Arduino Temperature Controlled PC Fan

Overview

The project illustrates the control of a PC fan speed and using an Arduino microcontroller to read a LM35 temperature sensor.

Explanation

The use of an optocoupler is covered along with Pulse Width Modulation (PWM) and reading analog voltages with the Arduino. A 12V from the Arduino is required to control a device without damaging it. A 12V PC fan cannot be directly driven by the Arduino since it is a 5V device. The optocoupler isolation keeps the Arduino safe.

The CNY75 optocoupler is used as a solution to separate the two voltages. The device consists of a phototransistor and an LED inside the same package. When the internal transistor lights, the phototransistor turns ON and/or conducts current. The brighter the internal LED, the more current can pass through the phototransistor. The phototransistor and LED are physically isolated from each other in order to protect the input side containing the LED from voltage spikes on the output side containing the phototransistor. It also provides the voltage translation needed for this project.

The phototransistor will turn ON and pass current for the 12V DC fan when the LED is activated from the 5V Arduino.