This is a standard TTL NOT gate with an open collector output found in the 7405 IC. The parts of the circuit are labeled with resistors. Abbreviations:
PB: Push Button
TTL: Transistor Transistor Logic (BJT technology)
OC: Open Collector
PU: Pullup Resistor
To explain how this works I will look at the two states:
1. The input is low (button not pressed). The first transistor is activated, which prevents the following transistor from receiving enough base current to turn on. The second transistor is therefore off, and the third transistor (the output transistor) is also off. The voltage on the emitter of the second transistor is about 0.195 V, which is not enough to turn on the third transistor, so the current instead goes through the 1k resistor. Since the output transistor is off, the output is not pulled low.
2. The input is high (button pressed). In this case, the first transistor changes state, allowing current to flow into the base of the second transistor. The second transistor turns on and provides base current to the third transistor. The third transistor then turns on and pulls the output low. Because this is an open-collector output, the IC does not drive the output high itself; the pull-up resistor provides the high output when the third transistor is off.
