Driving 5v Relay Module Using 3.3v

I am implementing a power-off function for my Ender 3 using OctoPi. For that, I need a relay and I happened to have one.

Unfortunately, after connection relay to Raspberry Pi GPIO pin, the relay was always on and not listening to any commands from OctoPi.

It appears this module was designed for 5v logic levels and Raspberry Pi works with 3.3v logic levels.

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Step 1: Original Relay Circut

After investigating relay one can see that its circuit is like this.

Step 2: Modified Relay and Circuit

Just desolder optocoupler and add 3K resistor like in a photo and now you can use it from Raspberry Pi or any other 3.3V board.

Resistor calculation and more descriptions.

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