An add-on numerical counter that plugs into the headphone jack of 1960s vintage geiger counters such as the Victoreen CDV700 and CDV700-6B. Also works with the Lionel ENI LENi counters or any other geiger counter having a headphone output pulse greater than -5V.
The main parts of the CARL circuit include an input pulse shaper, an audio click sounder, a microcontroller that runs the counter program, and a liquid crystal display module (LCD).?
DC power at approximately 5.7V is used to power the circuit. The 1N4001 diode drops that to around 5V. The 5.7V shown on the schematic comes from the low voltage dropout regulator circuit in the companion “Hot Rod CDV700” modification circuitry. The microcontroller and LCD module are capable of operating anyhere in the 3-6V range, 5 Volts is optimal.?
The CDV700 geiger counter produces a negative going pulse on the phone output with about a -20V swing. The pulse is limited, then fed into the LMC555N CMOS timer IC, which lengthens and squares the pulse. The 5V zener diode limiter prevents the negative going headphone click signal from going below 0V (5V less than +5V), this prevents the CMOS timer input from latching up or being damaged. The squared pulse on pin 3 of the 555 is inverted by the upper 2N3904 transistor to create a low-active pulse, that signal goes to the microcontroller interrupt pin for counting purposes. The pulse is also inverted by the lower 2N3904 transistor, that signal is used to drive the mini speaker for the clicking sound. The mini speaker volume is selected by the soft/off/loud switch and two resistors.?
One could easily extend the interrupt pulse signal to a ttl-compatible input on a computer. It should be possible to feed the signal to an input on a PC parallel port, for example.
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