Whilst doing this with discrete components is laudable I'd recommend you purchase a cheap'n'cheerful PIR sensor. These commonly use the BISS1000 chip and include all the parts you need at a price far below that of obtaining them individually. ...
After reviewing the OP's requirements for a
proximity sensor, I would not recommend a PIR sensor for such an application. PIR sensors rely on the movement of objects with contrasting infrared emission, compared to the background infrared emission, and have zero range discrimination. PIR sensors will detect the movement of objects hotter or colder than the background infrared radiation at
any distance within their field of view when the size and temperature of the objects provide enough contrast with the background radiation to present a usable signal.
An active infrared emitter (IR LED) and a co-located infrared-responsive photo-diode, both devices pointing upward, appear to be the simplest way to create a proximity detector, responsive to a finger placed over (but not covering) the infrared emitter. A finger placed above the infrared emitter will reflect
some of the infrared emitter radiation back toward the co-located photo-diode, which will then produce a signal that can be used to indicate proximity has occurred.
Notice there are potential problems with this simple scheme. First, there is no way to distinguish infrared emission by the IR LED from other nearby infrared emitters. This can lead to "false positives" where these other infrared emitters indicate proximity is present when nothing desirable to detect is there. Second, sensitivity to variations in proximity detection range may cause unreliable operation or require frequent "tweaking" to establish operation. Still, this is an interesting project for beginners to try out. Infrared (or near infrared, which is what IR LEDs produce) radiation is a fascinating subject to explore, and the results (compared to visible radiation) are not always intuitive or even similar.
One significant improvement that can be made in an infrared emitter and infrared photo-diode proximity detector is modulation of the infrared emitter with a square wave frequency of a few kilohertz. The signal from the photo-diode is then synchronously detected using the modulation frequency as a reference. The improvement in signal-to-noise ratio and detection sensitivity is enormous and usually well worth the extra effort.
