Astable 555 Timer Calculator Formula
Understand the math behind the astable 555 timer calculator. Each variable explained with a worked example.
Formulas Used
Frequency
frequency_hz = 1.44 / ((r1_ohm + 2 * r2_ohm) * c_farads)Period
period_ms = (r1_ohm + 2 * r2_ohm) * c_farads * 1000 / 1.44 * 1000Duty Cycle
duty_cycle_pct = (r1_kohm + r2_kohm) / (r1_kohm + 2 * r2_kohm) * 100Variables
| Variable | Description | Default |
|---|---|---|
r1_kohm | R1(kΩ) | 10 |
r2_kohm | R2(kΩ) | 10 |
c_uf | Capacitor(µF) | 0.1 |
r1_ohm | Derived value= r1_kohm * 1000 | calculated |
r2_ohm | Derived value= r2_kohm * 1000 | calculated |
c_farads | Derived value= c_uf / 1000000 | calculated |
How It Works
555 Timer Astable Mode
Formulas
f = 1.44 / ((R1 + 2R2) x C)
Duty Cycle = (R1 + R2) / (R1 + 2R2) x 100%
Duty cycle is always above 50% in standard configuration. Use a diode across R2 for below 50%.
Worked Example
R1 = 10k, R2 = 10k, C = 0.1 uF.
r1_kohm = 10r2_kohm = 10c_uf = 0.1
- 01f = 1.44 / ((10000 + 20000) x 0.0000001) = 1.44 / 0.003 = 480 Hz
- 02Duty cycle = (10 + 10) / (10 + 20) x 100 = 66.7%
Ready to run the numbers?
Open Astable 555 Timer Calculator