U1, designated as an LM324N, is a quad operational amplifier (op-amp) integrated circuit manufactured by Texas Instruments. In your schematic, it's utilized in multiple parts of your circuit due to its four individual op-amp units. Each op-amp section within the LM324N can be used independently for various functions such as amplification, filtering, or other analog signal processing tasks. Here's a brief overview of each section's role based on your project's connections:

The LM324N's versatility allows for it to be employed in various configurations tailored to the needs of your project. With its ability to operate from a single power supply and directly interface with other components like sensors, resistors, and potentiometers, it's an excellent choice for a wide range of analog signal processing applications in your Cheap Amber P.K.E. Meter project.