SD reader project won't work on battery power!

Other Hardware Storage
1

Hi all,

I have created a prototype datalogger consisting of a 5v pro mini, SD card module, RTC, humidity & temp sensor and light dependent resistor. The project works well when connected to my pc through an FTDI programmer however when i try and power it using a battery (7.4v, 2x 3.7v 18650 batteries) to the raw input it does not write to the SD. It completes the setup and creates a file called LOG.txt as expected but then doesn't write anything to the file.

I have measured the voltage received by the SD module in each case and it receives 5.03 V when supplied via battery and 4.97V when using FTDI. Is this tiny difference in voltage causing it not to write correctly? and how do i fix it!

I have attached my code and a schematic as a pdf.
sd card module - Recommendations For You - DealeXtreme

Arduino Light and Temperature Datalogger.pdf (93.4 KB)

Datalogger code.pdf (116 KB)

2

Post a link to the SD module you are using.

3

SurferTim:
Post a link to the SD module you are using.

I have added a link in the original problem post
x

4

Last I checked, that module is not designed for 5v logic. You are not the only one having problems with this type SD reader.
http://forum.arduino.cc/index.php?topic=214658.0

edit: Here is a site with the schematic. Only 4 resistors.
http://www.buyincoins.com/item/9032.html

If it uses an actual voltage divider, that requires 6 resistors, not 4. Here is a schematic with a voltage divider setup.
http://forum.arduino.cc/index.php?topic=8863.0

5

I understand this but if all that is true then why does mine function fine when powered by the 5v FDTI cable but not off batteries regulated through the pro mini?

6

You tell me. If the 5v pin shows more than 5 volts, and the 3.3v bus on the SD card reader shows 3.3 volts, then I presume it is the logic levels. Any time you use a higher logic voltage than the datasheet states, you are looking for a failure. It could be the difference in the 5v bus. I know it doesn't seem like much (5.05 vs 4.97) but that could be enough to do it.

If you have the parts available, try it with a voltage divider circuit.

I guess the alternative explanation is the battery doesn't like you. ??

7

SurferTim:
You tell me. If the 5v pin shows more than 5 volts, and the 3.3v bus on the SD card reader shows 3.3 volts, then I presume it is the logic levels. Any time you use a higher logic voltage than the datasheet states, you are looking for a failure. It could be the difference in the 5v bus. I know it doesn't seem like much (5.05 vs 4.97) but that could be enough to do it.

If you have the parts available, try it with a voltage divider circuit.

I guess the alternative explanation is the battery doesn't like you. ??

ha ha i wish it was that the particular battery didn't like me. I will try out a voltage divider in a bit and see if it makes any difference

8

That was actually incorrect on my part. It would not be the battery that doesn't like you, but the onboard voltage regulator. That is what makes the 5v bus a bit higher in voltage when using a battery to power it.