This is less than the error we would get from the tolerance in our resistors. On average, the output voltage will be off by a fraction of a percent. Here's what I got, from excel:Įach row shows the output voltage from a different key pressed, and compares it to an ideal exponential output. So if you've read this far, I encourage you to grind through the math to see what results you get. It's a ton of math! But also something any third level EE student can figure out. Because of the way the noninverting side of this opamp is set up, the virtual node will always be 1/3 the voltage of tuner voltage.įinally, the op amp will double the voltage at this node, with reference to 1/3 the tuner voltage. 3) a 100K ohm resistance to the virtual node of the exponential op amp. 2) A 37.4K ohm resistor to the tuner voltage. This node sees: 1) A resistor to ground that is a multiple of 1K, from the Keyboard. How can this be? Consider the node between the keyboard and the exponential opamp. With one button, we can double the tuning pitch, which doubles the output frequency, giving us an octave.Įxponential: Now for the real math! The op amp that creates the exponential voltage for a linear change in resistance has no components that act exponential. But if the octave switch is closed, the op amp acts as a non inverting amplifier with a gain of 2. When the octave switch is unpressed, the op amp is configured as a unity gain buffer. Octave: The output of the fine tuning opamp can be either buffered with a gain of x1, or doubled. This voltage is used to shift the pitch of the whole keyboard. If multiple keys are pressed, the highest pitch key will dominate, as all the lower keys will be shorted out.įine Tune: A potentiometer can adjust the voltage going into the op amp near where it says "Fine Tune". Each 1K ohm increment is one semitone (up one black or white key), with less resistance corresponding to higher pitch. Depending on which key is pressed, the keyboard will connect a different amount of resistance to ground.
#BEST MICROSYNTH SERIES#
Keyboard: As discussed in a previous log, the keyboard is a series of transistors to ground, simulated here as a switch. The joke on Hackaday is: why use a microprocessor when you can use a 555? I'm proud to announce there are times when even a 555 is too much! Technically the response is polynomial not exponential, but the polynomial is within a fraction of a percent of a true exponential. 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!Ģ 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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