A phototransistor is what is used now, LDRs are slow, insensitive and hard
to find now (many contain cadmium which is very toxic). Phototransistor
can probably rigged up to generate a logic signal direct with a high enough
load resistor, select on test. http://learn.parallax.com/sites/default/files/content/shield/robo_ch6/series_sch.jpg
However a neon is striking and breaking arc 100 or 120 times a second so
you'll get an ac signal out and need some de-bouncing in software.