The IR signal is processed by a voltage comparator. The photo-transistor's collector connects to +12. A 10k resistor connects to the emitter. The voltage at the emitter switches from 0v to 12v.
The comparator processes this signal. The comparator's output activates the base of an NPN transistor. As the comparator changes from open to closed, the voltage at the NPN's collector will change from 0 to 12v. This is the output signal.