Inverter circuit with NPN transistors. When switch is closed, QI, dumps to ground through the BLUE LED and is acting as a non-inverting buffer. When the switch is opened, Q1 does not conduct and the inverter output turns on the RED LED controlled by Q2 which is acting as a switch. The BLUE LED is simply to show current flow when the inverter has a logic zero output. The true inverter output is the input to the base of Q2.
