The analog meter, see Figure 3, will provide approximate radiation levels to 10 mR/hr. The analog meter may be set up in one of two ways. It may be permanently wired into the circuit, or used temporarily by plugging it into the headphone jack.
To connect the meter permanently to the Geiger counter circuit, wire the 6.8K ¼ watt resistor, 330 uF capacitor and meter across the LED as shown in the schematic.
A stand-alone analog meter that plugs into the headphone jack is shown in Figure 4 and Figure 5. Figure 4 also shows by adding a single 6.8K resistor and switch you can double the range of the Analog meter to 20 mR/hr. When SW1 is closed, it short circuits’ resistor R2. This provides a reading range of 0-10 mR/hr. When SW1 is open, resistor R2 is in series with R1 and the range doubles to approximately 0-20 mR/hr.