This circuit takes the audio output from a Vic CDV-700 6B Geiger counter and converts it to an RS232 pulse that can be counted on a computer port. Presumably it will work for other types of Geiger counters and is even configurable for both negative and positive going pulses. The trigger threshold can be adjusted although that may be removed in a future revision as it should not be necessary given that the input pulse can be inverted with the NPN transistor. The output pulse of the first 555 can be adjusted to give a longer or shorter RS232 pulse. The second 555 is used to stretch the pulse to give a bright LED rendition of the input pulse.
I mounted this in the following box which worked well and can be bought on Amazon:
Hammond 1591BSBK ABS Project Box Black
I first mounted this in a smaller box but this one is much easier and leaves room for future mods.