Radioactivity indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The article proposes a simple small-sized battery-powered radiation indicator. Its distinguishing feature is that a high stable voltage source for powering the ionizing radiation sensor is assembled on a pulse stabilizer microcircuit.

Simple battery-powered radioactivity indicators in most cases contain a step-up voltage converter necessary to power the ionizing radiation sensor, as a rule, this is a Geiger-Muller counter, as well as light and sound signaling devices.

To increase the reliability of registration of radioactive radiation, it is necessary to maintain the voltage on the Geiger-Muller counter within the required limits. Unfortunately, in most simple indicators of radioactivity, output voltage stabilization is not provided. At the same time, for normal operation, for example, a Geiger-Muller counter SBM-10, which has a nominal supply voltage of 400 V, it must not go out of the range of 350 ... 450 V. Thus, the voltage deviation from the nominal must not exceed ± 12,5%. Taking into account the fact that the indicators are mainly powered by batteries, and therefore unstable, this can cause a change in the voltage on the meter and, as a result, reduce the reliability of registration of ionizing radiation.

In the proposed indicator of radioactivity, the voltage on the Geiger-Muller counter is maintained within the required limits in the supply voltage range from 1 to 3,2 V. The indicator circuit is shown in fig. 1. The step-up voltage converter is assembled on a specialized NCP1400ASN50T1 microcircuit. In addition, the converter includes a storage inductor L1 and a diode-capacitive voltage multiplier on the elements VD2-VD5 and C2-C5. The principle of operation of the voltage converter on the NCP1400ASN50T1 chip is based on maintaining a constant voltage of 5 V at the output of the rectifier on the VD1 diode. And this means that when the supply voltage changes, the amplitude of the pulses on the winding I will remain approximately constant (5,5 ... 5,6 V). Therefore, the amplitude of the voltage pulses on the winding II weakly depends on the supply voltage of the converter and is determined by the ratio of the number of turns of these windings. The HL1 LED serves as an indicator of the normal operation of the converter.

Fig. 1

The output voltage of the voltage multiplier through resistors R3 and R4 is supplied to the Geiger-Muller counter BD1. At the moment of passage through the counter of a radioactive particle with a certain energy, ionization of the inert gas occurs in it, and the resistance of the counter decreases sharply. At this moment, a voltage pulse appears on the resistor R4, which opens the transistor VT1. As a result, the HL2 LED flashes, and a click is heard in the acoustic emitter HA1. With a normal natural radioactive background, there may be several flashes (and clicks) within a minute. Diode VD6 protects the gate of the field-effect transistor from breakdown.

All parts, with the exception of the battery, are installed on a printed circuit board made of fiberglass laminated on one side with a thickness of 1 ... 1,5 mm, its drawing is shown in fig. 2. LEDs, resistors and most of the diodes are installed on one side of the board, one of the diodes, capacitors, a microcircuit, an acoustic emitter and a counter are on the other. To attach the meter to the board, spring contacts are soldered. The choke and acoustic emitter are fixed on the board with hot glue. The appearance of the mounted board is shown in fig. 3.

Fig. 2

Fig. 3

The device uses fixed resistors R1-4, C2-23 (R3 - KIM), oxide capacitors - imported low-profile, the rest - K73-166. Light-emitting diodes - the increased brightness of various colors of a luminescence: HL1 - green, HL2 - red. It is desirable that their radiation angle be as large as possible. The KP505G transistor can be replaced with a 2N7000 or BSS88 transistor, but in this case, in parallel with the R4 resistor, it may be necessary to install a capacitor (K10-17) with a capacity of several hundred picofarads. This is due to the fact that the gate-source capacitance of the KP505G transistor is about 500 pF, and it shunts the R4 resistor, suppressing interference from the voltage converter and other pickups. And the gate-source capacitance of the 2N7000 and BSS88 transistors is several times less. Therefore, the installation of an additional capacitor is required. Acoustic emitter - DC winding 36 Ohm - taken from an electronic-mechanical alarm clock. Similar parameters for the electromagnetic emitter YFM-1238P. Since short voltage pulses are supplied to the emitter, the current consumed by it is small.

The accumulative choke is wound on an annular magnetic circuit from a transformer of a compact fluorescent lamp. The outer diameter of the magnetic circuit is 10 mm, the height is 3,5 mm. It is covered with a layer of insulation, which is very convenient for making a choke. First, winding II containing 2 ... 0,1 turns is wound with a wire PEV-300 320, it should occupy no more than 3/4 of the perimeter of the magnetic circuit. Then, near its end, winding I is wound - 10 ... 15 turns of PEV-2 wire with a diameter of 0,2 ... 0,3 mm. Before fixing the inductor on the board, the number of turns of this winding is selected experimentally. For a different number of turns in the supply voltage range from 1,2 to 3,2 V, the current consumed by the device and the output voltage of the multiplier are measured. It should be in the range of 350...450 V at the lowest possible current consumption. The Geiger-Muller counter is not installed, and the output voltage of the converter is measured with a voltmeter with an input resistance of at least 10 MΩ. The experimental data of the author's version of the device with a choke, the winding I of which contains 13 turns, are shown in fig. 4.

Fig. 4

For the body (it consists of two parts) of the indicator, a stand of an LED rechargeable lawn lamp is used - a plastic tube with an outer diameter of 18 mm. A printed circuit board is placed in one segment 118 mm long. On one side, it has two holes about 5 mm in diameter for LEDs (Fig. 5), and on the other, the same hole for an acoustic emitter and a window for the counter (Fig. 6), which is covered with transparent plastic (from a plastic bottle). In another segment of the tube there is a battery compartment with a power switch.

Fig. 5

Fig. 6

If you plan to use the indicator rarely and turn it on for a short time, you can use small batteries. The length of the segment of the tube with the battery compartment will depend on this. In the author's version, a segment of the metal case of a small-sized LED lamp with a switch is used as a battery compartment. This compartment is designed to install disk galvanic cells with a diameter of 12 mm. The length of the second section of the tube into which it is glued is about 40 mm. Both segments of the tube (with the board and the battery compartment) are connected using a plastic sleeve-adapter, a plastic plug is installed at the end of the first segment of the tube.

Author: I. Nechaev

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