understand electrical diagram of geiger counter

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nevergiveup
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Joined: Tue Jun 17, 2014 3:22 am

understand electrical diagram of geiger counter

Post by nevergiveup »

hi!
I' m doing on analysis of the geiger counter of mightyohm but I dont understand how the floor " impulse detection" of the electric diagram works.
- What is the role of the 2N3904 transistor in this floor?
-what is the signal shape to the output of the geiger tube?
- Why is a decoupling capacitor put on the output of this tube?
please explain me how woks the floor " impulse detecion"

Thank you
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mightyohm
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Re: understand electrical diagram of geiger counter

Post by mightyohm »

The schematic is here: http://mightyohm.com/files/geiger/geige ... edR5R6.png

When the tube detects an "event" (a single gamma ray or beta particle), it sends a pulse of current into the base of Q2 via the filter network of R4, R7, and C2. R4 and R7 serve to limit the amplitude of the pulse so as not to damage Q2. C2 is not a decoupling capacitor, it is a filter. When placed in series with the tube's equivalent resistance in the on state (several K ohms), it creates a low pass filter that reduces interference from the power supply and external sources.

The pulse of current turns Q2 on for a short interval (a few microseconds), and this causes a current to flow in the collector. The collector current causes a voltage drop across R3, pulling the voltage on the collector down to near zero. The sudden drop in voltage causes an interrupt on the MCU, which counts the pulse.

In the attached photo, the voltage on the tube negative terminal to gnd is shown in yellow, from Q2 base to ground is green, and from Q2 collector to ground is blue. This screen capture is shown with C2 = 470pF (2x the normal value), so the actual pulse will decay a bit more quickly than what is shown.
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gm_pulse_400v_r4_22k_c2_470p.png
gm_pulse_400v_r4_22k_c2_470p.png (8.45 KiB) Viewed 9505 times
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