A 4000 series CMOS Geiger Counter
2025
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Over the years I have designed and built some geiger counters using micros c/w Bluetooth. As a fan of old sci-fi I prefer the look of a moving coil meter and a healthy "click". I have decided not to include a microprocessor or any chips that need firmware. CMOS technology is well proven and very robust. The 4000 series is now over 50 years old and many of the oldest chips still function perfectly. This design uses robust 50 year old technology. Unlike most old geiger counters this one can count the clicks and display them on a moving coil meter so it can record the radiation dose since turn on. It is possible to set the counter such that background radiation in Adelaide for one day is about 1/5 full scale on the meter. The maximum number of clicks for full scale deflection of the meter is over 8 million clicks. The printed circuit board has two 50 milliamp meter drive outputs. Two meters or a single switched meter can be connected. One display is the total dose as a "click" count, the other is a conventional geiger counter display. One goal was very low battery consumption, when supplied from a 9V battery the current consumption will be around 0.22mA (background radiation), as the battery voltage goes down the current will increase up to around 0.67mA at 7V input. A small 9V battery will last many weeks on background radiation, for extended battery life the speaker can be disconnected or a series resistor can be fitted to reduce its volume . Very high click rates with the speaker on will increase the current to over 2.5mA.
Above is the schematic for the CMOS total dose counter.
Above is the total dose counters LTspice test.
The I(R_meter) image shows the linearity of the R-2R DAC used to drive the moving coil meter. Six bits were chosen as the small meter steps can be seen with the naked eye. The image is from the SPICE simulation with the click generator set at 1000Hz and Q1 on IC 2 selected. If the Q1 tap is jumper selected it takes 65 clicks for the meter needle to move one step. Visually the result is quite satisfying.
Looking at spice simulation 4 seconds at 1000Hz is 4000 "clicks" for full scale deflection of the meter. BTW tubes normally have specifications including the number of clicks per uSv (micro sievert).
The image above shows the signal conditioning, integrating meter drive and speaker drive circuits. Note: R19 is 330k
The image above shows the low current regulated high voltage supply. The base emitter junction of Q3 is used as the voltage reference. The temperature of Q3 changes the preset output voltage slightly, it keeps within the geiger-muller tubes voltage specification (on the plateau). The current consumption is very low, only 2.2mV measured across R28 at back ground radiation. I=V/R so 0.0022/22= 100uA is consumed by the HV power supply. With a radioactive watch placed next to the GM tube 3mV/R28 is measured so 136uA. The components R28, C7 and R33 are selected to suit the transistor type used. The MPSa42 is a 300V device so the maximum output voltage is limited by this. Note the high voltage output is twice the voltage of the positive going pulse on the collector on Q5 so high voltages of up to 500 volts should be fine, most of my tubes run at <430V.
Above is an image of the prototype CMOS geiger counter after running 24 hours. The meter is connected to the Total Dose output (DAC out). Jumper position 6 was selected, if the background radiation stays the same in 5 days the meter will reach 100 and the over range LED (not fitted) will light and the meter will start at zero again. If the jumper was set to position 7 the meter would have read 10 not 20. I intent to leave this running until the battery goes flat. I will record the meter reading every couple days. The over range LED would be soldered onto the two wires exiting on the left of the image near the speaker switch. I need to find a LED that only requires micro amps to light or maybe a push to test button.
This page created 26 May 2025, updated 01/06/25, updated 08/06/25,
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