Project:OD600 Measurement
From London Hackspace Wiki
The idea is to take a 600 nm source, (or a source including 600 nm and a BP filter) and a level detector, and calibrate it to match 0.4 at OD600 on a proper spectrophotometer..
At the moment the main issue is that the LDR (light dependent resistor) only sees about 20 ohms of difference between the initial inoculation and the mid-log phase, which shows as 0.4 OD600 on a spectrophotometer.