The astable mode acts as an oscillator, generating a continuous square wave output without any external trigger. Use this calculator to determine the frequency and duty cycle based on your chosen resistor and capacitor values.
Astable Mode Calculator
Frequency
7.21 Hz
Duty Cycle
52.38 %
Time High (Th)
72.1 ms
Time Low (Tl)
66.5 ms
Astable Circuit Diagram
+VCC
|
[R1]
|
.------+----(Pin 7)
| |
[R2] |
| |
.-----+------(Pin 6)
| | |
[C1] --- [555]----(Pin 3) - Output
| | |
=== .------(Pin 2)
GND |
|
===
GND
Output Waveform
The monostable mode, or "one-shot" mode, produces a single output pulse of a specific duration when it receives a trigger signal. Use this calculator to determine the pulse width (the 'on' time) based on your component values.
Monostable Mode Calculator
Pulse Width (Time High)
110 ms
Formula:
T = 1.1 * R * C
Monostable Circuit Diagram
+VCC
|
.------+------(Pin 8)
| |
[R] [555]----(Pin 3) - Output
| |
.------+----(Pin 7)
| |
| |
[C] ---+-----(Pin 6)
| |
=== |
GND |
|
Trigger--(Pin 2)
555 Timer Pinout
- 1. GND: Ground reference.
- 2. TRIG: Trigger pin. Starts the timing cycle in monostable mode.
- 3. OUT: The output pin.
- 4. RESET: Resets the timer. Active low.
- 5. CTRL: Control Voltage. Allows access to the internal voltage divider.
- 6. THRESH: Threshold. Ends the timing interval.
- 7. DISCH: Discharge. Used to discharge the external capacitor.
- 8. VCC: Supply voltage (+4.5V to +15V).
What is the 555 Timer?
The 555 timer is one of the most popular and versatile integrated circuits ever produced. Introduced in 1972 by Signetics, it's a highly stable device for generating accurate time delays or oscillations. It contains 25 transistors, 2 diodes, and 15 resistors on a silicon chip, all housed in an 8-pin package.
Its widespread use comes from its simplicity, low cost, and reliability. It can operate in three modes:
- Astable Mode: A free-running oscillator. It generates a continuous stream of rectangular pulses with a specific frequency. Common applications include LED flashers, tone generators, and pulse-width modulation (PWM).
- Monostable Mode: A "one-shot" pulse generator. It stays in a stable state until a trigger pulse is applied, at which point it outputs a single pulse of a set duration. This is useful for timers, bounce-free switches, and touch switches.
- Bistable Mode: A basic flip-flop. It has two stable states (high and low) and can be used as a simple memory cell. This mode is less common and doesn't require a timing capacitor.
A Note on Duty Cycle in Astable Mode
Due to the circuit's charging path (through R1 and R2) and discharging path (through R2 only), the time high will always be longer than the time low. This means a true 50% duty cycle is impossible with the standard astable configuration. To achieve duty cycles of 50% or less, a diode is typically placed in parallel with R2.
