Introduction:
This device serves as a digital radiometer and dosimeter. It allows measurement of absorbed dose and dose rate of ionizing radiation.
The detector is a GM (Geiger-Muller) tube with mica window, capable of measuring alpha, beta and gamma radiation.
It also allows the alarm when exceeding the chosen level of dose rate.
Circuit description:
This digital radiometer with a dosimeter is controlled by circuit Atmel AVR ATtiny2313A, ATtiny2313V or ATtiny2313.
It is clocked from the 8MHz internal RC oscillator.
It is displaying on a low-power 4-digit seven-segment red LED display.
It can be set up from two double digit displays LD-D028UR-C (character height 7mm).
The display has a common anode and is controlled by the multiplex frequency of approx 100Hz.
The buttons TL1, TL2 are used to control the Radiometer-Dosimeter. As a detector of ionizing radiation I used tube E1 - Geiger-Muller (GM) tube Philips 18504.
For its operation it requires a high anode voltage. The plateau of the tube begins at 425V (max) and the length is 225V (min).
The GM tube has a relatively broad plateau and it is not necessary to set the voltage to its center, it can be set closer to its lower end.
It seems that the most appropriate voltage is about 480V. This voltage is obtained by low power inverter.
You can also use a different type of tube. Anode voltage can be adjusted by changing ZD1 zener diode.
Transistors T2 and T3 are oscillator. Voltage is transformed by Tr1 transformer and doubled in the circuit with
D1, D2, C5 and C6. Operating frequency can be adjusted in the event of problems by changing C4 (especially if oscillator does not start).
Stabilization is made by feedback formed by ZD1 and T1. As ZD1 you can use zener diode or transil.
Since the zener diode with such a high voltage are not usually available, you can use more Zener diodes (or transils) in series.
Tr1 is transformer for powering CCFL lamps. It comes from a 17'' LCD monitor and was originally designed to power a pair of CCFL tubes.
GM tube is connected with a grounded cathode. This connection method allows better noise immunity.
The pulses are detected by capacitor C7 separating the DC component of the voltage and amplified by transistors T4 and T5.
Then they are indicated by the speaker Rep1 (small speaker with an impedance of at least 32 ohms) and evaluated by circuit IO1.
Counting pulses is done by the 16-bit timer/counter1 that is externally clocked by these pulses.
When exceeding the set level of dose rate the pin 5 of IO1 (port PA0) goes into logic 1 and lights up the warning LED1.
This pin can also operate a self oscillating siren to ensure acoustic warning.
Pin 4 (port PA1) is used for standby switching of the high voltage converter and indicator with Rep1.
If the device is turned off, it will go into logic 0.
At the same time IO1 put into sleep mode Power-down and the entire radiometer - dosimeter has almost zero off-state power consumption.
Consumption is approximately 10-30 mA in operating mode, in indication mode about 2-4 mA. The circuit can be powered by voltage about 2.7 to 5.5 V.
The power supply can be a 5V supply, battery or accumulator.
You can use 3pcs 1.5V batteries (AA or AAA), 3pcs 1.2V NiCd or NiMH or one Li-Ion or Li-Pol with voltage of 3.6 or 3.7 V.
Put a fuse in series.
Control, measurement:
To control the device two buttons - TL1 and TL2 - are used.
TL1 is used to switch on and switch modes.
TL2 is used to reset the measuring cycle, reset the cumulative dose and turn off the radiometer-dosimeter.
After switching on (press TL1) the radiometer-dosimeter will get into mode of indication and monitoring.
Acoustic pulses indicates GM tube pulses and the device monitors radiation levels and measures dose. The display is off, this state is indicated only by blinking dot.
TL2 button can now turn off the device.
Pressing TL1 you will enter the the dose display (dosimeter). In the mode of the dosimeter decimal point flashing, data is in mR.
Pressing TL2 can refresh the data. The data are also automatically refreshed at certain intervals. Long press of TL2 (1.5 s) resets the dosimeter.
The next press of TL1 puts the device into fast radiation measurement (search). Measurements are made with 10x higher refresh rate and resolution
is up to 0.01 mR. Press TL1 to get into an accurate measurement mode of radiation with a resolution
up to 0.001 mR. In dose rate modes (radiometer), decimal point is shining permanently, the data is displayed in mR/h
Measuring cycle can also be manually set to start by pressing TL2.
Tne next press of TL1 puts the radiometer-dosimeter back into power saving mode. Nou you can switch off by the TL2.
Turning off the device dose not clear the dose, but it is no more measured.
In all modes except "off" the monitoring of radiation level is active (increase in radiation levels above the set limit activates alarm) and dose measurements
is also active.
After replacing the batteries the dose may need to be reser (dosimeter can show meaningless value).
Calibration is done by adjusting the calibration constant. You can get into calibration mode by long press of both TL1 and TL2 at the same time (1.5 s).
Digits can be set using TL2. TL1 is switching between the digits. This is followed by setting alarm threshold level in mR/h.
Alarm level setting can also be reached by long pressing TL1.
The calibration constant (XXX.X) determines the exact measurement time in seconds (s).
Number of pulses during this period corresponds to the level of radiation in uR/h
Fast measurement time is 10 times shorter than the calibration constant.
Calibration constant and alarm level are stored in EEPROM memory, so they will not be lost even if the power is removed.
Theoretically maximum dose rate the device can display is 9999mR/h and dose 9999mR, but the actual maximum dose rate is limited by the GM tube.
The device is intended for the measurement of low levels of radiation only. A higher radiation values the tube may saturate and lead to underestimation of
dose rate and dose. For really correct measurement it would also be necessary to supplement the tube with the compensation shield to prevent
the overestimation of less penetrating types of radiation.
The program for free download:
Source code in assembler (ASM)
Compiled HEX file (1872 Bytes)
How to write the program into the AVR is described here.
