclosure).  Electrically connected audio is more universal, gives you a convenient way to ground the Theremin, chassis jacks are easier to mount, and it contains / concentrates all of the optical and powering mess inside.  But optical is just so clean. Could do both I suppose by driving a second TOSLINK SPDIF box off of another FPGA pin.

(BTW, is anyone else getting that heart stopping "an error occurred" message when posting to TW?  I've gotten one for each of my last 4 or so posts.)

Posted: 12/9/2018 10:49:47 AM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

Wow Roger, super nice!  Nothing says "real equipment" like internal metal chassis stuff going on.
Thank you! 

You probably won't need the USB Blaster in there much, but it's good that it has a spot in case you do.
It's mounted on 3M Command Strip velcros, so it can come and go at will.

I'm still not sure what I'll do with the SPDIF box (in or out of the enclosure).  Electrically connected audio is more universal, gives you a convenient way to ground the Theremin, chassis jacks are easier to mount, and it contains / concentrates all of the optical and powering mess inside.  But optical is just so clean. Could do both I suppose by driving a second TOSLINK SPDIF box off of another FPGA pin.

Personally I always prefer equipment that has an internal power supply with a standard IEC detachable power cord, and I would try to implement that for any final version of a homebuilt theremin as well (not this prototype, though).  This would provide the grounding without the use of a less-mainstream wall wart (as with the Etherwave) and would still allow the simplicity and noise immunity of an optical output.  A production theremin would be a different matter however; the added complexity (read:cost) of UL/ETL or CE certification with an internal power supply makes the use of pre-approved wall warts more attractive.

As a sidenote, since I play seated I have experimented with placing a foil "butt plate" under my seat that is then connected by a short wire to the theremin ground.  This provides a more well-defined ground return than that provided by a mains or audio cable ground, and the positive effect is most obvious when noting the much-reduced frequency shift when touching any metal on the theremin body itself (unlike the Etherwave, the Subscope has exposed metal switches). The volume and pitch sensitivities change slightly as well.  Clearly this is not for everyone, particularly standing players, unless you resort to conductive (chain-mail?) undergarments with a wrist-strap style snap connector. 

(BTW, is anyone else getting that heart stopping "an error occurred" message when posting to TW?  I've gotten one for each of my last 4 or so posts.)

Yes, every time recently.  And I still forget to grab and copy the text before hitting the "Post" button.  Sometimes I just write everything as a notepad file first and then copy/paste to the editor.  But I even forget to do this most of the time.

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