Oscilloscope Selection Calculator

In the standard 555 astable topology the timing capacitor always charges through both R1 and R2 in series but discharges through R2 alone, so the high time t_high = 0.693 × (R1+R2) × C can never be shorter than the low time t_low = 0.693 × R2 × C. That makes duty cycle mathematically bounded at 50% or above for any positive R1 — it's a limitation of the circuit itself, not a rounding artifact in this calculator. To get closer to a true square wave you'd need a diode across R2 to bypass it during charging, which is a different circuit than what this formula models.

When the calculated frequency falls between 16 Hz and 8 kHz — roughly the low end of audible bass through the top of a soprano's range — the calculator maps it to the closest musical semitone using 12 × log2(frequency / 440) relative to A4 (440 Hz), then names that note and octave. Outside that range the field is left blank because it stops being musically meaningful; this is a convenience for hobbyists building 555-based tone generators or simple beepers, not a precision tuning tool.

The formulas assume the 0.693 (ln 2) charge/discharge constant exactly, which relies on the 555's internal comparators switching at precisely 1/3 and 2/3 of Vcc — real chips have some threshold variance, and capacitor tolerance is often the bigger factor, with ceramic capacitors commonly rated ±20% from their marked value. Stray PCB or breadboard capacitance adds further drift at higher frequencies. Expect single-digit-percent deviation at minimum with a decent capacitor, and always verify on a scope before locking in a fixed-frequency design.

It's an approximation of the steady current flowing through R1 and R2 from the supply rail, calculated as Vcc / (R1 + 2×R2) assuming a fixed 5V supply — it does not include the 555's own internal supply current or whatever current your output load draws from pin 3. It's mainly useful for a rough power-budget check on the timing network itself, not a full circuit current estimate.