555 Timer Calculator

The 555 timer is one of the most versatile and widely used integrated circuits in electronics. Introduced in 1972, it can be configured as an oscillator, timer, or flip-flop, making it essential for countless applications.

Operating Modes

Astable Mode

Free-running oscillator that generates a continuous square wave output. Used for LED flashers, tone generators, PWM signals, and clock sources.

• Continuous oscillation
• No external trigger needed
• Duty cycle always > 50% (standard)

Monostable Mode

One-shot timer that produces a single output pulse when triggered. Used for debouncing, delay timers, and pulse stretching.

• Single pulse output
• Requires trigger input
• Precise timing control

555 Timer Pinout

Pin	Name	Function
1	GND	Ground (0V reference)
2	TRIG	Trigger input (active low, <1/3 VCC)
3	OUT	Output (can source/sink ~200mA)
4	RESET	Reset input (active low, tie to VCC if unused)
5	CTRL	Control voltage (bypass with 10nF to GND)
6	THR	Threshold input (resets when >2/3 VCC)
7	DIS	Discharge (open collector output)
8	VCC	Supply voltage (4.5V to 16V typical)

Common Applications

Timing & Delays

• Time delay relays
• Debounce circuits
• Touch switches

Oscillators

• LED flashers
• Tone generators
• Clock sources

Signal Processing

• PWM generation
• Pulse stretching
• Frequency dividers

Design Tips

Bypass capacitor: Always add a 100nF capacitor between VCC and GND close to the chip to filter power supply noise.
Control voltage: Add a 10nF capacitor from pin 5 to GND to improve noise immunity unless modulating the control voltage.
Reset pin: Connect to VCC through a 10kΩ resistor if not using the reset function.
CMOS variants: For lower power consumption and rail-to-rail output, consider TLC555 or LMC555 CMOS versions.
50% duty cycle: For astable mode with 50% duty cycle, add a diode across R2 or use complementary outputs with a flip-flop.

Circuit Configuration

555 Timer Formulas

Astable Mode (Oscillator)

Monostable Mode (One-Shot)

About the 555 Timer IC