The 74HC14 is a hex Schmitt-trigger inverter that can be easily configured as an RC relaxation oscillator. Because of its built-in hysteresis—switching at different upper ( VT+cap V sub cap T plus end-sub ) and lower ( VT−cap V sub cap T minus end-sub ) threshold voltages—a single resistor ( ) and capacitor (
) are all that's needed to create a stable square-wave output. Oscillator Frequency Formula
The standard approximation for calculating the oscillation frequency ( ) of a 74HC14 circuit is:
f≈10.8⋅R⋅Cf is approximately equal to the fraction with numerator 1 and denominator 0.8 center dot cap R center dot cap C end-fraction 74hc14 oscillator calculator
Alternatively, some simplified calculators use a constant of for specific supply voltages:
f≈1.2R⋅Cf is approximately equal to the fraction with numerator 1.2 and denominator cap R center dot cap C end-fraction : Frequency in Hertz (Hz) : Resistance in Ohms ( Ωcap omega : Capacitance in Farads (F). Step-by-Step Design Guide 74hc14 relaxation oscillator - NI Community
Inv1 (1→2) ––R1––+
| |
C1 +––Inv2 (3→4)
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GND out
Same formula but less load on timing node → more stable. The 74HC14 is a hex Schmitt-trigger inverter that
r = 18e3 # 18k ohm c = 100e-9 # 100 nF
freq_h, period_h = hc14_oscillator(r, c) print(f"Heuristic: freq_h:.2f Hz, Period: period_h*1000:.2f ms")
freq_e, period_e = hc14_oscillator(r, c, method="exact") print(f"Exact (5V): freq_e:.2f Hz, Period: period_e*1000:.2f ms") Stage 1: unstable (R1, C1) Stage 2: buffer
Output:
Heuristic: 1010.10 Hz
Exact (5V): 964.32 Hz
This shows a 4.5% error between the heuristic and exact methods—well within typical component tolerances.
Use τ = R × C (time constant).
f ≈ 0.455 / τ because 1/(2.2) = 0.4545.
Example:
R = 47kΩ, C = 10nF → τ = 470µs → f ≈ 0.455 / 470e-6 ≈ 968 Hz