Ok. So, obstacle detector. On first glance it is pretty easy to create - you have LED and photodiode, by measuring photodiode current you can determine brightness of light ahead of it. The only problem is, in real life is doesn't work that well. Actually, it just doesn't work. Lots of lights everywhere which create interferences and stuff like that. To avoid that we need some way to determine that we are receiving light which we are emitting and not just some kind of light. Basically, easiest way for that is to use pulsed signal for lighting. I.e. you measure light level with your LED turned on and off, and if those level are equal - its just an interference. But than, you need some kind of filters, amplifiers and stuff like that. To messy. At that moment we are starting that there are things that use pulsed IR light... remote controls for TV as example. And for that they use pretty specific part.
As example - TSOP17xx by Vishay Semiconductors. Two last digits tell you carrier frequency. Basically, it has high output when there is no signal, or low output when it senses signal on predefined frequency. (see figure 7 in datasheet). And best thing is that it is really protected from interferences of any kind.
So, all you need is mcu(any will work actually). Schematics could be found here (http://roboforum.ru/mediawiki/images/8/80/Ir_locator30.gif
). These were created by Russian RoboClub long time ago... but they work just perfectly.