Yes, changing the resisors worked fine. I noticed with the original R6 of 330 ohms, the pulse duty cycle was close to 50% at 3v. With the R6 change to 1K and 5v the pulse duty cycle was around 30%, but at 5v instead of 3v.
With this mod no level translators are required and I can feed the output of your serial port into the Arduino and display the data in the Arduino IDE serial monitor.
I plan to add a typical 16x2 LCD to display the data as well and will post the Arduino sketch at that time for others to implement.
Would this schematic work for dual voltage (3V and 5V) control? I plan to solder 2 small PCBs with 2 resistors and jumper in places of R6 and R11 so one can choose between voltages. In fact I don't think one for LED needed, it will probably just be slightly duller with 300 Ohm in place of 100 Ohm..
geiger_dual_voltage.png (4.21 KiB) Viewed 4781 times
You could do this (I would just use 330 for the LED).
The resistor value on Q1 affects how the HV supply works, in particular the minimum battery voltage at which the HV supply will still run. If you're using regulated supplies (eg. 5V and 3.3V) then you could just leave the 1K in there all the time. You'll need to adjust the HV supply at each voltage. The 330 ohm resistor is more important when you're running at low voltages, below 2.4V.
Yes, you should always count on realigning the HV supply when you change the circuit or supply voltage. You might be able to find a "magic" resistor combination that works at two fixed voltages, eg. 5V and 3.3V. It would be an interesting experiment.