Compute denominator term: A = (VCC − V_T−) = 5 − 1.9 = 3.1 B = (VCC − V_T+) = 5 − 3.1 = 1.9 f_factor = ln( (A·V_T+) / (B·V_T−) ) Plug numbers: A·V_T+ = 3.1·3.1 = 9.61 B·V_T− = 1.9·1.9 = 3.61 ln(9.61/3.61) = ln(2.662) ≈ 0.979
Then R = 1 / (f·C·f_factor) = 1 / (1000 · 100e-9 · 0.979) ≈ 10,215 Ω → choose 10 kΩ.
Compute t_charge = RC·ln(A/B) = (10k·100nF)·ln(3.1/1.9) = 0.001 · ln(1.632) = 0.001 · 0.490 = 0.00049 s t_discharge = RC·ln(V_T+/V_T−) = 0.001 · ln(3.1/1.9) = same = 0.00049 s Period ≈ 0.00098 s → f ≈ 1,020 Hz. Duty cycle ≈ 50%. 74hc14 oscillator calculator
Note: with these symmetric threshold ratios the duty cycle is near 50%; in general it will not be exactly 50%.
Multiply R×C, then look up f:
| R × C (seconds) | Frequency | |----------------|------------| | 0.000001 (1µs) | 454 kHz | | 0.00001 (10µs) | 45.4 kHz | | 0.0001 (100µs) | 4.54 kHz | | 0.001 (1ms) | 454 Hz | | 0.01 (10ms) | 45.4 Hz | | 0.1 (100ms) | 4.54 Hz | | 1 (1s) | 0.454 Hz |
Let's design an oscillator for a $1\textkHz$ square wave output at $5\textV$. Compute denominator term: A = (VCC − V_T−) = 5 − 1
Step 1: Choose Capacitor Let's select a common ceramic capacitor: $C = 100\textnF$ ($0.1\mu\textF$).
Step 2: Apply Formula Using the constant $0.8$: $$ R = \frac0.8f \cdot C $$ $$ R = \frac0.81000 \cdot (100 \times 10^-9) $$ $$ R = \frac0.80.0001 = 8,000\Omega $$ Let's design an oscillator for a $1\textkHz$ square
Step 3: Select Standard Resistor $8\textk\Omega$ is not a standard E12/E24 value.
This is close enough for non-critical applications like clocking a counter or flashing an LED.