Geiger Counter

2022

The first version of my Geiger counter board was USB powered (no battery) which was a bit annoying, so I added an 18650 battery to the second revision. I've designed a several lithium-ion powered boards before that use buck-boost converters to power their 3.3V microcontrollers, but here I was lazy and powered the 3.3V ESP32 with a low dropout linear regulator connected to the battery. I set an undervoltage protection limit of 3.2V (the ESP32 will get squirrely as you undervolt it) which results in something like 70% of the capacity of the lithium-ion battery being useable. My laziness was driven by the integrated buck-boost part that I had used in the past being out of stock and subsequently not feeling like finding a new part and adding it to my library.

Finished Geiger Counter

In addition to adding a battery, I also added a battery charger and an overvoltage protection circuit for the Geiger tube high voltage supply.

Hardware

Most of the circuits on this board are directly copied from the previous version. I'll cover the changes.

Battery Charge and Protection

Lithium ion battery require some additional protection for reliable and safe operation. You need to ensure that the voltage, current, and temperature limits of the battery are not exceeded. For current limits, I added a PCB fuse since the only situation where discharge current 100mA would be expected is short circuit, and the linear battery charger IC I selected sets the charge current limit. I ignored temperature limits which is not uncommon for low power products with slow charging circuits like this. Even if the board is left in the sun while charging, nothing should get hot enough to actually be dangerous. For voltage protection, the linear battery charge IC sets the maximum voltage and a comparator based under voltage protection circuit protects against overdischarge.

Battery Charge and Protection Circuits

The previous board had no use for a power switch since it was USB powered, so I had to add one here. The XYZ series of switches are pricy (like most electromechanical hardware) but are great quality and are available in many different configurations (mounting options, poles, momentary).

Battery Charge IC

MIC842LYC5-TR is a battery charger IC well suited for charging lithium ion batteries off of a 5V source. It provides a charging indicator light and has a resistor programmable charge current limit. In this case, I set the charge current to 100mA????? so that the IC didn't get too warm and so that I wouldn't have trouble charging off of typical USB ports. While the USB spec specifies that standard USB 2.0 peripherals can only draw 100mA without further negotiation, no OEM worth their salt will design a USB port not capable of supporting 500mA these days due to the proliferation of non-compliant USB accessories. The MIC842LYC5-TR in MIC842LYC5-TR sets the charge voltage to 4.2V. A number of other options are available.

Undervoltage Protection Comparator

An open drain comparator with an internal reference controls a P-channel MOSFET acting as a high side switch to enforce an undervoltage limit on the battery. The internal reference is nice here because it both eliminates an external reference part and also ensures low quiescent current: cheap shunt voltage references require significant bias current and low quiescent current series voltage references are more expensive. I set the undervoltage limit to about 3.2V (the schematic des not reflect what I ultimately came up with on the bench).

Boost Converter Overvoltage Protection

The output voltage of the high voltage supply for the Geiger tube is nominally set by the peak current of the inductor triggering reset of the 555 timer by the voltage across a current sense shunt exceeding the base to emitter threshold of an NPN BJT. As simulations for the last board showed, this results in a highly temperature dependent output voltage. This alone is OK, but because the output voltage needs to be adjusted with a potentiometer, the potential exists to set the output voltage too high (in excess of the rating sof the diode and BJT). In practice, it seems that non-destructive breakdown mechanisms in these parts limit the output voltage, but this is achieved by operating the parts past their absolute maximum ratings which is never good to bank on. To add some protection against this, I added a second reset NPN with its based connected to a voltage divider from the output. The voltage divider adds a load to the output, but this actually stabilizes the output voltage somewhat since with no-load the output voltage is defined by the switching losses and diode leakage (which will be temperature dependent) only. I set this limit to 500V to provide plenty of headroom above the nominal 400V output.

Software

Both this version of the board and this version of the board enumerate as USB virtual serial ports, but now that this board can operate without a USB cable connected, I added support in software for bluetooth. It's crappy copy pasta from examples, and it doesn't work with iPhones (they have neutered support for Bluetooth if you don't pay them) but it works reasonably well for Android. This lets you see real numbers on your phone instead of just listening to clicks.

This Arduino sketch can be found on Github here.

Radiation at Altitude

Dose from cosmic rays is a function of altitude. On the ground, many cosmic rays and secondary particles are shielded by the atmosphere. As altitude increases (to a point), flux increases. Because most dose from cosmic rays is from secondary neutrons produced in a hadronic cascade in which a single sufficiently high energy cosmic ray will cause a shower of secondary neutrons, the altitude w absorption and generation of these secondaries are in equilibrium is the altitude of peak flux. Above this altitude, flux decreases until you hit the lower Van Allen belt.

Background Radiation on a Commercial Flight

As a result, radiation on a commercial flight will typically be 10-100x higher than radiation on the ground. The plot here was recorded by one of my Geiger Counters on a flight between St. Louis, MO and Seattle, WA.