My old AS synth project...

[marinedalek]

Well, hello there! My first post here after lurking for several weeks... thought I'd share the synth that I built for my AS Design & Technology project.

It's a triangle-core synth based on a simple op-amp integrator design. Not having time for lots of prototyping I took the rather odd route of making it resistance-controlled rather than voltage-controlled. Each keypress on the copper touch-sensitive keyboard triggers a reed relay that switches in the tuning pot corresponding to that note. There's also a variable rate and length "sequencer" which plays each note of the keyboard in turn - by entering non-standard tunings various riffs can be programmed in.

As far as sound shaping goes, there's a PWM section with its own LFO driven by the triangle core, as well as pure triangle and square waves, with divided -1oct and -2oct square waves also included. Filtering is very basic, just two 6db/oct buffered passive lowpass/highpass combinations, which can be chained in series for stronger filtering.

Initially there were going to be two additional oscillators with non-keyboard-controlled tuning for fun swoopy sound effects, which is why I included 2 filter sections (one for osc1, the second for the other 2 oscillators). The first of these was to be a saw/pulse oscillator, and the second a sine/square oscillator - however time constraints meant these were never built .

Here's a flickr set with images of the project taken on my camera phone a few months ago.

http://www.flickr.com/photos/marinedalek/sets/72157607621408769/

During construction a fault developed which means the output is extremely distorted (~infinite gain) with a lovely DC offset just to make it even more unpalatable. I'm hoping to fix this some time when I get some equipment set up.

[RF]

Hey nice job - Congrats!

[fonik]

very nice, indeed.

what circuits did you use for this cute and sweet little handheld touchkeyboard? why did you etch the keys not as filled planes?

[marinedalek]

To detect keypresses I used a simple potential divider circuit based around the 4071 OR chip. Each touchpad consists of a copper "fork" which is connected to the touch circuit, and a surrounding area of copper connected to the positive voltage rail. Touching a pad pulls the voltage of the fork up and activates the OR gate, giving a clean digital gate signal.

I didn't do solid pads, because the keyboard relies on detecting conductance, rather than detecting induced EMF in someone's finger.

[fonik]

i see. thanks.
oh, and it's realy cute, the keyboard.

[Mubo]

we want sound clips!

[marinedalek]

The synth's currently residing at my grandparents' house due to lack of space here, so sadly there are no samples at the moment