{"id":52326,"date":"2026-11-01T10:50:58","date_gmt":"2026-11-01T10:50:58","guid":{"rendered":"http:\/\/siratek.net\/index.php\/tutorials\/pwm-and-analogwrite\/"},"modified":"2026-11-01T10:50:58","modified_gmt":"2026-11-01T10:50:58","slug":"pwm-and-analogwrite","status":"publish","type":"siratek_tutorial","link":"https:\/\/siratek.net\/index.php\/tutorials\/pwm-and-analogwrite\/","title":{"rendered":"Dimming and Speed Control with PWM"},"content":{"rendered":"

<\/p>\n

What this covers<\/h2>\n

A digital pin has two states, on and off. Yet analogWrite()<\/code> dims an LED and slows a motor. This explains how, which pins can do it, and the three places it behaves differently from what you would expect.<\/p>\n

Before you begin<\/h2>\n

An LED and a 220 ohm resistor on pin 9, wired as in Your First Circuit, an LED and a Resistor<\/em>. Everything here can be seen on that one LED.<\/p>\n

Switching fast instead of switching partly<\/h2>\n

The pin never sits at a middle voltage. It switches between 0 V and 5 V, hundreds of times a second, and spends a different proportion of each cycle high.<\/p>\n

That proportion is the duty cycle<\/strong>. At 25 percent the pin is high for a quarter of each cycle; the LED receives a quarter of the energy and looks dim. At 75 percent it looks bright. The switching is far too fast to see, so your eye averages it into steady brightness.<\/p>\n

analogWrite(pin, value)<\/code> sets the duty cycle, with value<\/code> from 0 to 255.<\/p>\n\n\n\n\n\n\n\n\n\n
analogWrite<\/code> value<\/th>\nDuty cycle<\/th>\nLED looks<\/th>\n<\/tr>\n<\/thead>\n
0<\/td>\n0 percent, always off<\/td>\nOff<\/td>\n<\/tr>\n
64<\/td>\n25 percent<\/td>\nDim<\/td>\n<\/tr>\n
128<\/td>\n50 percent<\/td>\nHalf<\/td>\n<\/tr>\n
191<\/td>\n75 percent<\/td>\nBright<\/td>\n<\/tr>\n
255<\/td>\n100 percent, always on<\/td>\nFull<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The name is misleading. It is not analog output, and the Uno cannot produce a true varying voltage. It produces a square wave whose average<\/em> is what you asked for.<\/p>\n

\"Four
The pin is always fully on or fully off. Only the proportion changes.<\/figcaption><\/figure>\n

Which pins<\/h2>\n

On an Arduino Uno, six pins only: 3, 5, 6, 9, 10 and 11<\/strong>. They are marked with a tilde on the board.<\/p>\n

analogWrite()<\/code> on any other pin does not produce an error. It treats anything from 128 upwards as fully on and anything below as off, which looks like a broken fade rather than a mistake. If your dimming only ever gives two brightnesses, this is why.<\/p>\n

Three things that surprise people<\/h2>\n

The frequency is not the same on every pin<\/h3>\n

Pins 5 and 6 switch at roughly 980 Hz. The other four switch at roughly 490 Hz. Both are far too fast to see on an LED, but they matter for a motor, which can whine audibly at the lower rate, and for anything that filters or measures the signal.<\/p>\n

Pins 5 and 6 do not quite reach zero<\/h3>\n

Those two share a timer with the Arduino core\u2019s own timekeeping, the one millis()<\/code> and delay()<\/code> depend on. The interaction leaves them slightly brighter than expected at very low values, and analogWrite(5, 0)<\/code> may not be completely dark.<\/p>\n

If you need a pin to go genuinely off, use digitalWrite(pin, LOW)<\/code> for that case, or put the LED on 3, 9, 10 or 11.<\/p>\n

tone()<\/code> takes pins 3 and 11 away<\/h3>\n

tone()<\/code> claims the timer that drives PWM on pins 3 and 11. While a tone is playing, analogWrite()<\/code> on those two does nothing useful, silently. A project that makes sounds and dims lights should put the lights on 5, 6, 9 or 10.<\/p>\n

Minimal working example<\/h2>\n
\/\/ Siratek: what a duty cycle looks like.\n\/\/ Steps an LED through five brightness levels, naming each one.\n\nconst int ledPin = 9;\n\nconst int levels[] = { 0, 64, 128, 191, 255 };\nconst int percent[] = { 0, 25, 50, 75, 100 };\nconst int levelCount = 5;\n\nvoid setup() {\n  pinMode(ledPin, OUTPUT);\n  Serial.begin(9600);\n  Serial.println(\"Siratek: stepping through duty cycles.\");\n}\n\nvoid loop() {\n  for (int i = 0; i < levelCount; i++) {\n    analogWrite(ledPin, levels[i]);\n\n    Serial.print(\"value=\");\n    Serial.print(levels[i]);\n    Serial.print(\"  duty=\");\n    Serial.print(percent[i]);\n    Serial.println(\"%\");\n\n    delay(1200);\n  }\n}<\/code><\/pre>\n
Stepping through fixed levels shows the relationship far better than a smooth fade, because you can hold each one and compare.<\/p>\n
Driving it from a potentiometer<\/h3>\n
analogRead()<\/code> gives 0 to 1023 and analogWrite()<\/code> wants 0 to 255, so the value has to be rescaled.<\/p>\n
const int potPin = A0;\nconst int ledPin = 9;\n\nvoid loop() {\n  int reading = analogRead(potPin);\n  analogWrite(ledPin, map(reading, 0, 1023, 0, 255));\n  delay(20);\n}<\/code><\/pre>\n
Dividing by four would also work, and is what map()<\/code> effectively does here. Use map()<\/code> anyway: it states the intent, and it keeps working when one of the ranges changes.<\/p>\n
What PWM can and cannot drive<\/h2>\n
PWM suits anything that responds to average power: LED brightness, small motor speed, heater output, a servo\u2019s control signal.<\/p>\n
It does not suit anything that needs a steady voltage, such as a reference for a sensor or an audio level. For those you need a real digital-to-analog converter, or a filter to smooth the square wave into a level.<\/p>\n
And the pin\u2019s current limit has not changed. A motor still needs a transistor or a driver module between the pin and the load, whatever the duty cycle. PWM controls when<\/em> the current flows, not how much the pin can supply.<\/p>\n
Check yourself<\/h2>\n
    \n
  1. analogWrite(7, 128)<\/code> on an Uno. What does the LED do?<\/li>\n
  2. Why might a motor whine on pin 9 but not on pin 5?<\/li>\n
  3. Your sketch plays a tone and dims an LED on pin 11. The LED stops dimming. Why?<\/li>\n
  4. Does analogWrite(9, 128)<\/code> put 2.5 V on pin 9?<\/li>\n<\/ol>\n
    Answers in order: pin 7 has no PWM, so 128 and above is fully on and the LED is simply lit. Pin 9 switches at about 490 Hz, which is within hearing; pin 5 runs at about 980 Hz. tone()<\/code> has taken the timer that drives pin 11. No, the pin is still switching between 0 V and 5 V; 2.5 V is only the average.<\/p>\n
    Troubleshooting<\/h2>\n\n\n\n\n\n\n\n\n\n\n
    Symptom<\/th>\nLikely cause<\/th>\nCheck or fix<\/th>\n<\/tr>\n<\/thead>\n
    Only two brightnesses<\/td>\nNot a PWM pin<\/td>\nUse 3, 5, 6, 9, 10 or 11<\/td>\n<\/tr>\n
    Will not go fully off on pin 5 or 6<\/td>\nTimer shared with the core timing<\/td>\nUse digitalWrite()<\/code> for off, or another pin<\/td>\n<\/tr>\n
    Dimming stopped when sound started<\/td>\ntone()<\/code> holds pins 3 and 11<\/td>\nMove the output to 5, 6, 9 or 10<\/td>\n<\/tr>\n
    Motor whines<\/td>\n490 Hz switching is audible<\/td>\nTry pin 5 or 6<\/td>\n<\/tr>\n
    Fade looks uneven<\/td>\nEye response, not a fault<\/td>\nExpected; curve the values if it matters<\/td>\n<\/tr>\n
    Measured voltage is wrong<\/td>\nA meter averages the square wave<\/td>\nThe pin is switching, not holding a level<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    Next steps<\/h2>\n
      \n
    • Easier first:<\/strong> Your First Circuit, an LED and a Resistor<\/em>.<\/li>\n
    • Next:<\/strong> Timing Without delay()<\/em>, so a fade can run while the rest of the project works.<\/li>\n
    • Later:<\/strong> the servo and motor driver tutorials, which are PWM applied to movement.<\/li>\n<\/ul>\n
      Sources and attribution<\/h2>\n
      All text and code in this tutorial are original Siratek work.<\/p>\n
        \n
      • Arduino core documentation and source for analogWrite()<\/code>, the PWM pin list and the timers behind them.<\/li>\n
      • ATmega328P datasheet for the timer behaviour on pins 5 and 6.<\/li>\n<\/ul>\n
        No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n
        <\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"
        A digital pin has two states, on and off. Yet analogWrite() dims an LED and slows a motor. This explains how, which pins can do it, and the three places it behaves differently from what you would expect.<\/p>\n","protected":false},"featured_media":0,"template":"","siratek_tutorial_level":[],"class_list":["post-52326","siratek_tutorial","type-siratek_tutorial","status-publish","hentry"],"yoast_head":"\nDimming and Speed Control with PWM - siratek<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"http:\/\/siratek.net\/index.php\/tutorials\/pwm-and-analogwrite\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Dimming and Speed Control with PWM - siratek\" \/>\n<meta property=\"og:description\" content=\"A digital pin has two states, on and off. 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