@AhtishamAshraf Optocoupler is good, so you see it using in relay front ends. I remember vividly that the day I fried my 12V 1A psu, because I forgot the flyback thing, the instant that back EMF happened was a bit scary: everything in the ciruit, including Rpi, got dark at the same time, sort of a "black out". I wrongly thought that all power supplies ware fried. Later I found only the psu for the relay was fried. The separate psu for Rpi could reset, and Rpi can also reset, meaning not fired.

This makes me thing that doing offline testing without Rpi is a good thing, not just because of using "Incremental TDD (Test Driven Development)", but also for "Structured Programming/Development/Troubleshooting" where making architecture as structured/modular as possible, and all modules as "decoupled" as possible. Just brainstorming aloud. Nothing urgent, no reply necessary.

@AhtishamAshraf HI there. I am catching up. My new toy, LY-03 mini magnetic door lock has arrived, in less than 24 hours after placing the order. I hope to test it sometime tomorrow. Cheers.

@AhtishamAshraf Now I am doing the preliminary static test fo the LY-03 350mA, 10mm travel distance solenoid lock. I usually do non destructive tear down, to make sure the insde things are properly assembled and soldered etc. The tear down photo is shown below.

@AhtishamAshraf I used my multimeter to measure the coil resistance to be 27Ω. So the static current should be V/R = 12V / 27Ω = 12 / 27 = 444mA, seems higher than the specified 350mA. I also wanted to measure the inductance, so later I can calculate the magnetic energy stored in the coil and see if it is big enough to create a back EMF to disturb the Rpi's python RFID program. But I have forgotten how to measure the inductance. So I will leave it later.

The high priority thing is to use a 12VDC source and operate the lock by hand. The most importance thing is to use a flyback diode to protect the solenoid and power supply which I already fried one last time. This time I should not forget it any more.

Now I am very carefully documenting my power supply and flyback protection (for both power supply and door lock) wiring for a record of successfully test, postmortem in case I am frying my psu for a second time! :)

Ah, bed time. Call it a day. Will test this new toy tomorrow. Cheers.