How Geiger Detector works.
To familiarize You with the way our detector works I will discuss the diagram of the prototype built by Szopler.
The device can be presented as 3 blocks:
- Power supply/inverter which powers the GM-20 Lamp with voltage 400V,
- Impulse forming block, which will provide impulses for the central unit,
- Central Unit, responsible for communication between detector and computer.
For our device to work properly - the GM-20 lamp must be provided with voltage 400V
Whole prototype in the first version is powered with 5 volts which is provided by the USB port of the computer. However our GM-20 lamp needs 400volts to work properly, so it is necessary to use an inverter. The MC34063 AP set needs only few pieces to become such an inverter.
Resistor R15 limits the peak current and protects the internal transistor against short circuit, the condenser C9 is responsible for the frequency of work. For the secondary winding of the high voltage transformer there is a Half-wave rectifier connected in form of quick rectifying diode D5, and condenser C6. Divider R24, R20 and R16 decides the output current.. Condenser C13 is stabilizing the voltage supplied from the divider. Current in the lamp during ionization is limited by resistors R22-23. Linkage of the inverter with central unit via D1, R9 serves as cutout of the generator. In this way we can limit the time the GM-20 lamp operates and increase its lifetime significantly.
We should mark that as a power supply unit can be used any other inverter type that provides 400V.
To form impulses in our prototype, we used double operational amplifier LM358N. Because its parameters are not critical, we can use any other operational amplifier with similar parameters.
The impulses are provided via condensers C11-12 for the input of IC4a amplifier, which works in Differentiator. The use of two condensers connected in parallel C11-12 is to protect the circuit in case one of them fails and avalanche breakdown occurs. Comparator IC4b forwards the impulses to central unit.
For the communication with the computer we use ATTINY2313-20P microcontroller. Depending on the mode of work, it can also display results on the connected LCD (yes, the device can be used in the future as a mobile Geiger detector). R12 sets the illumination of the display, T3 is responsible for the state of the display (on/off). In case we would like to "hear" single ions, it will inform us using T2 microcontroller and BUZZER. Diodes D2 and D3 secures the USB port of the computer, in case the circuit faces avalanche breakdown.
The diagram concerns a prototype. The device will surly face modifications and we will inform you about them ASAP. As soon as we will get past alpha phase of the project we will discuss in details the functioning of the circuit and the software of microcontroller.
The circuit diagram is available under http://radioactiveathome.org/images/Diagram1_1.png.