In this circuit, we are just turning ON/OFF an LED using BS170 n-channel MOSFET. Here, the Gate and Drain terminal are connected through a 5v DC source and LED is connected to the Source. As the gate pulse applied by turning ON the switch, MOSFET conduct current through Drain to Source and LED start glowing. LED remain ON until the gate pulse applied. As the gate pulse removed LED turns OFF.
https://components101.com/mosfets/bs170-n-channel-mosfet
Datenblatt BC 172