555 Timer Astable Calculator
Design an astable multivibrator circuit.
The Legendary 555 Timer
The 555 Timer is arguably the most successful integrated circuit in history. Designed in 1971 by Hans Camenzind, billions are produced every year.
This calculator is for the Astable Multivibrator configuration, where the 555 oscillates continuously, producing a square wave output. This is used for clocks, tone generation, and flashing LEDs.
Astable Frequency Formulas
The frequency depends on how fast the capacitor (C) charges through resistors Ra+Rb and discharges through Rb.
frequency (f) = 1.44 / ( (Ra + 2Rb) × C )
Period (T) = 0.693 × (Ra + 2Rb) × C
Duty Cycle % = ( (Ra + Rb) / (Ra + 2Rb) ) × 100
Timing Intervals
- High Time (Output On): Thigh = 0.693 × (Ra + Rb) × C
- Low Time (Output Off): Tlow = 0.693 × Rb × C
Practical Applications
- LED Flasher: A 1Hz blinker for warning signs or toys.
- PWM Motor Controller: Creating a crude Variable Frequency Drive for small DC motors.
- Tone Generator: Connecting a speaker to the output creates Audio. (e.g., Atari Punk Console).
FAQ
Can I get a 50% Duty Cycle?
Not easily with the standard circuit. Because Thigh includes Ra and Tlow does not, High time is always longer than Low time (Duty > 50%). To get 50%, you can add a diode across Rb, or just use a CMOS 555 which has different rail-to-rail characteristics.
CMOS (LMC555) vs Bipolar (NE555)?
The classic NE555 (Bipolar) is power hungry gives a massive current spike when switching. You need a big bypass capacitor. The LMC555 (CMOS) is low power, faster, and works at lower voltages (down to 1.5V).
What is Pin 5 (Control Voltage) for?
Usually, you just connect a 10nF capacitor to ground to filter noise. However, applying a voltage to Pin 5 changes the internal threshold, allowing you to modulate the output frequency (FM synthesis) or Pulse Width.