PROTO is a free and open-source circuit simulator app for Android devices. It allows users to design and simulate electronic circuits directly on their phones or tablets. You can build circuits using a variety of components, observe their behavior, and analyze their performance. It[SQ]s a useful tool for learning electronics, prototyping designs, and educational purposes. Think of it as a simplified version of desktop circuit simulators like LTSpice or Multisim, adapted for mobile use.