It’s a project called snapVCC, and it turns a 9 volt battery ... 3.3 to 5 volts. An LED indicates the output active, and another switch disconnects the battery from the circuit.

Building a portable lamp with a 9V battery and LED bulb is a simple, creative DIY project that anyone can try. This video provides clear, step-by-step instructions for assembling a functional lamp ...

The transistor then switches on a second BC547, which lights an LED. The whole circuit is built on top of a battery clip so it can be run straight from the top of a standard 9 volt battery.