Optocoupler interfacing with STM microcontroller

                          

The main purpose of opto-coupler is to provide electrical isolation between two circuit.The reference voltage for STM32F407 is 3.3V. That mean we can not generate voltage greater than this nor we can apply voltage greater than this to any pin of controller. While the triggering circuit for relay is operating at 12V. So, due to fault in circuit a high voltage could be applied to micro-controller. Similarly, a high current can pass through the microcontroller pin, that can damage the controller. That is why, it is better to use opto-coupler, that gives isolation between two circuits. An opto-coupler use light for communication between two circuit. There are several opto-coupler present in the market. PC817C is the cheapest one and it is sufficeint for this technique. Its all specification can be found from datasheet.Some important specificatons  of interest of this opto-coupler is given below.

·         High isolation voltage between input and output (Viso)=5000Vrms

·         Maximum operating frequency=1K

·         Power dissipation=70mW

·         Input forward current (Rated)=50 mA

·         Output collector current (Rated)=50mA

·         Temperatuer Range=(-10 to 100 degree celcius)