555 astable: R1=10kΩ, R2=68kΩ, C=10µF → 0.988 Hz
Worked answer for a 555 timer free-running (astable) with R1=10kΩ, R2=68kΩ and C=10µF.
Oscillates at 0.988 Hz period 1.012 s · duty 53.4%
| R1 | 10kΩ |
| R2 | 68kΩ |
| C | 10µF |
| Frequency f = 1/(ln2·(R1+2R2)·C) | 0.988 Hz |
| Period T = 1/f | 1.012 s |
| t_high = ln2·(R1+R2)·C | 540.655 ms |
| t_low = ln2·R2·C | 471.34 ms |
| Duty cycle = t_high / T | 53.4% |
In the classic two-resistor astable, the cap charges through R1+R2 but discharges only through R2, so the duty cycle is always > 50%. To approach a 50% square wave, add a diode across R2 so charging bypasses it.
Different values? Change R1, R2 or C in the interactive tool:
Open the 555 Timer Calculator →Read more: NE555 pinout, specs & circuits · NE555 pinout reference
Disclaimer: These are ideal values from the textbook astable formula. Real capacitor tolerance and leakage shift the actual frequency — trim by measurement for anything time-critical.