When input is on, the output is off, and vice versa.
Also know as a NOT gate
Can be used to connect two different logic levels.
Useful if only one gate needed.
This is a useful little circuit if you need to invert a signal. For example if the output from a CONTROL goes high and you want to trigger a monostable and need a low signal, this will invert the signal.
Any general purpose low power NPN transistor should work in this circuit.