After building out the Fuzz Calculators I decided I wanted a helper to calculate the capacitor and resistor combos for simple hex-schmitt oscillators. These calculations are especially helpful when building polyphonic synths, and stuff that isn’t just a random noise source.
Schmitt RC Oscillator
CD40106 / 74HC14 • note + audio
α
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β
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lnK
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Result
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Note
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Select mode and enter values to compute. T = R·C·ln(K), K = ((1−β)α)/((1−α)β).
Potentiometer Helper
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ℹ️ About Schmitt RC Oscillators
The hex Schmitt trigger (CD40106 / 74HC14) forms a relaxation oscillator. The cap charges/discharges between thresholds:
VT+ = α·VDD, VT− = β·VDD
The period is T = R·C·ln(K), so f = 1/T where
K = ((1−β)·α)/((1−α)·β).
- CD40106: α≈0.65, β≈0.35
- 74HC14: α≈0.60, β≈0.40
For audio-range: try R = 10k–1M, C = 1 nF–1 µF. Use the Pot helper to size a sweep and estimate note range.