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I'm using a "Logomatic v2 Serial SD Datalogger" to measure the voltage output of this piezo-resistive sensor, "Force Sensitive Resistor - Square." The values are logged in a .txt file in a microSD. (I would link the datasheets and such to both of them, but I cannot post more than two links. Google should direct you to them).

I configured it first to ASCII ADC mode (mode 2, ASCII = Y) and I got this log file. I also configured it to binary ADC mode (mode 2, ASCII = N) and I got this weird file.

However, I'm not sure how to analyze the values in the log. How do I translate these values to a certain voltage output? I ultimately want to correlate a certain voltage output or resistance with the amount of pressure being exerted upon the sensor.

As for my circuit, I'm supplying 3.3V to the sensor, connected the sensor to a ADC port of the data logger, added a voltage divider (resistor) between power and GND.

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  • \$\begingroup\$ Please post a schematic of the circuit you are using. The users guide off sparkfun seams pretty clear about how to use the device. Have you read it? sparkfun.com/datasheets/Sensors/Pressure/fsrguide.pdf \$\endgroup\$ Sep 19, 2013 at 0:26
  • \$\begingroup\$ If you take a look at the "FSR Voltage Divider" on page 18 of that data sheet, the schematic of my circuit is there. I have read the data sheet, it's just that I don't know how to interpret the values I'm getting from the circuit. \$\endgroup\$
    – user29269
    Sep 19, 2013 at 2:10
  • \$\begingroup\$ Use a program to dump the binary logfile in hexadecimal codes, then see if you can make anything from that. (Or read the documentation of the device ...) \$\endgroup\$
    – jippie
    Sep 19, 2013 at 5:19

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It looks like it's a 10 bit ADC supplied by 3.3V (checkout the datasheet for the ARM7 part on the logomatic: http://www.nxp.com/documents/data_sheet/LPC2141_42_44_46_48.pdf). That means that a value of 1023 corresponds to 3.3V. 0 corresponds to GND. It's a linear scale so all you have to do is scale the numbers you see in the ASCII file (e.g. Voltage = Value * 3.3V / 1023). As to the "weird" file I don't think there's anything weird about it. Without digging in too hard it's dumping binary data to the file, which isn't readable in clear text (e.g. ASCII). So your text editor interprets the binary data as ASCII and gets "weird" characters. You'd have to write a script/program or something to parse the binary to interpret the data. Scaling would be the same.

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  • \$\begingroup\$ BTW I haven't looked at your sensor, but it could be that there's some nominal offset voltage you should subtract to get absolute force. I've played with a lot of pressure sensors that have some bias voltage. You can easily subtract this baseline from the numbers in your text file and scale as I suggest in the above. \$\endgroup\$
    – Doov
    Sep 19, 2013 at 2:12

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