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I am working on an rfid door lock. I have this circuit and I need some help. It works as is. But my goal is to use only one adapter/power supply instead of two.

On image 1, as you can see, right now I have a 9V source that powers the relay and a 12V (1A) that powers the arduino and the door lock. The rest of the circuit is not shown because it works stand-alone.. they are just some additional things connected to the arduino (rfid reader and leds). There are two grounds, marked as 0 and 1, because of the two power supplies.

And there is a ground pin in the Arduino that closes the circuit for the output at pin 2 to work, using ground 0 (the transistor acts as a switch).

I tried to get rid of the relay and just use a transistor (image 2), connecting the 12VDC (1Amp) to the door lock, arduino and transistor. This seemed like a bad idea anyways, but I wasn’t sure what to try. It did not work, as I expected. So the circuit for that is on the second attached image. I tried using both the 2N3904 (from the first working circuit) and the TIP31C for it.

My theory is that for the first, the 2N3904 goes into saturation and stops acting as a switch, and the TIP31C needs more current, since the power supply starts blinking and does not supply current.

I’m a little rough on my electronics since I haven’t done anything since college and would like to start again. I know that for both circuits, and even more for the second, I may be either shorting something or putting the arduino in trouble since I’m using the GND pin to close the out from pin 2 into the same ground as the power supply.

Either way, I would like your input on both circuits, if I have done a bad practice, but most importantly if there are any ideas on how to make the circuit use 1 power supply only.

Your help is appreciated.

Thanks! So much info I'll have to take a look at each with time. But I will definitely read it all this week and will investigate about the diode.