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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radiation level indicator
Encyclopedia of radio electronics and electrical engineering / Dosimeters A distinctive feature of the proposed indicator of the level of radioactive radiation is that it is controlled by the PIC12F683 microcontroller. When developing the device, the author got acquainted with many existing industrial and amateur radio designs on this topic. For example, a description of one of them was published in the magazine "Radio" No. 10 last year. By creating this device, the author intended to bring its capabilities closer to the needs of an ordinary person. The device offered to the attention of readers has the following characteristics: - LED (by the number of flashes) indication of the level of radioactive radiation directly in μR/h; - forced sound and light (flashes) indication of the registered pulses of the radiation source (in normal mode, it is disabled in order to save battery power and remove annoying psychological effects); - automatic inclusion of sound and light indication of the registered pulses of the radiation source when the threshold of 50 μR/h is exceeded; - automatic activation of the alarm when the second threshold of 75 μR/h is exceeded; - the values of the first and second thresholds, as well as the parameters of the battery used and the specific type of Geiger counter necessary for the operation of the device, are stored in the non-volatile memory of the microcontroller (EEPROM) and can be easily changed in accordance with individual requirements; - current consumption during operation in conditions of natural radioactive background - less than 1 mA (actually measured - 0,86 mA), operating time with a used lithium-ion battery with a capacity of 750 mAh - more than 35 days; - LED indication of the remaining days of battery life; - control of a condition of tension of the accumulator; - battery charging via standard USB connection; - maximum dimensions (determined primarily by the used Geiger counter SBM-20) 120x30x25 mm. Thus, the proposed device has a long (more than a month) duration of operation without recharging the battery, gives an alarm in case of exceeding the specified level of radioactive radiation and indicates the level of radiation directly in microroentgens per hour. A photograph of the indicator meter is shown in fig. 1. The scheme of the device is shown in fig. 2.
Before describing the operation of the device, it is necessary to consider how the level of radioactive radiation is determined by the pulses of the Geiger counter, in our case SBM-20. According to the manufacturer's data [1], the sensitivity of this counter to gamma radiation is 420 ± 20 pulses/s at a radiation intensity of 4 μR/s, which corresponds to 14,4 mR/h. Accordingly, the radiation level of 1 mR / h will correspond to 420 ± 20 / 14,4 = 29,17 ± 1,39 imp. / s or, which is the same, 1750 ± 83 imp. / min. Let us decompose 1 mR/h into factors, for example, 50x20 μR/h, in this case, at a radiation level of 20 μR/h, the Geiger counter SBM-20 will produce 1750±83/50 = 35±1,7 pulses/min. Having found the time during which the Geiger counter will give out 20 pulses at the calculated rate of 35 ± 1,7 pulses / min, we obtain the time interval during which the number of pulses of the Geiger counter corresponds to the radiation level in microroentgens per hour: (60 s / 35 ± 1,7, 20 pulses) x 34,3 = 32,7 s (taking into account the spread - from 36 to XNUMX s). This time interval for counting pulses is formed by the timer 12 built into the PIC683F1 microcontroller. Taking into account the software settings, the period of timer 1 is 0,524288 s, which means that the required measurement period consists of 34,3 s / 0,524288 s = 65 (taking into account rounding) timer periods 1. In hexadecimal form 65 = 0x41, the number 41 is written to the zero (first in a row) cell of the non-volatile memory of the microcontroller EEPROM, and it can be easily changed if another type of Geiger counter is used. The next, first (second in a row) EEPROM memory cell stores the hexadecimal value of the planned number of days for the battery to work: (750 mAh / 0,9 mA) / 24 hours = 35 (rounded off) = 0x23. The second cell of the EEPROM is the value of the first threshold (it turns on the sound and light indication of the Geiger counter pulses) 50 μR/h = 0x32. The third cell of the EEPROM is the second threshold (alarm) 75 μR / h = 0x4V. The fourth cell of the EEPROM is the pulse duration for generating the necessary voltage on the Geiger counter; for the SBM-20, the operating voltage should be 400 V [1]. The formula for calculating the pulse duration is K x 3 µs + 5 µs, where K is the decimal value of the fourth cell. It makes no sense to calculate the duration of the "pump" pulse, since the voltage will depend on the real parameters of the forming circuit. This coefficient must be selected experimentally by measuring the resulting voltage. It is important to note that since the supply voltage source of the Geiger counter is low-power (another is not needed, since the maximum current of the counter does not exceed 20 μA [1]), this voltage must be measured through a high-resistance divider. For this purpose, the author used a divider with a gigaohm input resistance, the measurement was made with a TDS-210 oscilloscope. In the fifth, sixth and seventh (sixth-eighth in order, respectively) cells of the EEPROM, the coefficients are recorded that provide the daily interval. This is necessary to calculate the battery life. The product of these three numbers must be equal to the number of measurement periods per day. The duration of the day in seconds 60x60x24 = 86400 s is translated into the number of measurement intervals (the actual value is 65 x 0,524288 s = 34,07872 s), we get 86400 s / 34,07872 s = 2535 integer intervals. We factorize the number 2535 \u13d 13x 15x 13, respectively, in the cells we write 0 \u0d 13x0D, 0 \u15d 0x0D, XNUMX \uXNUMXd XNUMXxXNUMXF. Important note. For the normal operation of the program embedded in the microcontroller, it is necessary that the initial data satisfy the condition 0 < X < 127, since this condition must be met for some commands used in the program. It is convenient to use the site calc-x.ru/conversion_number.php to convert numbers to different number systems. Now consider the device circuit. The device is powered by a lithium-ion battery, a ready-made board with dimensions of 20x25 mm made in China is used to charge it, if desired, it can be made independently using the TP4056 microcircuit. To power the device with a stabilized voltage of 3,3 V, an LP2980-3.3 chip is used. Its important feature is operation at a low load current and a small intrinsic current consumption (at a load current of 1 mA, it does not exceed 170 μA). The node for obtaining the supply voltage of the Geiger counter is fully consistent with the circuit from a similar device [2]. At pin 7 of the microcontroller (GP0), a short pulse is generated with a duration determined by the contents of the fourth EEPROM cell. This is followed by a pause of 250 μs, and the execution of the program again returns to the formation of the pulse. Initially, the author planned to use a separate block to form a high voltage (there are many circuits of such blocks), this would free up one output of the microcontroller, but practical tests showed that such nodes consume a current of 1 mA or more, microcurrent could not be achieved. The counting of Geiger counter pulses (pin 4) and the response to the measurement button SB1 (pin 3) are implemented by enabling the corresponding program interrupts in the microcontroller. Timer 1 interrupts are also allowed, providing the formation of the measurement interval. The light and sound indication of the registered pulses of the Geiger counter is carried out as follows. In the case when there is no need to indicate the input pulses, at the outputs GP1, GP2 (pins 6, 5) the indication pulses with a frequency of about 4 kHz are in phase, therefore neither the red LED HL2 glow, nor the HA1 piezo emitter react to them. When you press the forced indication button SB2, one of the outputs of the LED and the piezo emitter is connected to a common wire and the indication is forced to turn on. It is important to note that the resistor R9 in this case prevents the failure of the GP1 output of the microcontroller, so it cannot be excluded (for example, to increase the sound volume). When the first threshold of the level of radioactive radiation is exceeded, the indication pulses at the outputs GP1, GP2 are out of phase, the indication is automatically turned on. In the next measurement cycle, the indication will remain on, and this continues until the measured level is below the first threshold. If the second threshold is exceeded, an alarm signal is displayed, which is a three-time flash of the HL2 LED with a duration of 0,25 s, accompanied by a two-frequency (about 4 kHz) sound signal. After that, the measurement of the radiation level resumes. A short (no more than 0,25 s) pressing the SB1 button initiates the mode of indicating the measured level of radioactive radiation in microroentgens per hour by flashing the HL1 LED (blue in the author's version). First, tens are displayed with second light pulses, and then, with quarter-second pulses, the units of the measurement obtained. In order to avoid confusion in the case of zero units (eg 10 or 20 µR/h), the zero values of the units are displayed in one short pulse. When the SB1 button is pressed for more than a quarter of a second, the device switches to the display mode of the remaining predicted days of battery operation. First, the HL2 LED (red) flashes briefly, signaling the transition to the battery control indication mode, after a pause, the same LED indicates the battery status. After the predicted battery life is over, in this mode the number of "recycled" days will be displayed, the processing will be signaled by a short flash of the blue LED HL1. Tens and units are displayed similarly to the previous display mode. Button SB3 allows you to control the current state of the battery. To do this, resistors R13, R14 are selected so that at a nominal operating voltage (3,3 V) the green LED HL3 is lit, and at a voltage of about 3 V (discharged battery level) it does not. Transistor VT1 leads the amplitude of the Geiger counter pulses to the level required for the operation of the microcontroller. Transistor VT3, inductor L2 and a diode multiplier on diodes VD1, VD2, VD5-VD9 and capacitors C2-C4, C6, C7, C9, C10 provide the necessary supply voltage to the Geiger counter. The use of transistor VT2 is caused by the need for initial initialization of the microcontroller. The PIC12F683 microcontroller has six options for the initial installation, however, either the author came across such an instance, or an error was made in the program, but when the interrupt mode was initialized, the microcontroller "refused" to work without a "reset" when turned on. Since the dimensions of the board allowed, it was decided to leave the transistor VT2. The device is assembled on a universal board 100x15 mm in size with a cutout for the battery (Fig. 3), the necessary connections are made with a mounting wire.
The high-voltage output of the Geiger counter is located inside the case, the low-voltage output is closed from the outside with a decorative cap (Fig. 4). The USB battery charging board and the piezo emitter are located under the main board. To control the charging of the battery, two holes with a diameter of 1 mm are drilled in the bottom of the case using the indicators of the charging board. The microcontroller is installed on the board through a standard panel, which allows it to be reprogrammed if necessary. The Geiger counter is installed in fuse holders soldered into the board; in the absence of such, holders can be made from rigid copper wires. Soldering the meter leads can damage it. A view of the device with the cover removed is shown in Fig. 5.
There are no special requirements for the parts used, except that the VT3 transistor must be high-voltage (for KSP42, the maximum allowable collector-emitter voltage is 300 V), the nominal voltage of capacitor C1 must be at least 40 V (with a Geiger counter supply voltage of 400 V) . It should be noted that despite the symmetry of the SBM-20 meter body, it has a polarity and must be installed in accordance with it. In conclusion, I would like to draw attention to the following. Despite the full functional performance of the proposed device (the test was carried out using a source of radioactive radiation from an industrial device DP-5A), it can be improved, namely: - exclude transistor VT2 with additional elements; - eliminate the transistor VT1 with additional elements, replacing it with a conventional resistive divider with diode voltage protection of the microcontroller input, changing the polarity of the input pulses programmatically; - if the device is not planned to operate around the clock, program automatic recording of the current battery operation time into the non-volatile memory of the microcontroller so that the correct data is displayed the next time it is turned on. In this case, it is also necessary to program an additional mode for the SB1 button in order to carry out the initial setting after charging the battery, and automatic initial initialization based on signals from the charging board is also possible. In the proposed variant, each switching on leads to zeroing of the battery operation counter; - generate a voltage for the Geiger counter using a separate micropower unit, in this case one output of the microcontroller is released, which can be used, for example, for the built-in analog comparator. This will allow more precise control of the battery voltage. But more importantly, in this case, the microcontroller can be put into the "Sleep" mode with an interrupt on the pulses of the Geiger counter and the timer. The current consumed by the microcontroller in this mode does not exceed 100 μA; - using a smaller Geiger counter, for example, SBM-21, to create a key fob based on this device, which will control radiation safety for a year or more without recharging; - using a microcontroller with a large number of outputs, implement the output of the level of radioactive radiation to a digital indicator, but then it will be a different device. The program and firmware of the microcontroller can be downloaded from ftp://ftp.radio.ru/pub/2015/05/ind_rad.zip. Literature
Author: S. Makaretz
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