The Complete Guide to 555 Timer Monostable Mode
The 555 Timer IC in monostable mode (also called one-shot mode) produces a single, precisely-timed output pulse each time it receives a trigger. It has exactly one stable state (LOW), and any negative trigger at Pin 2 flips it into a temporary HIGH state for a duration determined entirely by an external resistor R and capacitor C. Our 555 Timer Monostable Calculator helps engineers, students, and hobbyists accurately predict that pulse width for time-delay, debouncing, missing-pulse detection, and pulse-stretching applications.
How the Monostable Circuit Works
In monostable mode, the 555 sits idle until triggered. Here's the sequence of events:
- Idle State: Output (Pin 3) is LOW. The internal discharge transistor at Pin 7 is ON, holding capacitor C fully discharged.
- Trigger: A negative pulse on Pin 2 (below ⅓ VCC) sets the internal flip-flop. Output goes HIGH and Pin 7 releases the capacitor.
- Charging Phase: Capacitor C charges through resistor R toward VCC. The output stays HIGH while this happens.
- End of Pulse: When the capacitor voltage reaches ⅔ VCC, the upper comparator resets the flip-flop. Output snaps back to LOW and Pin 7 discharges C — ready for the next trigger.
Key Formulas
Output Pulse Width
The duration the output stays HIGH after each trigger:
Time Constant (RC)
The fundamental charging time constant of the RC network:
Threshold Voltage
The voltage at which the pulse ends, set by the internal voltage divider:
Equivalent Frequency
If continuously re-triggered, the maximum repetition rate:
Why the Magic Number 1.1?
The "1.1" coefficient isn't arbitrary — it comes from the natural log relationship of RC charging. The capacitor charges according to Vc(t) = VCC × (1 − e−t/RC). Setting Vc = ⅔ VCC and solving for t gives t = RC × ln(3) ≈ 1.0986 × RC, rounded to 1.1 RC. Because both VCC terms cancel out, the pulse width depends only on R and C — not on supply voltage.
The 555 Timer Pin Configuration (Monostable)
- Pin 1 (GND): Ground connection
- Pin 2 (TRIG): Trigger input — pulse this LOW to start the timer
- Pin 3 (OUT): The output pulse
- Pin 4 (RESET): Active-low reset (tie to VCC for normal operation)
- Pin 5 (CTRL): Control Voltage — bypass with 10nF to GND for stability
- Pin 6 (THR): Threshold — tied to Pin 7 and the top of the capacitor
- Pin 7 (DIS): Discharge — open-collector, dumps C when output is LOW
- Pin 8 (VCC): Power supply (4.5V to 16V for standard NE555)
Common Applications
Frequently Asked Questions
Does VCC affect the pulse width?
No — and that's the beauty of the 555 monostable. The threshold ⅔ VCC scales with the supply voltage, so both VCC terms cancel in the charging equation. You can run the same R and C on 5V or 15V and get the same pulse width. Only the output voltage swing changes.
What's the maximum pulse width I can get?
Practically, several minutes. The limit is capacitor leakage: if leakage current is comparable to the charging current through R, the capacitor may never reach ⅔ VCC. For long delays use low-leakage capacitors (film, tantalum) and keep R below about 10MΩ.
How do I trigger the 555 monostable?
Pull Pin 2 below ⅓ VCC with a short pulse. A typical trigger circuit is a 10kΩ pull-up to VCC with a push-button to GND, plus a small (10nF) coupling capacitor so the trigger is edge-sensitive. The trigger pulse must be shorter than the desired output pulse, otherwise the output stays HIGH as long as Pin 2 is held LOW.
Why is Pin 5 connected to a capacitor?
Pin 5 (Control Voltage) is connected to a 10nF capacitor to ground for noise immunity. This bypasses supply noise that could destabilize the internal ⅔ VCC comparator threshold and jitter the pulse width.
How do I choose R and C values?
For most applications, start with R between 1kΩ and 1MΩ, then pick C for the desired pulse width using T = 1.1 × R × C. Use small caps (nF range) for microsecond pulses, and µF caps for millisecond-to-second delays. Avoid R below ~1kΩ (excess supply current) and above ~10MΩ (leakage error).
Quick Reference: All 555 Monostable Formulas
| Parameter | Formula | Notes |
|---|---|---|
| Pulse Width | T = 1.1 × R × C | Seconds; output stays HIGH for this time |
| RC Time Constant | τ = R × C | T = 1.1 × τ |
| Threshold Voltage | VTH = ⅔ × VCC | Capacitor charges to this to end pulse |
| Trigger Level | VTRIG = ⅓ × VCC | Pin 2 must go below this |
| Max Rep. Frequency | f = 1 / (1.1 × R × C) | If continuously re-triggered |
| Supply Voltage Range | 4.5 – 16 V (bipolar) / 2 – 18 V (CMOS) | T is independent of VCC |
Worked Example: 1 Second Time Delay
Pulse Width Reference Table
| R (kΩ) | C (µF) | Pulse Width T | Typical Use |
|---|---|---|---|
| 10 | 0.1 | 1.1 ms | Switch debounce |
| 100 | 0.1 | 11 ms | Short time delay |
| 100 | 1 | 110 ms | LED on-time, relay hold |
| 100 | 10 | 1.1 s | 1-second delay |
| 1000 | 10 | 11 s | Long delay timer |
| 1000 | 100 | 110 s ≈ 1.8 min | Automatic shutoff |
Design Tips & Best Practices
| Tip | Action |
|---|---|
| 🔇 Noise immunity | Put 10 nF ceramic on Pin 5 to GND |
| ⚡ Decoupling | Add 100 nF from VCC (Pin 8) to GND |
| ⚠️ Trigger longer than T | Add RC differentiator on Pin 2 so trigger is always brief |
| 🔄 Re-triggerable | Add NPN transistor across C to discharge on each new trigger |
| 🕐 Long delays (>60 s) | Use low-leakage tantalum or film capacitor; avoid electrolytics |
| 🔋 Low power | Use CMOS 555 (LMC555, ICM7555) — µA supply current in idle |
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