⏲ 555 Timer Calculator
Calculate component values for the NE555/LM555 in astable (oscillator) and monostable (one-shot) modes.
Astable Mode — Oscillator
f = 1.44 / ((Ra + 2Rb) × C)
Monostable Mode — One-Shot
t = 1.1 × R × C
Astable — Target Frequency Solver
Given f & duty, solve for Ra, Rb
%
555 Pin Functions
| Pin | Name | Function |
|---|---|---|
| 1 | GND | Ground (0V) |
| 2 | TRIGGER | Starts timing when pulled below ⅓ Vcc. Active LOW. |
| 3 | OUTPUT | HIGH during timing. Can source/sink up to 200 mA. |
| 4 | RESET | Forces output LOW when pulled LOW. Tie to Vcc if unused. |
| 5 | CONTROL | Sets internal threshold. Bypass to GND with 10 nF if unused. |
| 6 | THRESHOLD | Resets flip-flop when voltage exceeds ⅔ Vcc. |
| 7 | DISCHARGE | Open-collector transistor that discharges timing capacitor. |
| 8 | Vcc | Supply: 4.5–15 V (NE555) or 1.5–15 V (CMOS 7555) |
Tip: The 555 cannot reach exactly 50% duty cycle in standard astable mode (Ra must be > 0). For 50% duty, add a diode across Rb. The CMOS 7555 is pin-compatible with much lower power consumption.