DRSB-01 Geiger Counter is an inexpensive Russian-made Geiger counter, obtainable from eg. eBay, for prices often as low as 17 USD, with 9 USD air mail postage. It is a fairly sensitive detector of gamma and high-energy beta radiation.
It is powered by two AA batteries; it is fairly insensitive to drop in the battery voltage, as the lower-voltage NiMH cells will work reliably. The battery life is claimed to be in excess of 200 hours with normal radiation background. The device is built with typical Russian idiotenzicher and soldatenfest design.
It indicates photons and beta particles with an audible click (somewhat quiet, especially if the batteries are almost run-down), a green LED blinking in more or less synchronous way with the audio indication, and a red LED indicating attention-requiring level of radiation. There is no measurement of radiation nor integration of the dose, however that can be easily added using a cheap microcontroller.
The counter is fairly sensitive; under normal Central-European conditions it gives about 10-40 counts per minute. Some sources claim sensitivity down to 0.003 microsievert.
The weak point of the device is the step-up transformer for the Geiger tube. It is improperly sealed, and acts as a speaker, emitting a quiet high-pitched whistle. Some people do not hear it at all, few - especially when younger - do and consider it highly annoying. However it may be considered an indicator of the unit's operation.
The quality of the soldering leaves a bit to be desired. But it is manufactured according to the well-developed principles of Russian engineering, and its robustness makes up for that. Besides, it is cheap.
The Geiger tube used inside is a fairly large kind, providing high volume of ionizable space for the particles to strike, contributing to the device's sensitivity. The chips inside are generic CMOS logic, the K561IE14 being an equivalent of CD4029 (a presettable counter, likely to drive the green LED) and the K561LE5 being an equivalent of a CD4001, a quad 2-input NOR gate. The odd boxy-shaped three-legged parts are some Russian transistors. The simplicity of the construction makes the device relatively easy to repair under field conditions. The weak point of the device is the power switch, realized as a strip of metal soldered to the board, pressed against by a slider in the case. But if it becomes a problem, its replacement by a better kind is a non-issue.
Seal the transformer. May prove difficult, as the ferrite core is glued together really well. Maybe use a highly diluted beeswax, and let it seep into the windings?
Add an integrator unit with ATmega88, to measure radiation levels.
Add an audio jack connector to attach headphones.