NE555 Astable vs Monostable

In astable mode, the 555 timer continuously oscillates between high and low, generating a square wave. Frequency and duty cycle are set by R1, R2, and C.

In monostable mode, a trigger pulse starts a single output pulse whose width is determined by R and C alone (T = 1.1 × R × C).

Astable mode formulas

In astable mode, the oscillation frequency and duty cycle are determined by the resistors R1 and R2, and the capacitor C:

Frequency: f = 1.44 / ((R1 + 2×R2) × C)
Duty cycle: D = (R1 + R2) / (R1 + 2×R2)

Worked example: With R1 = 10 kΩ, R2 = 10 kΩ, C = 100 nF:
f = 1.44 / ((10,000 + 20,000) × 0.0000001) = 1.44 / 0.003 ≈ 480 Hz
D = (10,000 + 10,000) / (10,000 + 20,000) ≈ 0.667 (66.7% duty cycle)

Monostable mode formula

In monostable mode, a single trigger pulse produces an output pulse of duration T:

T = 1.1 × R × C

Worked example - 1-second pulse: To get T = 1 s, choose R = 910 kΩ and C = 1 µF:
T = 1.1 × 910,000 × 0.000001 = 1.001 s ≈ 1 second.

555 timer pin reference

Pin	Name	Astable function	Monostable function
1	GND	Ground (0 V)	Ground (0 V)
2	Trigger	Connected to pin 6 (threshold)	Active-low trigger input; starts timing pulse
3	Output	Square wave output	Pulse output
4	Reset	Tie to VCC to enable; pull low to reset	Tie to VCC to enable
5	Control Voltage	Bypass with 10 nF cap to GND if unused	Bypass with 10 nF cap to GND if unused
6	Threshold	Connected to R2/C junction; resets flip-flop at ⅔ VCC	Connected to timing capacitor; resets at ⅔ VCC
7	Discharge	Discharges C through R2 during low phase	Discharges timing capacitor at end of pulse
8	VCC	Supply voltage (4.5–16 V)	Supply voltage (4.5–16 V)

Common 555 applications

LED flasher (astable): R1 = 1 kΩ, R2 = 47 kΩ, C = 10 µF -> ~1.4 Hz blink rate. Connect LED with current-limiting resistor (470 Ω) on pin 3.
Debounced switch (monostable): R = 100 kΩ, C = 10 µF -> 1.1 s output pulse, long enough to ignore contact bounce on a mechanical switch.
PWM generation (astable): Add a potentiometer between R1 and R2 to vary duty cycle. Use a diode to separate charge and discharge paths for asymmetric PWM below 50%.
Tone / beeper (astable): R1 = 1 kΩ, R2 = 10 kΩ, C = 10 nF -> ~4.8 kHz audible tone. Connect a small speaker or buzzer on pin 3 with a 100 Ω series resistor.