A few weeks ago, I bought one of these sound level meters in
order to get a better sense of the noise level around my house.
I chose this specific model because it had a serial port,
figuring it would be easy to hook the meter up to a small
embedded system and gather ongoing samples throughout the day.
Nope. I wasted quite a bit of time trying to get the meter
working. For a while I could only get it to work on a Windows
machine using their special software. After pulling out the
oscilloscope and multimeter, I finally figured it out. The
documentation is wrong. The 1/8" TRS jack doesn't have TX, RX,
and GND. Instead: tip: logic 0 (positive voltage) ring: tx data
sleeve: logic 1 (negative voltage) Internally, these three pins
are connected to an optocoupler in order to separate the
external serial connection from the on-board 5v logic. For
embedded systems, this turns out to be somewhat convenient,
because one can provide whatever voltages are desired between
tip and sleeve and then read the data which uses th