The DRSB-01 is a Geiger Counter that gives audible (clicks) and visible indications (a green led and a red led) when elevated levels of radioactivity are detected: ionizing gamma rays and hard beta-rays.
You can use it to check the presence of ionizing radiation and get an idea on the dose (based on the frequency of clicks), but this device is not capable of doing any precise measurements.
A very simple example of a radiation dosimeter, can be used to learn the basic radiation detection principles: a Geiger-Muller tube, a small power source, an inverter, and a detector circuit, connected to the led and the speaker.

Scale range ambient dose rate equivalent:	0.003 … 0.6 μSv/h
Range of registered energy	1) gamma radiation, unknown 2) X-ray radiation, unknown 3) beta radiation, unknown 4) alpha radiation, not detected
Time of calculations	Instant
Time of indication	continuous
Power elements	2 AA batteries
Overall dimensions	150x65x21 mm
Weight (w/o batteries)	0.125 KG

It is bigger then my Radex 1706 or the Terra-P MKS 05, but smaller then the CDV 700 or the CDV 717.

The dosimeter is intended for use at temperatures from -10°C to 40 °C, relative humidity up to 80%, normal atmospheric pressure (650-800 mm Hg). Dampness will interfere with its high voltage generating circuit. Keep the radiation dosimeter clean and protect it from impact, dampness, and harsh environments.
One set of batteries will operate the radiation dosimeter continuously for at least 100 hours under natural background radiation conditions.
It came with two pages of instructions, you can see them here: DRSB instructions.

My uranium glass marbles, give higher-than-background clicks, and the green led lights frequently:

The CHECK Source DU on the CDV 700 casing, brings the RED led on:

As I said, this is a very simple Geiger Counter device. You can see the schematics here:

I plan to add a Microcontroller and an LCD to “enhance” this dosimeter. But more on that later!