Faking Analog: Master PWM to Fade LEDs and Control Motors

Challenge: The Electronic Candle

Fading is cool. But Randomness is better. Can you make an LED flicker like a candle in the wind? Hardware: Yellow LED on Pin 9. Code:

void loop() {
  analogWrite(9, random(100, 255));
  delay(random(10, 100));
}

random(min, max) picks a random number. This creates a jittery, organic light. Put it inside a translucent cup. You have a safe flameless candle.

Challenge: The Mood Lamp

You built a breathing light. Now build a Mood Lamp. Goal:

• Start at Red.
• Fade seamlessly to Green.
• Fade seamlessly to Blue.
• Fade back to Red. Hint: You will need nested loops or a long sequence of loops. Red -> Green means: Red goes Down, Green goes Up (at the same time).

for (int i=0; i<255; i++) {
   analogWrite(redPin, 255-i);
   analogWrite(greenPin, i);
   delay(10);
}

If you put this inside a paper lantern, you have a $50 Philips Hue bulb for $2.

Hardware Deep Dive: $490\text{Hz}$ vs $980\text{Hz}$

Not all PWM pins are created equal.

• Pins 3, 9, 10, 11: Run at $490\text{Hz}$.
• Pins 5, 6: Run at $980\text{Hz}$ (Double speed). Why? Because Pins 5/6 are connected to the internal Timer0, which also controls millis() and delay(). This is why messing with PWM frequencies (advanced hacking) can sometimes break your timing functions. For LEDs, you won’t notice. For DC Motors, $980\text{Hz}$ is often smoother and quieter!

The Bill of Materials (BOM)

Deep Dive: The Motor Connection

Why do we care about fading LEDs? Because the exact same logic controls Motors. If you connect a DC motor to a pin (via a Transistor!), analogWrite(127) makes it spin at half speed. This is how Drones stabilize themselves. They adjust the PWM of each propeller thousands of times a second. This is how your car’s fan adjusts speed. PWM is the language of power control.

Advanced Academy: The Lie of Linearity (Gamma Correction)

If you run the fade code above, you might notice something weird. The LED seems to get bright REALLY fast, and then stays bright for a long time. Why? Because the human eye is Logarithmic (just like we learned with Potentiometers yesterday). Twice the power does NOT look like twice the brightness to us. To make a fade look “smooth” to a human, you actually need an Exponential curve. You have to increase brightness slowly at first (1, 2, 4, 8) and fast at the end (…, 64, 128, 255). This math fixes the “Gamma” of the LED. Professional lighting controllers use lookup tables to fix this. For now, just know your eyes are deceiving you.

Math Class: PWM Power Calculation

Here is a physics trick. If you run an LED at 50% Duty Cycle (2.5V effective), does it use 50% Power? Power = Voltage * Current. Or: $P = \frac{V^2}{R}$. Since the Voltage is square-waved, the Average Power is indeed 50%. But the Peak Power is still 100% during the ON phase. This is important for resistors. Even if you dim an LED to 1%, you MUST still use a resistor that can handle 100% current. Because for that tiny 1% slice of time, the full 5V is hitting the LED. Rule: Calculate resistors based on Peak Voltage (5V), never Average Voltage.

Safety: The Heat Trap (MOSFETs)

When driving Motors with PWM (via a MOSFET), a dangerous thing happens. MOSFETs hate being “halfway” on. They love being Fully ON (0 Resistance) or Fully OFF (Infinite Resistance). During the transition (the rising edge of the square wave), they act like resistors and generate Heat. If you PWM a massive motor at $20\text{kHz}$, the MOSFET spends a lot of time in that “transition zone”. It might get hot enough to melt your breadboard. Solution: Use a “Gate Driver” chip to snap the MOSFET on/off instantly, minimizing the transition time. Or just add a heatsink.

History: Where did PWM come from?

It wasn’t invented for LEDs. It was invented for Communication. In the 1940s, ITT engineers used Pulse Time Modulation to squeeze multiple phone calls onto a single wire. By shrinking the pulses, they could fit silence between them for other signals. Today, we use it to drive Class-D Audio Amplifiers (which are 90% efficient compared to old 50% Class-AB amps).

Challenges with PWM

• The Frequency Noise: Standard Arduino PWM runs at $490\text{Hz}$ or $980\text{Hz}$. This is audible to some human ears (a faint whine) and definitely dogs.
• The Camera Flicker: If you film your project with a camera, the LEDs might flicker on video. This is because the camera’s shutter speed clashes with the PWM frequency.
• Not True Analog: If you need true smooth voltage (for audio), you need a “Low Pass Filter” (Capacitor + Resistor) to smooth out the square waves.

Troubleshooting Guide

Nerd Corner: Why 255?

Why is the limit 255? Why not 100 or 1000? Because 255 is the maximum value of an 8-bit byte. 2^8 = 256. (0 to 255). The ATMega328 registers are 8 bits wide. To store a value like 256, it would need TWO registers (16 bits). Using 8 bits makes the PWM incredibly fast and memory efficient. We live within the limits of the hardware.

Optimization: The Bitwise Hack

If you want to divide a number by 2 very fast: Don’t do x / 2. Do x >> 1. This shifts the bits to the right. 0000 1000 (8) becomes 0000 0100 (4). This is how the internal timers handle PWM scaling.

The Engineer’s Glossary (Day 15)

• PWM: Pulse Width Modulation. Tricking the eye with fast switching.
• Duty Cycle: The percentage of time the signal is HIGH.
• RGB: Red Green Blue color model.
• Common Cathode: All LEDs share a Ground connection.
• Tilde (~): The symbol indicating PWM capability on Arduino.

Advanced Application: Servo Motors

We said PWM controls “Speed”. But for Servo Motors, it controls Position. A Servo expects a pulse every 20ms.

• A 1ms pulse means “Go to 0 degrees”.
• A 1.5ms pulse means “Go to 90 degrees”.
• A 2ms pulse means “Go to 180 degrees”. This is a specific type of PWM. The Arduino Servo library handles it for you, but under the hood, it is just analogWrite logic running with very precise timing.

Advanced Application: The Fan Controller

If you want to control a PC Fan (12V):

• You CANNOT plug it into the Arduino (5V).
• You CAN plug it into a 12V supply.
• You CAN put a MOSFET Transistor (Day 25) between the Fan and Ground.
• You CAN connect the Arduino PWM pin to the MOSFET Gate. Now, when you analogWrite(128), the MOSFET flickers the Fan’s power 490 times a second. The fan spins at 50% speed. This is how every variable-speed drill, fan, and drone works.

Hack: Creating “True” Analog (RC Filter)

If you really need a steady 2.5V line (not flickering):

• Add a Resistor in series with the PWM pin.
• Add a Capacitor from the resistor to Ground. This creates an RC Low Pass Filter. The Capacitor stores the charge during the ON pulse and releases it during the OFF pulse. It smooths out the square waves into a wobbly line. If your PWM is fast enough, the line becomes flat. This is how Arduinos can “Talk” (play audio files) by faking sound waves.

Conclusion

You have mastered the art of the fake. You can now create smooth transitions, organic pulsing lights, and unlimited colors. Between digitalWrite (Day 11), analogRead (Day 14), and analogWrite (Day 15), you have complete control over the input and output of energy.

Tomorrow, on Day 16, we combine everything. We will take the following sensors:

• LDRs: Light levels (Day 16).
• Thermistors: Temperature.
• Microphones: Sound volume.
• Flex Sensors: Finger bending. —and build a Theremin—a musical instrument you play without touching, manipulating sound waves with light.

See you on Day 16.