Don't Burn Your Arduino! The Ultimate Guide to Controlling DC Motors

The Code Phase 2: Speed Control (PWM)

Because we used Pin 9 (a PWM pin), we can control the speed! Just like fading an LED. We are pulsing the motor power. The motor’s inertia smooths out the pulses, so it just spins slower.

void loop() {
  // Accelerate
  for (int speed = 0; speed <= 255; speed++) {
    analogWrite(motorPin, speed);
    delay(10);
  }
  // Decelerate
  for (int speed = 255; speed >= 0; speed--) {
    analogWrite(motorPin, speed);
    delay(10);
  }
}

Torque Warning: At very low PWM values (e.g., 20 or 30), the motor might hum but not move. It needs a minimum amount of power to overcome static friction. Usually, motors start spinning around PWM 50-60.

The Trap: Common Ground

This is the #1 mistake beginners make. You have a $9\text{V}$ battery for the motor. You have a USB cable for the Arduino. You MUST connect the $9\text{V}$ Battery Negative to the Arduino GND. Voltage is relative. If you don’t connect the grounds, the Arduino’s “$5\text{V}$” signal has no reference point for the transistor to understand. Nothing will work. Rule: Always tie the grounds together (but NEVER the Positives!).

Deep Dive: MOSFET vs BJT

We used a BJT (2N2222) today. It is cheap and good for small motors ($< 600\text{mA}$). But it runs hot. It wastes power. For bigger motors (Drills, Drones, RC Cars), we use MOSFETs (Metal Oxide Semiconductor Field Effect Transistor). MOSFETs are voltage-controlled (Gate). BJTs are current-controlled (Base). MOSFETs are much more efficient for high power. We will use an IRLZ44N Logic Level MOSFET in future high-power projects. For now, the 2N2222 is your friend.

Deep Dive: The Saturation Region

You might hear engineers talk about “Biasing” a transistor. For motors, we don’t care about linear amplification (like in an audio amplifier). We want the transistor to be fully ON or fully OFF. This “Fully ON” state is called Saturation.

• Cut-Off Region: Base current is $0$. Switch is OFF. Motor has $0\text{V}$.
• Active Region: Base current is low. Transistor is “half-open”. It gets extremely hot because it is resisting the flow. Avoid this.
• Saturation Region: Base current is high enough ($> 1\text{mA}$). Transistor is fully open. $V_{CE}$ (Voltage between Collector and Emitter) drops to $\sim 0.2\text{V}$.

Why does this matter? If you don’t use enough Base Current (e.g. you use a $100k\Omega$ resistor instead of $1k\Omega$), the transistor won’t fully open. It will act like a bottleneck. Your motor will run slow, and the transistor will burn up. Everything we do today relies on achieving Saturation.

Safety Upgrade: The Optocoupler (4N35)

What if your motor is huge? Like a treadmill motor? What if it generates so much electrical noise that it crashes your Arduino even with a diode? You need Total Isolation. You need an Optocoupler (like the 4N35).

An Optocoupler is a chip with an LED inside and a light-sensitive transistor inside.

• Arduino turns on the internal LED.
• Light hits the internal transistor.
• Transistor opens. There is NO electrical connection between the Arduino and the Motor. It is just light. If the motor explodes and sends $1000\text{V}$ backwards, it burns the Optocoupler, but your Arduino remains untouched. For expensive projects, always use Optocouplers.

Engineering Spotlight: The Darlington Pair

The 2N2222 has a gain (hFE) of about 100. If you put $1\text{mA}$ into the Base, you get $100\text{mA}$ at the Collector. What if you need $5\text{ A}$? You would need $50\text{mA}$ at the Base, which might kill the Arduino pin. The solution is a Darlington Pair (like the TIP120). It is two transistors inside one package, chained together. Gain 1 * Gain 2 = Total Gain. 100 * 100 = 10,000. You can drive a massive motor with a whisper of current. Note: They have a higher voltage drop (1.4V), so they get hotter than single transistors. Why did we put a resistor between the Arduino and the Transistor Base? Because the Base-Emitter junction inside the transistor looks like a short circuit (technically a diode) to the Arduino. Without the resistor, the Arduino would dump unlimited current into the Base, protecting the Transistor but killing the Arduino pin. The $1k\Omega$ limits current to a safe $5\text{mA}$.

Challenge: The Automated Fan

You have an LDR (from Day 16). You have a Motor (Fan). Combine them! Build a system that turns the Fan ON when it gets bright (simulating a hot sunny day). Or, if you have a temperature sensor (Thermistor), use that.

int light = analogRead(A0);
if (light > 600) {
  analogWrite(9, 255); // Full blast
} else if (light > 400) {
  analogWrite(9, 128); // Half speed
} else {
  digitalWrite(9, LOW); // Off
}

Troubleshooting Guide: The Diagnostic Checklist

If your motor is dead, don’t panic. Follow this order:

The “Click” Test:

Do you hear a faint “whine” or “click” when power is applied?

• Yes: PWM is too low. Increase softStart minimum.
• No: Proceed to step 2.

The Common Ground Check:

Put your Multimeter logic on Continuity Mode (Beep).

• Touch one probe to Arduino GND.
• Touch the other to Battery Negative.
• No Beep? You found the problem. Tie them together.

The Transistor Orientation:

Look at the 2N2222. Is the Flat Side facing you?

• If you wired it backwards, it might still work poorly but get incredibly hot. Flip it.

The Base Resistor Check:

Did you accidently use $10k\Omega$ or $100k\Omega$ instead of $1k\Omega$?

• Too much resistance = No Saturation = Weak Motor.

The Diode Direction:

If the diode is backwards (Stripe to Negative), you have created a Short Circuit.

• Your battery will get hot, and the motor will not spin.

The “Magic Smoke” Check:

Does the Arduino still run the “Blink” sketch?

• If not, you might have fried the pin. Try using Pin 10 instead of Pin 9.

The Bill of Materials (BOM)

Pro Tip: Noise Suppression (The Ghost in the Machine)

DC Motors are “noisy”. As the brushes spin inside, they spark. This creates Radio Frequency (RF) interference. It can confuse your sensors or reset the Arduino. The Fix: Solder a small 0.1uF Ceramic Capacitor across the motor terminals. It acts like a shock absorber for the electrical noise, shorting the high-frequency sparks before they travel down the wires. If your robot builds are “glitchy”, this is usually the missing piece.

Advanced Theory: Reversing Direction (The H-Bridge)

You might have noticed something. Our Transistor circuit allows us to turn the motor ON and OFF. It allows us to control SPEED (PWM). But it does NOT allow us to Reverse Direction. Why? Because the current always flows from Collector to Emitter. One way. To reverse a DC motor, you must swap the Positive and Negative wires.

To do this electronically, we need Four Transistors arranged in an “H” shape. This is called an H-Bridge.

• Forward: Top-Left and Bottom-Right transistors open.
• Reverse: Top-Right and Bottom-Left transistors open.
• Brake: Top-Left and Top-Right open (short circuit).

Building an H-Bridge from scratch is complex (and prone to “Shoot-Through” short circuits). In the future, we will use dedicated H-Bridge Chips like the L293D or L298N. For today, master one direction first!

Pro Tip: The Soft-Start Algorithm

When a motor starts from 0 RPM, it is “Stalled” for a split second. It draws maximum current (Inrush Current). This spike can dim your lights or reset your Arduino. To prevent this, never jump from 0 to 255 instantly. Use a “Soft Start” wrapper function:

void softStart(int targetSpeed) {
  for (int i = 0; i <= targetSpeed; i += 5) {
    analogWrite(9, i);
    delay(10); // Ramp up over ~500ms
  }
}

Use this whenever you turn the motor ON. Your power supply will thank you.

Safety Check: Measuring Stall Current

Before you use ANY distinct scavenged motor with a 2N2222, check its Stall Current.

• Take your Multimeter. Set it to 10A Mode (Move the red probe to the 10A socket).
• Connect the probes directly to the Battery.
• Briefly touch the motor terminals to the probes.
• Hold the motor shaft so it cannot spin (Stall).
• Read the Amps immediately and let go (do not hold for > 1 second).
• If < 0.6A: Safe for 2N2222.
• If > 0.6A: Danger! Do not use 2N2222. Use a MOSFET.

Conclusion

You have successfully broken the 40mA barrier. You are now controlling High Power with Low Power. This is the fundamental concept behind every machine in the world. From the tiny vibrator in your phone to the massive motors in a Tesla Model S to the hydraulics of a Space Shuttle… It is all just small signals controlling big valves.

Tomorrow, on Day 18, we go even bigger. We will control Alternating Current (AC). We will learn about the Relay. We will learn how to turn on a real lightbulb or a coffee machine safely. (Don’t actually plug in current yet, we will stay safe with $12\text{V}$, but the theory is the same!).

See you on Day 18.