Construction Instructions, What’s Your Function?

While all the components are trickling in from the postman, I thought it would be a good idea to get the PhotoSynTheremin assembly instructions in order. Instructions that the ten year old me would appreciate.

I have very fond memories of putting kits together as a kid back in the days before Al Gore invented the internet (yes, I’m that old). Instructions were one of those things that were hit or miss. Either simple enough for a noob to understand off-the-bat, or conceived for the grey-bearded, flux-breathing, seasoned technician – of which I was certainly not. One of the first kits that I put together was an FM transmitter. You know, the kind with an electret microphone, planar antenna and drunk-driver of a slaloming tank circuit, sure to spray spurious emissions about the RF spectrum in some illegal fashion. The instructions however, were clear as spin on glass. Assembly was a breeze. This was one of my first kits and I recall gaining all the confidence in the world, limited only by my Radio Shack 30-watt, cold-solder-joint-inducing soldering iron would allow.

My second kit was a high-voltage power supply for a 5mW HeNe laser project outlined in RadioElectronics magazine. Now that I think about it, why did my parents let me play with this stuff? The instructions were clear enough, but I recall spending a good hour on the phone with the support staff, because it used every type of capacitor under the sun and the instructions had no indication of voltage rating, dielectric type, or what each capacitor should look like. Granted, at that age I would’ve been safer playing with a baseball bat and a beehive, but this was the latchkey generation after all. Eventually I got things sorted out, but fortunately never powered the thing up as I didn’t understand the input current requirements to get the thing producing voltage higher than 12V.

After several other kit building experiences, I think in 1997, I had gotten my ham license. This opened up a whole new universe of kits and propagation modes that I hadn’t been exposed to before. I’d never put together a proper transceiver before so I snagged up one of those TenTec 6 meter kits so I could chew the rag with people on the other side of the earth through the magic of sporadic E propagation. Upon opening the kit and disgorging all of the components upon my ESD-unsafe work surface, I realized this would be quite the difficult endeavour.  This kit was a MONSTER! Usually I spend about an hour or two banging a kit together, but in this case I can remember the eye strain, sweating, second guessing, the winding of toroidal inductors, troubleshooting and days, not hours of painstaking assembly. After completion, I took it to my college’s electronics lab in an effort to follow the alignment instructions. But, I did something wrong. Somewhere, in that vast box of color coded and faintly stamped ratsnest of passive components laid some flaw; some mistake I would never find, nor care to search for. And on the shelf it went for eternity, eventually finding its way into my junk box.

I suppose the goal here was to come up with a kit that was just the right amount of components. Harking back to those days when I was learning about electronics, building kits that were “just the right size”, where I could spend a couple of hours on the weekend warming my lungs with the fumes of molten lead and singing my fingers with the fiery tip of an unregulated Chinese soldering iron. Using integrated circuits from a decade when Ronald Reagan was president, phones were anchored in place by a helical cord, and you could walk into a proper electronics shop and buy a fistful of resistors and perfboard to take on the next challenge.

I hope I can get back to that place and make building this particular kit an enjoyable experience for others. It is neither too small of an effort, nor too complex. My goal was to make this kit experience somewhere between the spurious RF transmitter and that horrific TenTec radio. Hopefully I’ve achieved the kit-building butter zone.

Prototype Built, Parts Ordered!

circuit boards

I am happy to report that the PhotoSynTheremin prototype is built, and I am starting with a small batch of qty 25 boards. The next batch of 25 (last one had a slightly small LED footprint – Unacceptable!) will arrive in a week, electronics arrive today, and plexiglass cover should be here in about a week. Once everything comes in I will make the kits available on Tindie.com.

The original prototype had some issues which required a second board-turn:

  • As the gain was turned up, the frequency would oscillate and shift up an octave.
  • The frequency range of the audio oscillator behaved differently than the breadboard. What a surprise.
  • In an effort to mitigate ambient light & saturation of the photodiode, I opted for a different pair of photodiode and IR LED; This needed testing.

In the above picture you can probably see a bunch of MLCC capacitors strung together in a rats-nest. This was an effort to find the right capacitor for the integrator circuit and eliminate the unpredictable oscillator action. I ended up increasing the capacitance from the breadboarded 0.001uF to 0.01uF on the PCB. I suspect that parasitics in the breadboard caused skewed results (which is usually the case) when adapting to the “real” PCB, but in doing so we trade off stability with frequency range. That’s ok, because it’s ear-piercing enough!

The gain of the LM386 was reduced by adding some resistance (or attenuation) at the amplifier input which helped with the gain & range issues. Lastly, the new photodiode and IR LED combination worked a treat.

Now, to just finish up drafting the instructions, start the new YouTube channel, get some boxes and other ancillary items and it’ll be ready to annoy The World!