![]() ![]() So, I found a way to combine two op amps and a few resistors in such a way that a linear increase in resistance from the keyboard turns into an almost exponential voltage that the oscillator can use. But 555 timers cost much more than an op amp, and it's hard to find ones that run at low voltage. Other cheap synthesizer keyboards use 555 timers and rely on the exponential discharge of an resistor and capacitor to get an exponential output. There are many things that have to accounted for, or else the notes will sound too close or too far apart. But this requires matched resistors, thermal compensation, and circuit by circuit tuning. Most synthesizers use the exponential gain of BJT transistors to turn a linear voltage into an exponential voltage. It's a lot trickier to design a system that outputs 10, 20, 40, 80 Hz. For example, It's easy to design a system that will output 10, then 20, then 30, then 40 Hz. But the keyboard increases resistance linearly for each key pressed, and the VCO oscillator outputs a frequency that's linearly proportional to an input voltage. Musical notes increase in frequency exponentially. It's like a membrane button you'd see if you took apart a calculator, but your finger is the button pad. This current is just enough to turn on a transistor. Pressing a key with your fingertip allows a tiny amount of current to flow across the touch pad. Instead of mechanical buttons, each key is a resistive touch sensor. It's just 4 op amps, a few NMOS's, a speaker driver, and passives! It also doubles as my business card!ΔΆ Layer PCB, but all traces are on the top layer, the bottom layer is all ground plane. But, while working on the larger synth kit, I made the MicroSynth to test some of the circuits out. ![]() I'm not ready to tell you about the synth kit yet. MicroKits started out making low cost and easy to build theremin kits, and now I'm working on a synth kit. About Me: I'm David, owner and inventor at MicroKits. ![]()
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