The light corona monitoring and alarm circuit I designed is pretty simple.  The circuit is powered by +5v and has an on-board voltage regulator as a reference voltage.  The current from a reversed biased photodiode is converted into a voltage using an op Amp circuit.  That output voltage is fed to a voltage comparator circuit.  I included an adjustment in the current to voltage converter.  After setting the gain to produce a one volt output signal for a typical corona discharge, the comparator’s threshold voltage could also be adjusted.  The device then changes states when the corona intensity dropped by about 20%. 

The alarm output of the monitoring circuit is connected to the company’s computer through a three pin connector.  The alarm signal controlled the treatment process.  When the computer sensed the corona alarm signal, it would shut down the system.  A human operator would then back up the plastic film a few feet and rethread it into the take-up rollers.  Finally, he would press the reset button, which told the computer to start up the film treating process again.