Geiger Counter I Circuit

The circuit is built around a 4049 hex inverter (Ul), a pair of 555 osciliator/timers (U2 arid U3), two transistors, a Geiger-Muller tube, and a few additional support components. The first 555 (U2) is configured for astable operation. The output of U2 (a series of negative-going pulses) at pin 3 is fed to three parallel-connected inverters (Ul-a, Ul-b, and Ul-c). The positive-going output pulses of the inverters are fed to the gate of Ql, causing it to toggle on and off. The output of Ql, which is connected in series with the primary of step-up transformer Tl, produces a stepped-up series of pulses in Tl`s secondary.
Geiger Counter I Circuit - schematic

The output of Tl (approximately 300 V) is fed through a voltage doubler (consisting of Dl, D2, C3, and C4), producing a voltage of around 600 V Three series-connected Zener diodes (D3, D4, and D5) are placed across the output of the voltage doubler to regulate the output to 500 V, fed through R4 (a 10- current-limiting resistor) and J2 to the anode of the GM tube. The limiting resistor also allows the detection ionization to be quenchcd. The cathode side of the tube is connected to ground through a 100-kQ resistor, R5. When a particle is detected by the GM tube, the gases within the tube ionize, producing a pulse across R5. That pulse is also fed through C5 and applied to the base of Q2 (a TIP120 npn transistor), where it is amplified and clamped to 9 V. The output of Q2 is inverted by gate Ul-d, then it is used to trigger U3 (the second 555, which is configured for monostable operation). The output of U3 at pin 3 causes LED1 to flash, and produces a click that can be heard through speaker SPKR1 or headphones. The circuit is powered by a 9-V alkaline battery and draws about 28 mA when not detecting radiation.

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