CHOUR - Very low power Geiger-Muller counter PC-GM6
Device: Geiger counter
Main parts: GM Tube SBM20, MSP430 Texas Instrument family processor, Imex module for High Voltage
Main characteristic: Average power consumption arround 25~30µA.
Apart from the Gamma-Scout, most of the mass-market Geiger-Muller counter available (in 2013), have a limited range, at least for those who use a microprocessor to manage the data processing. The reasons are multiple:
- the high voltage generator is often poorly designed. Still, there are diagrams on the net offering generators announcing consumption of a few uA. Some modules already built announce a power consumption between 0.5 and 25µA.
- the microprocessor used is not efficient from the power management point of view. But again, some processors (Texas-Instrument for example) are very efficient in this area.
- how is the microprocessor used is not optimal. A common mistake is to delegate the counting task to a software running on the microprocessor. This forces it either to be in continuous operation, or be in sleep and be awakened every time there is a pulse.
The most obvious solution to avoid this problem is to use a CPU counter (which is the case in PC-CGM5 or 6) and raise this counter periodically (every second, every 10 seconds ...)
But sometimes, this option does not take advantage of the most efficient low power modes. PC-GM3 and PC-GM4 propose an other solution with a Real Time Clock PCF8583. PC-GM3 and PC-GM4 using this solution consume less power than PC-GM6 using the counter of the processor.
Overview of PC-GM6
The circuit described here is an improvement of PC-GM5.
It uses the architecture of PC-GM5 for the analog part (signal recovered at the anode) but adds an improvement for the signal shaped.
The analog signal is injected on a CD4098 (dual monostable). The first monostable gives a clean signal to the processor. The duration of this signal can be adjusted to correspond the dead time of the GM tube.
The second monostable is used to drive a moving-coil galvanometer. The pulse duration can also be adjusted to suit the characteristics of the galvanometer (in this case, the galvanometer consumes around 50µA at full scale).
Obviously, the power consumption increases. But not too much. On average, the power consumption of the Geiger-Muller counter is approximately 25µA to 30µA which allows an operation time of about 9 to 10 years with a battery of 2400 mA.h.
As with the previous counters, I've used the following:
- A high voltage generator IMEX-38-56-1 (in French) with a rated consumption given for 0.5 to 10 uA at 3.3V for generate 400V.
- A MSP430F449 Texas-Instrument microprocessor (Olimex development board MSP430F449-STK2) purchased around twenty euros and manages a display suitable for this type of application
- A Russian Geiger-Muller tube SBM20.
This GMC stores the number of pulses within a certain time interval. Every second, the processor which is normally in standby (mode 3), wakes up, did some checking (keyboard management, USB ...) and return to standby. Every 10 seconds, the processor reads the counter, made his statistical processing, checks alarm conditions and if there is no alarm, return to standby. In case of alarm, the frequency of measurement is one second.
This counter also includes:
- A battery charging module.
- A USB-Serial TTL communication module
- A disconnectable galvanometer which allows to have an idea of the instantaneous measurement of radioactivity. On this model, the galvanometer is calibrated in dB. So not much to do with radioactivity!
The processor runs on 3.6V max. The lower the voltage, the less it consumes. The HT generator operates 2.5V to 5V. The LIPO battery provides 3.7V. Under load, the voltage can reach 4.2V. To lower the voltage for the processor, I used, as for the other counters, a MCP1702 regulator whose consumption at rest is barely measurable (2µA from the documentation).
The diagram of the analog part
The output of the high voltage generator is connected closer to the Geiger-Muller tube. The ground of the tube is directly connected to ground. Detection of the signal is done by the anode. When the tube detects a particle, a voltage spike appears across C4. This peak is amplified by the Darlington formed by T1 and T2 (in my case, I used BC548C because this is what I had in stock but the BC547 and 549 must also be suitable).
On the T1 / T2 collector, the positive potential via R4 falls to 0V and then returns to the positive potential when the pulse disappeared.
This signal is connected across -TR1/2 of CD4098 via a 100kOhms resistor. On a falling edge, a pulse is generated on /Q1 (first timer) whose duration depends C5 and P1. Another pulse is generated on Q2 (second shot) to drive the galvanometer. This output can be disconnected via a switch if one does not want Interacting with the galvanometer. The duration of this pulse depends on C6 and P2.
The value displayed by the galvanometer depends on C7. This capacitor according to the pulses it receives via D3 and R8 and discharges into the base of T3 which supplies the galvanometer. R11 should be chosen so that the galvanometer needle is at maximum when the capacitor is fully charged which is the maximum count rate. For example, if the pulse duration /Q1 is 500µS (> dead time of the tube) this corresponds to the detection of a particle throughout the 500µS or 2,000 particles per second. Run away!
The Sleep pin of the T232R is connected to the GPIO P2.2 of the processor. It shows whether an external power supply is connected or not.
TX and RX of the USB module are connected to UTXD0 and URXD0 (P2.5 and P2.4) of the processor and are used for communication between the GMC and a PC. Do not forget to configure the USB module for TX and RX operate on 3.3V and not 5V.
I consider that the module MSP430f449STK2 Olimex has a bug design that does not allow to use all the functionnalities of the bargraph. It is possible to fix this bug as follows:
- Desoldering or cut the pins 42 and 35 of the display.
- Unsoldered the resistors R9 and R10 0 ohm.
- At the PCB, connect pin 42 with pin 40 and pin 35 with pin 37. Thus, the bargraph of the display is controlled by S0, S1 and S3.
By doing this, you remove the symbol display "µmHF" and arrows that are not really helpful for the GM counter. Doing that, you have a bargraph to visualize the counting rate in the manner of an analog galvanometer.
The GMC program has two versions. One in which the bargraph is used as a simple warning indicator (unmodified PCB) and in which the arrows flash to the rhythm of the CPU alarm, the other in which the bar graph is used as count rate indicator. The arrows are no longer used..
The indications of these modifications are presented below.
inside the Geiger counter.
inside the Geiger counter.
The program is developed using IAR workbench. It is necessary to have a JTAG probe to load the program in the Olimex development board. You can download the software from the download page download page.