This piece is the 33rd in my ongoing series.  This one uses small digital volt meters that I found for about $4 each.  I was delighted that they cam in 3 colors and immediately bought 3 of each.  I hooked a photocell to each one so it reads ambient light level, and then I "de-calibrated" each one to make them all read different values.  They show 0.1 in a dark room and up to 12.0 in a very bright room.  They are sometimes stable, but mostly the numbers jump around in the slightest light variation.