How to Control a 4-Wire PC Fan Using ESP8266 (PWM + RPM Monitoring Guide)

What is fan RPM wire and how the pulses show the speed?

Understanding how the RPM (tachometer) wire works will help you measure fan speed correctly and design reliable circuits.

Let’s go step by step:

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🌀 1. What the Tach (RPM) Wire Does

In a 4-wire fan, the tachometer wire (usually green) is a feedback signal from the fan’s internal controller. It tells you how fast the fan blades are rotating — but not by sending an analog voltage or direct RPM value. Instead, it sends digital pulses (a square wave) that correspond to the fan’s rotation speed.

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⚙️ 2. Signal Characteristics

Parameter	Typical value
Voltage type	Open-collector (transistor output, needs pull-up)
Logic level	5 V or 3.3 V (depending on your pull-up resistor voltage)
Pulses per revolution	2 pulses per revolution (for most fans)
Output frequency	Proportional to fan speed

So when the fan spins, its internal Hall sensor or controller switches a transistor on/off twice per revolution of the blades.

This transistor pulls the tach wire low when active, and your pull-up resistor brings it high otherwise — forming a square wave.

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📊 3. How Pulses Represent RPM

Let’s say your fan generates 2 pulses per revolution.

If you measure the frequency (number of pulses per second) of that signal:

RPM = (Pulses per second × 60) / Pulses per revolution

For 2 pulses/rev, this simplifies to:

RPM = Frequency (Hz) × 30

✅ Example:

• Tach signal = 100 Hz

• Each rev = 2 pulses → 100 Hz × 30 = 3000 RPM

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🔌 4. Why It’s “Open-Collector”

The tach output inside the fan doesn’t actively drive the line high — only low. That’s why you must add a pull-up resistor (e.g., 10 kΩ) to bring it up to 3.3 V when inactive.

This design allows:

• Multiple fans’ tach lines to be combined safely (in some systems)

• Isolation between the fan’s 12 V logic and your microcontroller’s 3.3 V logic

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🔍 5. Reading Tach Signal on Microcontroller

• The ESP8266 or ESP32 pin detects edges (rising or falling) of this square wave.

• You count how many edges (pulses) happen in a time window (like 1 second).

• Then apply the formula to get RPM.

Your MicroPython code already does this:

rpm = (pulse_count / 2) * 60

because it counts all falling edges in 1 second (pulse_count = pulses/sec).

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🧩 6. Typical Signal Timing (Visual)

Tach (green):  _|‾‾‾|_|‾‾‾|_|‾‾‾|_
                ↑    ↑    ↑
                |    |    |
                Each low pulse pair = 1 revolution

Each pair of low pulses corresponds to one full rotation of the fan blades.

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✅ Summary

Concept	Description
Output type	Open-collector square wave
Needs pull-up	Yes (typically 10 kΩ → 3.3 V)
Pulses per rev	Usually 2
Frequency relation	1 Hz = 30 RPM
Read method	Count pulses with microcontroller timer or interrupt

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Explanation about PWM wire and how to control it by ESP8266 PWM

This is the heart of 4-wire fan control 🔧🌀 You already control the RPM signal; now let’s understand and master the PWM control wire.

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🧠 1. What the PWM Wire Does

The PWM wire (usually blue) is how the motherboard or controller tells the fan what speed to spin at.

• It’s an input to the fan’s internal controller.

• It carries a PWM (Pulse Width Modulation) signal — a fast square wave.

• The duty cycle (how long the signal stays HIGH vs. LOW) tells the fan what percentage of its maximum speed to run at.

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⚙️ 2. Electrical Characteristics (Intel PWM Fan Spec)

Parameter	Typical Value
Signal Type	5 V, open-collector compatible
Input Voltage Range	0–5 V logic
Frequency	25 kHz (±10%)
Logic “High”	> 2.1 V (fan detects this as ON)
Logic “Low”	< 0.8 V
Recommended Driver	Open-collector (NPN transistor) or open-drain

👉 The fan already has an internal pull-up resistor to 5 V, so the controlling circuit (e.g. microcontroller) must pull the line LOW to modulate speed — not drive it high.

That’s why we use a transistor like 2N3904 or 2N2222 as a level shifter and open-collector driver.

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⚡ 3. How PWM Controls the Speed

PWM = Pulse Width Modulation

• The fan reads how long the signal stays high within each cycle (duty cycle).

• Example:

  • 100% duty → Full speed

  • 50% duty → Half speed

  • 20% duty → Very slow or stopped

Duty Cycle	Fan Speed (approx.)
0%	Off
25%	~25% speed
50%	~50% speed
75%	~75% speed
100%	Full speed

Each fan can respond slightly differently, but this is typical.

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🔌 4. Circuit (Using ESP8266 + 2N3904)

    +5V (fan internal pull-up)
      |
      +----> PWM (blue wire)
                 |
                 | collector
               2N3904
                 |
                 | emitter
                 |
                GND (common with ESP8266)
                 |
ESP8266 PWM pin (e.g., D1) --[10 kΩ]--> base of 2N3904

✅ Notes:

• The fan internally pulls the PWM line up to 5 V.

• The 2N3904 transistor pulls it low according to ESP8266 PWM signal.

• The ESP8266 never outputs 5 V, but that’s fine — the fan only cares about when it’s pulled low.

• The 10 kΩ base resistor limits base current.

This way, the ESP8266’s 3.3 V PWM safely controls the 5 V fan input.