EZDAQ

Kevin Cuzner

A Teensy 3.1-based DAQ designed for simple data collection.

I found that I needed something to log a long term (several days long) test that I was performing on an analog circuit of mine. This is a very simple program that uses the ADC on the Teensy 3.1 to simply gather data continuously on all topside analog channels (A0-A9). It communicates over USB to some host software written in Python which performs the conversion from raw ADC output into useful voltage values. It supports logging all 10 channels at once, a variable time step between sample records, and per-channel attenuation correction values for any resistor divider used in front of the analog channels.

This is not an accurate system by any means. Getting it calibrated so that two channels output the same value for a given voltage may be impossible unless some seriously precise resistors are used. I am mainly using this to see a trend in a voltage over time, so it basically functions as a very inaccurate long time data logging voltmeter for me. Perhaps you have an idea to make this thing more useful?

Installation and Host Usage

1. Clone the repository
2. Run make to build (requires arm-none-eabi GCC installed)
3. Run make install to upload the hex file to an attached Teensy 3.1 (requires teensy_loader_cli to be on the PATH)
4. Run the host/ezdaq python script (requires Python3 and PyUSB)

Circuit Setup

This uses the pins marked as A0-A9 on the Teensy 3.1 schematic to measure any voltages present on those pins. All that needs to be done is to place a capacitor (I used 0.1uF because I literally have 100's of those) and a resistor divider on that pin. The resistor divider is optional, but I needed it so that I could measure voltages above 3.3V. Make sure that your resistor divider has a high enough impedance to minimize the observer effect on any circuit you may connect this device to. I used 2x 1meg resistors.

To calibrate your resistor dividers, plug them into a known voltage (such as the 3.3V coming off of the Teensy). Then, run the ezdaq host program for the channels you are interested in calibrating. Note the values for each channel and divide those values by the known voltage. The resulting percentage is your "attenuation" value that you can plug in using the -a argument to ezdaq.

For example, if I plugged my resistor divider on channel A0 into 3.3V, I would run the following command:

$ ./ezdaq 0
Time,Channel 0
1467771464.9665403,1.7990478515625
...

We now have 1.799 for the "voltage" seen at the pin with an attenuation factor of 1. If we divide 1.799 by 3.3 we get 0.545 for our attenuation value. Now we run the following to get our newly calibrated value:

$ ./ezdaq -a 0 0.545 0
Time,Channel 0
1467771571.2447994,3.301005232
...

Things to note

• Unless you are using high-precision resistors, resistor dividers will be quite inaccurate. I used two resistors drawn from the same batch and ended up with a 45% divider instead of a 50% divider. To make matters worse, my second divider from that batch was a 47% divider, so things will vary greatly.
• I have no clue what I'm doing with the ADC on the Teensy 3.1. It seems calibrated enough, but I couldn't quite get 16-bit precision to work over the full 0-65535 range. Therefore, I am simply using 12-bit. If someone wants to put the time into getting that working, I would appreciate it, though 12 bits was sufficient for me.