While testing a slight modification on the PU-2 prototype, I decided to test the current draw of the SD Card reader described in the previous post. I placed my multimeter in series with the 3.3 volt pin and measured 11 milli-Ampères. Even while reading data, the current draw was constant. I then decided to measure the current draw when using 5 Volts. I disconnected the 3.3v pin and connected the 5v pin in series, again, with the multimeter. The current draw was 0 mA. !?
I disconnected the 5v pin and launched the reading program on the Arduino. It works. Really! The SD card reader is linked to ground, and MISO, MOSI, SCK and CS pins. NO power.
I can only guess that the SPI pins provide the necessary power to read the SD card. Here’s the schematic for this particular adapter:
Anyone has any idea? Anyone wants to try and replicate this?
Neat trick, Robert!
But don’t expect any ideas from me, this is NOT my area of expertise. If it works, great 🙂
I got my first Arduino Nano last week and the same SD card reader you use here. What I can tell you is that the resistors are supposed to be pull up to 3.3V to give the signal a nice clean edge that the newer faster cards need. When you write data to the card the current consumption can go up quite a bit over 100mA so i guess it wont work under that scenario. Ideally you should use a resistor divider to get the signal levels down to 3.3V or better still a level shifter. For my solution I am using the the 3.3V output of the SD reader to drive my Arduino, I have a 2 cell lipo driving the 5V input to the SD card, the 3.3V output is then fed back to the 5V ref of the Arduino, ie on the regulated side of the 5V, that way the whole system runs of 3.3V so I dont have to do any level shifting.
There’s a missing ground connection on the voltage regulator. I don’t know if you use it, or if your riddle is related to that, but it’s missing anyway.
You can fix it by providing a wire link between the rightmost pin of the regulator (next to C4) down to the right end of capacitor C2, that conveniently extends a bit to the right. You can then use the 3.3V for a simple level shifter (as I did).
Thanks for the tip! I’ll look into that.