555 Timer Monostable
t = 1.1 × R × C
Quick answer
This calculator solves t = 1.1 × R × C for Pulse Width. With Resistance 10 kΩ, Capacitance 100 µF, the result is 1.1 s. It solves in 3 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
In monostable (one-shot) mode, the 555 produces a single output pulse of a precise duration each time it receives a trigger input. The output goes high for exactly 1.1 × R × C seconds, regardless of the trigger duration, then returns low and waits for the next trigger. The factor 1.1 comes from the natural logarithm of 3 (the ratio of Vcc to the 1/3 Vcc threshold), which is ln(3) ≈ 1.0986. Monostable mode is used for debouncing switches, generating fixed-duration timing pulses, and missing pulse detection.
Variables
- t — Output pulse duration (s)
- R — Timing resistor (Ω)
- C — Timing capacitor (F)
Practical Notes
The monostable is not retriggerable in the standard 555 configuration. For retriggerable operation, use a 74HC123 or similar dedicated monostable IC. For very long timing intervals (minutes to hours), consider a microcontroller timer instead, as large RC values suffer from leakage current and tolerance issues. The timing accuracy of the 555 is typically 1-2% with good quality components.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| t | Pulse Width | ms | ps, ns, µs, ms, s, min, h |
| R | Resistance | kΩ | mΩ, Ω, kΩ, MΩ |
| C | Capacitance | µF | pF, nF, µF, mF, F |
Step by step
- 1. What you know: Resistance 10 kΩ, Capacitance 100 µF
- 2. The relation: t = 1.1 × R × C
- 3. Substitute: R = 10 kΩ, C = 100 µF
- 4. Result: Pulse Width = 1.1 s
Worked examples
Minimum range
Given: Resistance 100 Ω, Capacitance 100 nF
Result: Pulse Width = 11 µs
Low range
Given: Resistance 1 kΩ, Capacitance 1 µF
Result: Pulse Width = 1.1 ms
Typical
Given: Resistance 10 kΩ, Capacitance 100 µF
Result: Pulse Width = 1.1 s
High range
Given: Resistance 100 kΩ, Capacitance 1 mF
Result: Pulse Width = 1.833 min
Maximum range
Given: Resistance 1 MΩ, Capacitance 10 mF
Result: Pulse Width = 3.056 h
Solving for each variable
Solve for Pulse Width
Resistance 100 Ω, Capacitance 100 nF → 11 µs
Solve for Resistance
Pulse Width 100 µs, Capacitance 100 nF → 0.9091 kΩ
Solve for Capacitance
Pulse Width 100 µs, Resistance 100 Ω → 0.9091 µF
How Pulse Width changes with Resistance
| Resistance (input) | Pulse Width (result) |
|---|---|
| 1 kΩ | 110 ms |
| 2.5 kΩ | 0.275 s |
| 5 kΩ | 0.55 s |
| 7.5 kΩ | 0.825 s |
| 10 kΩ | 1.1 s |
| 15 kΩ | 1.65 s |
| 20 kΩ | 2.2 s |
| 40 kΩ | 4.4 s |
| 70 kΩ | 7.7 s |
| 100 kΩ | 11 s |
The same result in other units
| Unit | Value |
|---|---|
| µs | 1100000 |
| ms | 1100 |
| s | 1.1 |
| min | 0.01833 |
| h | 0.0003056 |
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<p><a href="https://vbatpower.com/formulas/555-monostable/">555 Timer Monostable</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate 555 Timer Monostable?
Use t = 1.1 × R × C. With Resistance 10 kΩ, Capacitance 100 µF, Pulse Width works out to 1.1 s.
What is the formula for 555 Timer Monostable?
The formula is t = 1.1 × R × C. In monostable (one-shot) mode, the 555 produces a single output pulse of a precise duration each time it receives a trigger input.
How do you find Resistance when you know the other values?
Rearrange for Resistance: with Pulse Width 100 µs, Capacitance 100 nF, Resistance is 0.9091 kΩ.
How do you find Capacitance when you know the other values?
Rearrange for Capacitance: with Pulse Width 100 µs, Resistance 100 Ω, Capacitance is 0.9091 µF.
What units does 555 Timer Monostable use?
Pulse Width is shown in µs, ms, s, min, h. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
Related Concepts
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