A circuit for a motion sensor receiver

Hey guys, im making a motion sensor which ignores ambient light sources and I have found a circuit which can receive the infrared beam which is transmitted at a certain frequency from a lm555 timer. But I would like to have someone to tell me in details how the receiver part works.

The circuit is down below

Thanks.

The first amp is tuned to have its best gain at a certain frequency, then it acts as a precision rectifier with D1, putting some negative volts into the capacitor. The second amp has a zero to negative 12 volt adjustment which is called a threshold. Anything less negative than the threshold voltage will not pass through the second amp. Anything more negative than the threshold voltage will be inverted and amplified to nearly 12 volts positive.

My teacher gave me some hints as to how I can tackle the 12 V, -12V problem, I have the circuit down below but I dont know how on earth you are supposed to get -12V out of it.

anyone knows the answer?