The time it takes for the capacitor to charge and discharge determines the frequency. The general formula to calculate the output frequency (
For R = 1 MΩ, C = 100 nF: RC = 1e6 · 1e-7 = 0.1 s f ≈ 1.233 / 0.1 = 12.33 Hz
T=thigh+tlow=R⋅C⋅[ln(VCC−VT−VCC−VT+)+ln(VT+VT−)]cap T equals t sub h i g h end-sub plus t sub l o w end-sub equals cap R center dot cap C center dot open bracket l n open paren the fraction with numerator cap V sub cap C cap C end-sub minus cap V sub cap T minus end-sub and denominator cap V sub cap C cap C end-sub minus cap V sub cap T plus end-sub end-fraction close paren plus l n open paren the fraction with numerator cap V sub cap T plus end-sub and denominator cap V sub cap T minus end-sub end-fraction close paren close bracket 74hc14 oscillator calculator full
To prevent the resistor values from being too small (overloading the output) or too large (susceptible to stray capacitance), choose a standard capacitor value between . Let's select ( Step 2: Calculate the Required Resistance Using our simplified frequency equation:
To turn this circuit into an adjustable clock source, swap the fixed resistor The time it takes for the capacitor to
To build an oscillator with the 74HC14 Hex Schmitt-trigger Inverter, you create a relaxation circuit where a capacitor charges and discharges through a resistor. Because the 74HC14 has , it waits for the capacitor to reach a high threshold ( VT+cap V sub cap T plus end-sub ) before switching its output low, and a low threshold ( VT−cap V sub cap T minus end-sub ) before switching high again. Quick Calculator Formula For most practical 74HC14 designs at 5V, the frequency ( ) can be approximated with:
Avoid calculators that simply print ( f = 0.72/RC ) without explaining their assumptions. Because the 74HC14 has , it waits for
Charging (output high): capacitor charges from Vth− to Vth+ toward Vcc. Voltage across capacitor during charge: Vc(t) = Vcc − (Vcc − Vth−)·e^(−t/(R·C)) Solve for charge time tch when Vc(tch) = Vth+: Vth+ = Vcc − (Vcc − Vth−)·e^(−tch/(R·C)) => e^(−tch/(R·C)) = (Vcc − Vth+) / (Vcc − Vth−) => tch = R·C · ln[(Vcc − Vth−)/(Vcc − Vth+)]