DIY Sim reader - im worried about "ground loop"

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1

Hey.

Im trying to make this sim reader.

https://www.ladyada.net/make/simreader/ … erv1_0.png

https://www.ladyada.net/make/simreader/make.html

It uses a 9v battery. the ST78M05 steps it down to 5v, verified by multimeter.

I am adding an sp3232ebcn-L/TR to eventually connect it to my rpi4b.

Datasheet:https://assets.maxlinear.com/web/docume … 3232eb.pdf

I hooked up all the charge capacitors in the appropriate locations, according to this: https://www.sparkfun.com/products/11189 … gIWfvD_BwE

If i did it right, i hooked up the 5vcc line to pin 2 of the sp3232 chip. (V+)

R1 and T1 lines read 3.3v.

My questions:

  1. pin 16 is reading 3.3v. Should it? So, i should connect a jumper from the rpi 3.3 rail to pin 16 of the sp3232, right?

  2. pin 15 of the sp3232 is ground. Is this a separate ground? Or do i link it together with the sim reader’s circuit ground, which is connected to the 9v battery?

  3. the rpi has its own ground. Should i link this ground with it all?

  4. Do i reverse the TX/RX lines in the circuit at any point? How about at the final point when connecting to the rpi.

I didnt think i should.

I was reading about ground looping. I dont want to fry my rpi.

Thanks for reading.

2

I doubt very many folks are going to have recent experience with this project (~15 years old)…the best thing we can advise is to test/try as the guide instructs and perhaps report your results back here (in case anyone else tries it too!)