555 Timer Astable/Monostable Calculator mechanism

Model scope

The two timing equations are standard ideal-555 approximations, and the astable duty-cycle equation is known, but the specification is incomplete because duty-cycle computation is not stated and the monostable frequency/duty-cycle outputs are undefined without trigger timing. The honest label should state that results are approximate ideal calculations and should identify the supported 50%-duty topology rather than implying that the standard two-resistor circuit provides exactly 50% duty cycle.

• The astable frequency formula f=1.44/((R1+2R2)C) is a standard approximate ideal-555 formula; some references use 1.443 instead of 1.44, so the precision and approximation should be labeled.
• The astable duty-cycle formula is omitted even though duty cycle is listed as an output. For the standard two-resistor astable, duty cycle=(R1+R2)/(R1+2R2).
• The monostable formula T=1.1RC is the standard approximate ideal-555 pulse-width formula, but the approximation and 555 component tolerances should be disclosed.
• The declared outputs f, duty cycle, and T are mode-ambiguous: a monostable normally produces pulse width T, while frequency and duty cycle require a specified trigger repetition period.
• The ios27_notes mention a 50%-duty variant, but no topology or formula is supplied. A standard two-resistor 555 astable does not inherently produce exactly 50% duty cycle unless an additional diode or separate charge/discharge path is used.
• No valid-input constraints are specified for R1, R2, C, or mode.

Units and conventions

• R1 and R2 are positive resistances in ohms after conversion.
• C is a positive capacitance in farads after conversion.
• With resistance in ohms and capacitance in farads, astable f is in hertz and monostable T is in seconds.
• Astable duty cycle is dimensionless and may be reported as a fraction or multiplied by 100 for percent.
• The constants 1.44 and 1.1 are dimensionless engineering approximations.
• The calculations assume the ideal standard 555 astable or monostable topology, not CMOS-specific or datasheet-specific timing corrections.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "mode": "astable",
    "R1_ohm": 1000,
    "R2_ohm": 10000,
    "C_F": 1e-7
  },
  "expected_outputs": {
    "f_Hz": 685.7142857,
    "duty_cycle_fraction": 0.5238095,
    "duty_cycle_percent": 52.38095
  },
  "working": "R1+2R2=1000+2(10000)=21000 ohm. C=100 nF=100 x 10^-9 F=1.0 x 10^-7 F. (R1+2R2)C=21000 x 1.0 x 10^-7=0.0021 s. f=1.44/0.0021=685.7142857 Hz. Duty cycle=(R1+R2)/(R1+2R2)=(1000+10000)/21000=11000/21000=0.5238095=52.38095%."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

The zero-resistance astable input produces undefined frequency and duty cycle, so the calculator must validate the denominator and reject the input rather than emit Infinity or NaN. Unitless component values also produce a 1000-fold frequency discrepancy unless the unit convention is explicit.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche3/tranche3-results.json
• BOUNDARY_HAZARD_CATALOG.json