555 Timer Calculator

Method, formulas and references

Astable oscillator

The capacitor charges through RA and RB until it reaches 2/3 VCC, then discharges through RB into the DISCH pin until it falls to 1/3 VCC. Each interval is an RC exponential between those two levels, which takes ln 2 ≈ 0.693 time constants, so the timing does not depend on the supply voltage:

The output is high while the capacitor charges, so its duty cycle is always above 50 % in this circuit. A diode across RB (anode at DISCH) bypasses RB during charging: the high time becomes 0.693 RA C and any duty cycle is possible.

Monostable (one-shot)

A trigger below 1/3 VCC starts the pulse; the capacitor then charges through RA from 0 to 2/3 VCC, which takes ln 3 ≈ 1.1 time constants:

Limits of the bipolar 555

• TI recommends astable operation at 100 kHz or below; use a CMOS timer such as the TLC555 above that.
• The shortest monostable pulse is about 10 µs.
• The threshold input current flows through the timing resistors, which limits RA + RB to about 3.4 MΩ at 5 V and 10 MΩ at 15 V.
• Timing drifts by roughly 50 ppm/°C (monostable) to 150 ppm/°C (astable) for the NE555, before the capacitor's own tolerance and drift. Use a film or C0G capacitor for stable timing.

References

1. TI SLFS022K, NA555, NE555, SA555, SE555 Precision Timers, sections 5.5, 5.6 and 6.3.