Simple Proximity or Distance sensor:
The basic design of Proximity or a distance sensor is easy to make. Basic circuit diagram is as follows.

To make it useful for different ranges use a variable resister in place of the resister across photo transistor. For better accuracy we use an Op-Amp as a buffer in the circuit. So by changing the variable resistance we can change the range of operation. This is shown in video.

Operation:

Connect LED for detecting output in the circuit. The light from Led in primary loop falls on an obstacle and reflects to light dependent resister( LDR ) or Photo transister (depending on what we use) making them conduct.So as the variable resistence changes the current flowing changes through them changes and the photo transistor becomes more sensitive to light varying the range of operation. Care should be taken about the light that directly falls on photo transistor. This should be avoided by keeping a baffle across led and photo transistor.

Basic design is demonstrated, this can be modified depending on the purpose.

Enjoy.. :D