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NE555 astable calculator

Timing resistors of 4.7 kΩ and 10 kΩ with a 100 nF capacitor make an NE555 astable run at a duty cycle of 59.5 %. Enter the two resistors and the capacitor to get frequency, period and both pulse widths. Free in your browser.

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Worked example

If you enter

R1: 4,700
R2: 10,000
C: 0.0000001

You get

Frequency hz: 584.087061089
Duty cycle: 0.5951417004
Duty cycle percent: 59.5141700405
Period s: 0.0017120735

The NE555 timer in astable (free-running) mode generates a continuous square wave whose frequency and duty cycle are set by two resistors and one capacitor.

How the 555 sets its rhythm

In astable mode the NE555 is a self-running oscillator that turns two resistors and one capacitor into a continuous square wave. The first resistor runs from the supply to the discharge pin 7, the second from pin 7 to the joined threshold and trigger pins 6 and 2, and the capacitor from that node to ground. The capacitor voltage ramps between one third and two thirds of the supply. The output is high for the natural logarithm of two times the sum of both resistors times the capacitance, and low for the same logarithm times the second resistor times the capacitance. The two times add up to the period, and the frequency is its inverse.

Using the result and avoiding mistakes

With 4.7 kΩ, 10 kΩ and 100 nF, the high time is the longer one and the duty cycle comes out at 59.5 %. Figures like these are what you need to set a buzzer tone or a rough PWM signal for a MOSFET gate. The most common slip is the capacitor unit: it goes in farads, so 100 nF is 1e-7. The second is relying on the rounded 1.44 constant from a datasheet, which makes frequency, period and pulse widths disagree slightly. A resistor or capacitor of zero or below cannot set any timing and is rejected.

What you can do with it

Tone and crude PWM estimators

Firmware and audio hobby tools estimate square-wave frequency and high-time fraction for buzzers or MOSFET gates driven from a NE555 astable stage.

FAQ

What happens if a resistor or the capacitor is zero or negative?

A zero or negative timing resistor or capacitor cannot set NE555 astable oscillation; those failures are not billed.

Is the NE555 astable calculator free?

The browser tool runs free on your device.

Why is the duty cycle always above 50 %?

Because the output stays high while the capacitor charges through both resistors and goes low while it discharges through the second one only. With the first resistor above zero, the high time is always the longer of the two.

Why does my result differ slightly from the datasheet formula?

Many datasheets print the rounded constant 1.44 instead of one over the natural logarithm of two. This calculator uses the exact logarithm, so frequency, period and both pulse widths stay consistent with one another.

How do I enter a 100 nF capacitor?

Enter the capacitance in farads: 1e-7 for 100 nF, or 1e-6 for 1 µF. The resistors go in ohms, so 4.7 kΩ is 4700 and 10 kΩ is 10000.

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

POSThttps://api.kit.forhosting.com/phys/ne555-astable

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/phys/ne555-astable \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"r1":4700,"r2":10000,"c":1e-7}'

const res = await fetch("https://api.kit.forhosting.com/phys/ne555-astable", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "r1": 4700,
    "r2": 10000,
    "c": 1e-7
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/phys/ne555-astable",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "r1": 4700,
    "r2": 10000,
    "c": 1e-7
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/ne555-astable", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"r1":4700,"r2":10000,"c":1e-7}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"r1":4700,"r2":10000,"c":1e-7}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/ne555-astable", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "r1": 4700,
  "r2": 10000,
  "c": 1e-7
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "phys.ne555_astable",
  "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

Per request$0.002

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

Astable topology, R1 R2 C units, and the ln(2) timing identities

A bipolar NE555 in astable mode is a relaxation oscillator whose timing node ramps between one third and two thirds of the supply. Pin seven is the discharge transistor, pin six is threshold, pin two is trigger, and they share one capacitor to ground in the textbook drawing. Resistor R1 connects VCC to pin seven. Capacitor C connects that node to ground. Datasheets often print the rounded constant 1.44 in place of one over ln(2); this engine uses the true natural logarithm of two so frequency, period, and the two pulse widths remain consistent with one another. The calculation is pure arithmetic: identical inputs always produce identical JSON, with no network, no random seed, and no wall clock.

Input contract, response fields, and zero or negative errors

Missing fields, empty strings, non-finite values, and magnitudes beyond the published isolate bound fail with invalid_input naming the field. Any resistor or the capacitor that is zero or negative also fails with invalid_input: a non-positive timing part cannot set NE555 astable frequency or duty cycle, and those failures are not billed. On success the body echoes the three inputs after stable rounding, then frequency_hz, duty_cycle as a unit fraction, duty_cycle_percent, period_s, t_high_s, t_low_s, and a formula string restating the identities. Pair the numbers with your own schematic if you need pin-out art; the endpoint solves timing only.

Labs, firmware clocks, pricing, and practical NE555 limits

Call the free in-browser NE555 astable calculator for breadboard clocks, LED flashers, buzzer tones, and lab reports without opening a spreadsheet or a SPICE deck. CMOS variants change supply current and output swing but keep the same RC identities, so the same three numbers still apply. Use the prepaid API when an electronics homework oracle, parts configurator, firmware timing estimator, or CAD plugin needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with unit converters when catalogs list kilohms and microfarads, and convert those prefixes yourself before the call. Authors shipping SDKs can map frequency_hz and duty_cycle_percent directly into UI fields that match what students write from the datasheet formulas by hand.

Integration use cases

Breadboard clocks and LED flashers

Enter R1, R2, and C for a free-running NE555 blinker, read frequency and duty cycle, and adjust resistor values before soldering a permanent board.

Classroom and homework oracles

Electronics courses pin deterministic JSON for frequency and duty from given R1, R2, and C, including intentional zero or negative parts that must fail with invalid_input.

Technical questions

How do I calculate NE555 astable frequency and duty cycle?

Send JSON with r1 and r2 in ohms and c in farads (for example r1: 4700, r2: 10000, c: 1e-7). The response includes frequency_hz, duty_cycle, duty_cycle_percent, period_s, t_high_s, and t_low_s.

Why is duty cycle always above 50%?

In the classic two-resistor astable without a discharge diode, high time uses R1+R2 while low time uses only R2, so the high fraction (R1+R2)/(R1+2·R2) is always greater than one half.

What units should I use for R1, R2, and C?

Use ohms for both resistors and farads for the capacitor. Convert microfarads yourself (1 µF = 1e-6 F) before calling the API or widget.

Read the full KIT documentation →

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