QM1538 ==> 4Hz Data Logging for Fifty Bucks

Everyone wants data logging.

You want data logging... (yes you really do).

Keep track of real events in the real world and look at them over time. Feed your data-processing algorithms with something a bit more realistic than

/dev/urandom

. Do science. Learn about the environment you are living in.

Just for the record, I'm a happy customer who bought one of these meters and liked it. I'm not getting any kickback off this. Swear.

Why Qm1538?

Spending money is the best fun when you are spending someone else's money. On the other hand, if you are spending your own money then it's a compromise between spending little and buying complete junk and spending a lot and overkilling your problem. There are some really nice scientific data loggers out there but not much good gear at the low-end of the market. The QM1538 stands out as good value, versatile, easy to use and robust.

Good Points

Standard 9-pin RS232 data output 2400 baud.
Output is opto-isolated for safety and convenient floating-earth (so the specification says, I've not tested how much voltage it takes to break it down).
Reads a wide range of analog signals (voltage, current, frequency, temperature, duty-cycle, resistance, etc).
Runs on batteries -- measuring floating voltages, no need to struggle with common earth issues or interface circuits.
Doesn't cost too much.

Bad Points

Runs on batteries -- they run out and need to be replaced (however, cobbling up some sort of battery replacement is not difficult, but the unit does not provide any handy power supply jack so you have to wire directly into the meter).
You only get 4Hz samples (although the meter internally samples much faster and gives you handed the consolidated value). Whether this is a problem really depends on what you are measuring.
Strictly single channel. If you want more channels you will have to buy more meters (and find a computer with more serial ports).
All controls are manual, RS232 is data output ONLY, to change settings you must manually turn the selector and/or press the buttons.

Temperature -- Is My Server-Room Cooking?

The most obvious thing that every computer owner wants to keep track of is temperature. The QM1538 kit comes with a temperature probe and the dial has a setting for "degrees C" which is pre-calibrated to the probe. It gives a precision of 1/10 of a degree, I have nothing better to compare it with for accuracy.

Usage

In simple terms, this program is a stream filter that attaches to a serial port and dumps output to the standard out. The serial port is currently hard-coded into a global variable -- a command line option could easily be added or just edit the source code and compile.

char *serial_filename = "/dev/ttyS0";

Options

-n Output floating point numbers rather than a simple reconstruction of the screen display (in particular apply the SI scale modifiers to the output number instead of just outputting those scale modifiers, also throw away leading zeros). Less human readable, better for piping to a program.

Probably, more options would be useful, see where we go from here. Write them yourself or consider hiring an expert to make your project really zoom (hmmm, where to find such an expert).

Download

Links to source code:

qm1538_2007-01-18.c