Calculate frequency, duty cycle, and timing for the NE555 timer IC in astable (oscillator) and monostable (one-shot) modes.
Frequency in Hz. Output continuously oscillates between HIGH and LOW.
Pulse duration in seconds. Triggered by a negative-going pulse on pin 2.
| Pin | Name | Function |
|---|---|---|
| 1 | GND | Ground (0V reference) |
| 2 | TRIG | Trigger input (starts timing when < ⅓ Vcc) |
| 3 | OUT | Output (can sink/source ~200mA) |
| 4 | RESET | Active-low reset (tie to Vcc if unused) |
| 5 | CV | Control Voltage (bypass to GND with 10nF) |
| 6 | THR | Threshold (ends timing when > ⅔ Vcc) |
| 7 | DIS | Discharge (discharges timing capacitor) |
| 8 | Vcc | Supply voltage (4.5V to 16V) |
⚠️ Design Tips: Always add a 10nF capacitor between pin 5 (CV) and ground for stability. Use a 0.1µF decoupling capacitor near pin 8 (Vcc). For frequencies above 100kHz, consider using a CMOS 555 (TLC555, LMC555).
Standard 555 astable always has duty cycle >50% because charging goes through R1+R2 but discharging only through R2. For 50% duty cycle, add a diode in parallel with R2, or use a CD4047 IC instead.
The standard NE555 works reliably up to about 100kHz-500kHz. CMOS versions (TLC555, LMC555) can go higher and draw less current, making them better for high-frequency or battery-powered applications.
If the trigger pulse is longer than the output pulse, the 555 will retrigger. Make sure your trigger pulse is short (use a differentiator circuit: capacitor + resistor to ground on pin 2).
The output can typically sink or source around 200mA. For higher current loads, use a transistor or MOSFET as a driver. The output swings close to Vcc (HIGH) and GND (LOW), minus small saturation voltages.