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arduino-cli/libraries/TimerOne/examples/FanSpeed/FanSpeed.pde

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+#include <TimerOne.h>
+
+// This example creates a PWM signal with 25 kHz carrier.
+//
+// Arduino's analogWrite() gives you PWM output, but no control over the
+// carrier frequency.  The default frequency is low, typically 490 or
+// 3920 Hz.  Sometimes you may need a faster carrier frequency.
+//
+// The specification for 4-wire PWM fans recommends a 25 kHz frequency
+// and allows 21 to 28 kHz.  The default from analogWrite() might work
+// with some fans, but to follow the specification we need 25 kHz.
+//
+// http://www.formfactors.org/developer/specs/REV1_2_Public.pdf
+//
+// Connect the PWM pin to the fan's control wire (usually blue).  The
+// board's ground must be connected to the fan's ground, and the fan
+// needs +12 volt power from the computer or a separate power supply.
+
+const int fanPin = 4;
+
+void setup(void)
+{
+  Timer1.initialize(40);  // 40 us = 25 kHz
+  Serial.begin(9600);
+}
+
+void loop(void)
+{
+  // slowly increase the PWM fan speed
+  //
+  for (float dutyCycle = 30.0; dutyCycle < 100.0; dutyCycle++) {
+    Serial.print("PWM Fan, Duty Cycle = ");
+    Serial.println(dutyCycle);
+    Timer1.pwm(fanPin, (dutyCycle / 100) * 1023);
+    delay(500);
+  }
+}