Documentation sections

555 Timer Calculator (Astable & Monostable)

POSThttps://rftools.io/api/py/v1/calculate

Slug 555-timer. Calculate 555 timer frequency, period, duty cycle, and pulse width for both astable oscillator and monostable one-shot configurations

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of 555-timer
Name	Unit	Range	Default	Description
modenumber	none	0 … 1	0	Timer ModeOne of: 0 Astable (oscillator) · 1 Monostable (one-shot)
ranumber	kΩ	at least 0.001	10	Timing resistor RA (connected between VCC and discharge pin)
rbnumber	kΩ	at least 0.001	10	Timing resistor RB (astable: between discharge and threshold; monostable: not used)
cnumber	μF	at least 0.0001	0.1	Timing Capacitor
targetTimenumber	ms	at least 0.000001	2	The period the oscillator is for (astable), or the pulse width (monostable)
errorBudgetnumber	%	0.01 … 100	5	How far from the target the time may land, ±
rTolerancenumber	%	0 … 50	0	The resistors' tolerance; 0 takes the nominal values
cTolerancenumber	%	0 … 50	0	The capacitor's tolerance: 1–5 % for film or C0G, 10–20 % for X7R, 20 % for electrolytic; 0 takes the nominal value

Retired, still accepted and ignored with a warning: vcc (Supply Voltage, 2026-10-12).

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of 555-timer
Field	Unit	Description
frequencyheadline	Hz	Frequency
period	ms	Period
pulseWidth	ms	Pulse Width (high time)
dutyCycle	%	Duty Cycle
highTime	ms	High Time
lowTime	ms	Low Time
timeMin	ms	The period (astable) or pulse width (monostable) with every part at its low extreme
timeMax	ms	The same with every part at its high extreme
timingError_pct	%	The extreme farther from the target, over the target, signed; the 555's own initial error adds to it

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "555-timer",
    "inputs": {
      "mode": 0,
      "ra": 10,
      "rb": 10,
      "c": 0.1,
      "targetTime": 2,
      "errorBudget": 5,
      "rTolerance": 0,
      "cTolerance": 0
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("555-timer", {
    "mode": 0,
    "ra": 10,
    "rb": 10,
    "c": 0.1,
    "targetTime": 2,
    "errorBudget": 5,
    "rTolerance": 0,
    "cTolerance": 0,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "555-timer",
    "inputs": {
      "mode": 0,
      "ra": 10,
      "rb": 10,
      "c": 0.1,
      "targetTime": 2,
      "errorBudget": 5,
      "rTolerance": 0,
      "cTolerance": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "555-timer",
	  "inputs": {
	    "mode": 0,
	    "ra": 10,
	    "rb": 10,
	    "c": 0.1,
	    "targetTime": 2,
	    "errorBudget": 5,
	    "rTolerance": 0,
	    "cTolerance": 0
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "555-timer",
  "values": {
    "frequency": 480.89834696298794,
    "period": 2.0794415416798353,
    "pulseWidth": 1.3862943611198904,
    "dutyCycle": 66.66666666666667,
    "highTime": 1.3862943611198904,
    "lowTime": 0.6931471805599452,
    "timeMin": 2.0794415416798353,
    "timeMax": 2.0794415416798353,
    "timingError_pct": 3.972077083991765
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:555-timer",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "mode": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "ra": {
          "min": 0.001,
          "max": null,
          "unit": "kΩ"
        },
        "rb": {
          "min": 0.001,
          "max": null,
          "unit": "kΩ"
        },
        "c": {
          "min": 0.0001,
          "max": null,
          "unit": "μF"
        },
        "targetTime": {
          "min": 0.000001,
          "max": null,
          "unit": "ms"
        },
        "errorBudget": {
          "min": 0.01,
          "max": 100,
          "unit": "%"
        },
        "rTolerance": {
          "min": 0,
          "max": 50,
          "unit": "%"
        },
        "cTolerance": {
          "min": 0,
          "max": 50,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "mode": 0,
      "ra": 10,
      "rb": 10,
      "c": 0.1,
      "targetTime": 2,
      "errorBudget": 5,
      "rTolerance": 0,
      "cTolerance": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "mode"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator page555 Timer Calculator (Astable & Monostable), at these values