555 timer monostable calculator
10 kΩ and 10 µF give a 555 one-shot a 110 ms pulse with the classic 1.1 × R × C rule. Enter the timing resistor and capacitor to get the pulse width in seconds, plus the exact ln 3 value. Free in your browser.
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- Your data stays on your device
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Worked example
- Resistance
- 10,000
- Capacitance
- 0.00001
- Pulse width
- 0.11 second
- Pulse width seconds
- 0.11
- T
- 0.11
- Pulse width ln3
- 0.1098612289
A monostable 555 timer, often called a one-shot, produces a single high output pulse of predictable duration when its trigger pin is driven low. Designers set that duration with one external resistor and one external capacitor using the classic datasheet relation t equals one point one times R times C, where R is the timing resistance in ohms, C is the timing capacitance in farads, and t is the pulse width in seconds. Zero or negative capacitance is rejected with a clear validation error because a monostable network has no defined timing without a real capacitor.
How the monostable 555 pulse width formula works
In the standard NE555 monostable configuration the timing capacitor charges toward the supply through the timing resistor after a negative trigger pulse starts the one-shot. The internal threshold comparator trips near two thirds of the supply voltage and ends the high output pulse, so the continuous-time derivation is t equals R C times the natural log of three, which is about one point zero nine eight six R C.
Datasheets and almost every classroom worksheet round that constant to one point one, writing t equals one point one R C for quick hand design. Capacitance must be in farads and resistance in ohms so the product lands in seconds: ten kilohms times ten microfarads gives t of one hundred ten milliseconds with the one point one factor.
Real silicon has propagation delay, threshold offset, leakage, and layout parasitics that shift the measured width by a few percent, but the first BOM step is still the arithmetic product of nominal R and C with that monostable constant. Astable free-running period and duty cycle use a different resistor network and a different formula and belong on a separate landing so search intent for monostable pulse width does not collide with square-wave frequency tools.
What you send and what you get back
Send a finite non-negative resistance under the field resistance, expressed in ohms, and a finite positive capacitance under the field capacitance, expressed in farads. Numeric values and numeric strings are both accepted so form widgets and spreadsheet paste stay convenient; SI prefix strings such as ten kilohms written as text are not parsed, so pass ten thousand for a ten kilohm part and one times ten to the minus five for a ten microfarad part.
Resistance must be greater than or equal to zero; capacitance must be strictly greater than zero. Classic classroom checks include ten thousand ohms with ten microfarads giving one hundred ten milliseconds, and one thousand ohms with one microfarad giving one point one milliseconds. If you need astable frequency, RC time constants alone, or stored charge, treat those as separate tools rather than extending this pure monostable product.
Where monostable 555 calculators show up in real work
Introductory electronics labs constantly assign monostable 555 drills so students convert datasheet kilohms and microfarads into milliseconds of high pulse before they scope the output pin. Embedded and power bring-up teams size one-shot blanking, debounce, and soft-start windows with a single resistor and capacitor around a 555 or a compatible CMOS timer. Industrial controls and test jigs fire fixed-duration enable pulses into valves, LEDs, or camera triggers, and a trusted one point one R C product keeps notebooks aligned with firmware comments.
Keep astable period, plain RC tau, charge-only, and multi-timer cascade landings separate so each search intent has one clear page rather than an overloaded multi-mode timer tool that confuses both students and search engines.
What you can do with it
Blanking, debounce, and one-shot windows
Size a 555 one-shot for fixed-duration blanking or button debounce by choosing R and C that hit the target milliseconds.
FAQ
What happens if capacitance is zero?
A monostable 555 needs a positive timing capacitor; zero or negative capacitance is not allowed.
Is this the same as the RC time constant τ = RC?
No. The monostable pulse ends near two thirds of VCC, so the width is about 1.1RC (or exactly RC ln(3)), not the simple product RC used for a one-tau charge fraction.
What units should I use?
Pass SI values as numbers; prefix strings like '10k' or '10uF' are not parsed.
What does it cost?
The browser tool is free and runs locally.
Why does it use 1.1 instead of ln 3?
The exact derivation gives ln 3, about 1.0986, because the comparator ends the pulse near two thirds of the supply. Datasheets and almost every worksheet round it to 1.1 for hand design, and the result gives you both versions.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/elec/ne555-monostable \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"resistance":10000,"capacitance":0.00001}'const res = await fetch("https://api.kit.forhosting.com/elec/ne555-monostable", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"resistance": 10000,
"capacitance": 0.00001
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/ne555-monostable",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"resistance": 10000,
"capacitance": 0.00001
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/ne555-monostable", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"resistance":10000,"capacitance":0.00001}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"resistance":10000,"capacitance":0.00001}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/ne555-monostable", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"resistance": 10000,
"capacitance": 0.00001
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.ne555_monostable",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What you send and what you get back
The same resistance is accepted under the aliases r, R, or ohms, and capacitance under c, C, or farads, but only one key may appear for each quantity in a request. Zero capacitance, negative capacitance, NaN, and Infinity fail with invalid_input and are not charged. On success the response includes cleaned resistance and capacitance, pulse_width, pulse_width_seconds, and t as the same product one point one times R times C, pulse_width_ln3 as ln of three times R C, k_factor set to one point one, unit set to second, mode monostable, a short expression string, method ne555_monostable, and the formula string for the monostable pulse width. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where monostable 555 calculators show up in real work
Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Integration use cases
Lab homework and scope estimates
Convert a timing resistor and capacitor into monostable pulse width t = 1.1RC so students can predict the high time before measuring on an oscilloscope.
CI fixtures and answer keys
Script the same monostable product used in the browser so automated graders and BOM checks share one ground truth for pulse width.
Technical questions
What formula does this 555 monostable calculator use?
t = 1.1 × R × C for the datasheet monostable output pulse width in seconds. The response also includes pulse_width_ln3 = ln(3) × R × C, the continuous-time threshold derivation.