NE555 Timer Calculator
Calculate resistor and capacitor values for 555 timer circuits in astable (oscillator) and monostable (one-shot) modes.
Configuration
Percentage of time output is HIGH (greater than 50%, up to 99%)
Results
Enter values and click Calculate
About the 555 Timer
TheNE555 is one of the most popular integrated circuits ever made. It can operate in two main modes:
Astable Mode (Oscillator)
Generates a continuous square wave output. Perfect for:
- LED blinkers and flashers
- Clock signal generators
- Tone generators and buzzers
- PWM signal generation
Formulas:
Frequency = 1.44 / ((R1 + 2×R2) × C)
Duty Cycle = (R1 + R2) / (R1 + 2×R2) × 100%
Astable Mode Pin Connection Diagram
+Vcc (4.5-15V)
|
+---------------+---+---+---------------+
| | | |
| [R1] Pin 8 (Vcc) [10nF] (bypass cap)
| | | |
| Pin 4 ---+ +-------+ |
| (Reset) | | GND
| | NE555 |
Pin 7 ---+---[R2]---+ +--- Pin 3 (Output) --> Load
(Discharge) | | |
| +-------+
[C] | |
| +--- Pin 2,6 | Pin 1
GND (Trig,Thr) GND
Important: Pin 2 and Pin 6 must be connected together!
Pin 4 (Reset) must be connected to Vcc for normal operation.
Add 10nF ceramic capacitor between Pin 5 (Control) and GND for stability.
+Vcc (4.5-15V)
|
+---------------+---+---+---------------+
| | | |
| [R1] Pin 8 (Vcc) [10nF] (bypass cap)
| | | |
| Pin 4 ---+ +-------+ |
| (Reset) | | GND
| | NE555 |
Pin 7 ---+---[R2]---+ +--- Pin 3 (Output) --> Load
(Discharge) | | |
| +-------+
[C] | |
| +--- Pin 2,6 | Pin 1
GND (Trig,Thr) GND
Important: Pin 2 and Pin 6 must be connected together!
Pin 4 (Reset) must be connected to Vcc for normal operation.
Add 10nF ceramic capacitor between Pin 5 (Control) and GND for stability.
Monostable Mode (One-Shot)
Produces a single output pulse when triggered. Used for:
- Debouncing switches
- Timed delays
- Pulse stretching
- Missing pulse detection
Formula:
Pulse Width = 1.1 × R × C
Monostable Mode Pin Connection Diagram
+Vcc (4.5-15V)
|
+---------------+---+---+---------------+
| | | |
| [R] Pin 8 (Vcc) [10nF] (bypass cap)
| | | |
| Pin 4 ---+ +-------+ |
| (Reset) | | GND
| | NE555 |
Pin 7 ---+----------+ +--- Pin 3 (Output) --> Load
(Discharge) | | |
[C] +-------+
| Pin 6 |
+-- (Thr) Pin 1,5
GND GND
|
Trigger: Pin 2 ---[10kΩ]---+Vcc
| (pull-up)
[Button to GND]
Important: Pin 2 needs pull-up resistor (10kΩ) to Vcc.
Trigger on falling edge (momentary connection to GND).
Pin 4 (Reset) to Vcc, Pin 5 to GND via 10nF capacitor.
+Vcc (4.5-15V)
|
+---------------+---+---+---------------+
| | | |
| [R] Pin 8 (Vcc) [10nF] (bypass cap)
| | | |
| Pin 4 ---+ +-------+ |
| (Reset) | | GND
| | NE555 |
Pin 7 ---+----------+ +--- Pin 3 (Output) --> Load
(Discharge) | | |
[C] +-------+
| Pin 6 |
+-- (Thr) Pin 1,5
GND GND
|
Trigger: Pin 2 ---[10kΩ]---+Vcc
| (pull-up)
[Button to GND]
Important: Pin 2 needs pull-up resistor (10kΩ) to Vcc.
Trigger on falling edge (momentary connection to GND).
Pin 4 (Reset) to Vcc, Pin 5 to GND via 10nF capacitor.
Component selection tips
- Resistors: Use values between 1kΩ and 10MΩ. Higher values = lower current consumption
- Capacitors: Ceramic for high frequencies (>10kHz), electrolytic for low frequencies (<10Hz)
- Duty cycle: A standard astable circuit cannot reach exactly 50% (HIGH is always longer because R1 > 0). Keep R1 small relative to R2 to get close, or add a diode across R2 for a near-50% square wave
- Accuracy: Use 1% resistors and quality capacitors for precise timing
- Power supply: NE555 works with 4.5-15V. Use decoupling capacitor (10nF-100nF) near Pin 8
- Output: Pin 3 can source/sink up to 200mA. Use current limiting resistor for LEDs