[Minisystem] has a thing for dynamo powered bike lights. He
wanted to measure how well his latest is working, but just
logging the current flow through the LEDs wasn’t enough for him.
He picked up a cheap Lux meter and hacked into the circuit to
log measurements while he rides. He started by cracking open the
case to see what the meter held inside. There’s a Texas
Instruments Op-Amp that connects to the light sensor. The
datasheet for the part didn’t help much, but [Minisystem] did
find that the current output on one of the pins changes with
light intensity. Further testing led him to discover that the
signal is a multiple of 10 for what is shown on the Lux meter’s
readout. All he needs to do is take regular measurement of this
current and save that data. To do this, he grabbed his trusty
Arduino and made a connection between one of its analog inputs
and the op-amp pin. It should be easy enough to dump
measurements into the Arduino’s own EEPROM, or use an external
storage chip or SD card.