While it may not give you the same kind of audio quality as a several-hundred-dollar mic, this Arduino Pro Mini “spy bug” setup can give you up to 9 hours of battery-powered recording time in a very compact package. Hacker Great Scott! lays out how the circuit is made in the video below. He also notes that the device’s abilities are based on the ATmega328P’s analog to digital conversion capabilities which allow it to record a sufficient number of data points in order to record human speech.
To simplify the project, Great Scott! uses an electret microphone on a pre-built amplifier module as an input to the Pro Mini. This amp module conveniently features automatic gain control so that it can accommodate for both soft and loud noises. The setup includes a micro SD card to store audio, and optimize battery life, the voltage regulator, as well as the two LEDs on the Arduino board are stripped off.
The result is a nice compact setup, which you’re advised not to use for unethical purposes. Given its lengthy recording abilities and compact size, it could be turned into a convenient personal note-taking device, perhaps even with its own custom case.