microcontroller driving a relay with transistor and Opto-isolator


Posted on Feb 7, 2014

Controlling a relay with stm32 micro controller. For the isolation I have used a 4N33 opto-isolator. As my relay coil needs around 250 mA, I used a mps222 transistor. I figured that the logic gets inverted when I used the gpio as the source voltage for the opto-isolator led. I just used a pull up instead and connected the other end of the led part to the gpio. Gpio is open drain, I will drain around 3mA


microcontroller driving a relay with transistor and Opto-isolator
Click here to download the full size of the above Circuit.

in the pin. I do not have the R2 resistor grounded at this moment. My problem is if in case my stm gets disconnected then my relay will be turned on - which I want to avoid. Could anyone please tell me if adding this R2 will fix the issue




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