Lesson 3 of 4

Fade with a knob

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// Fade an LED on D9 with a knob on A0. const int LED_PIN = 9; // D9 has PWM (Timer1) const int KNOB_PIN = A0; void setup() { pinMode(LED_PIN, OUTPUT); Serial.begin(9600); } void loop() { int raw = analogRead(KNOB_PIN); // 0..1023 int duty = map(raw, 0, 1023, 0, 255); // 0..1023 becomes 0..255 analogWrite(LED_PIN, duty); Serial.print("raw="); Serial.print(raw); Serial.print(" duty="); Serial.println(duty); delay(200); }

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PWM fakes an analog output: D9 switches on and off 490 times a second, and the share of time it is on sets the LED's brightness. The knob on A0 chooses that share.

analogWrite() on a PWM pin
map() from 0-1023 to 0-255
Why some pins can't fade

Pin	Connects to	What it does
`D9`	Resistor → LED anode (+)	Carries the PWM signal.
`A0`	Knob wiper	Sets the brightness.
`5V`	One end of the knob	The top of the divider.
`GND`	LED cathode and the knob	The common return.

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 5 V
A 5 mm LED from a PWM pin through a 220 Ω resistor: about 12 mA on USB, bright and safe.
Part pin Board pin
A D9Wire colour orange
K GNDWire colour black
Careful
Never without the resistor: an LED straight on a pin pulls far more than the 40 mA maximum and kills the pin or the LED.
PotentiometerSupply 5 V
A 10 kΩ knob as a voltage divider: ends to GND and 5V, wiper to A0. analogRead gives 0-1023.
Part pin Board pin
1 GNDWire colour black
2 A0Wire colour yellow
3 5VWire colour red
Light sensor (LDR)Supply 5 V
A light-dependent resistor from 5V to A0 plus a 10 kΩ resistor from A0 to GND: more light, higher reading.
Part pin Board pin
1 5VWire colour red
2 A0Wire colour yellow
SG90 micro servoSupply 5 V
Signal on D9 with the Servo library (it takes Timer1, so PWM on D10 stops too).
Part pin Board pin
SIG D9Wire colour orange
GND GNDWire colour brown
Careful
Feed the red wire from a separate 5 V supply whose GND joins the Nano's GND. A stalling servo pulls about 1 A, which browns out the Nano on USB and trips the polyfuse.

All 10 parts on the Arduino Nano page

Connects to

What it does

D9

Resistor → LED anode (+)

Carries the PWM signal.

A0

Knob wiper

Sets the brightness.

5V

One end of the knob

The top of the divider.

GND

LED cathode and the knob

The common return.

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 5 V

A 5 mm LED from a PWM pin through a 220 Ω resistor: about 12 mA on USB, bright and safe.

Part pin	Board pin
A	D9Wire colour orange
K	GNDWire colour black

Careful

Never without the resistor: an LED straight on a pin pulls far more than the 40 mA maximum and kills the pin or the LED.

PotentiometerSupply 5 V

A 10 kΩ knob as a voltage divider: ends to GND and 5V, wiper to A0. analogRead gives 0-1023.

Part pin	Board pin
1	GNDWire colour black
2	A0Wire colour yellow
3	5VWire colour red

Light sensor (LDR)Supply 5 V

A light-dependent resistor from 5V to A0 plus a 10 kΩ resistor from A0 to GND: more light, higher reading.

Part pin	Board pin
1	5VWire colour red
2	A0Wire colour yellow

SG90 micro servoSupply 5 V

Signal on D9 with the Servo library (it takes Timer1, so PWM on D10 stops too).

Part pin	Board pin
SIG	D9Wire colour orange
GND	GNDWire colour brown

Careful

Feed the red wire from a separate 5 V supply whose GND joins the Nano's GND. A stalling servo pulls about 1 A, which browns out the Nano on USB and trips the polyfuse.

All 10 parts on the Arduino Nano page

Fade with a knob

Try it yourself

Serial monitor

Checks

Break it

Inputs

Parameters

Wire it in

Fade with a knob

Try it yourself

Serial monitor

Checks

Break it

Inputs

Parameters

Wire it in