Raspberry Pi is a versatile low cost single board computer that plugs into your TV and a keyboard. It can be used for many of the things that your desktop PC does, like spreadsheets, word-processing and games. It also plays high-definition video. Besides that it has many GPIOs to to receive signals and outputs to control or activate other devices or peripherals.
There are several great features in the Raspberry Pi board which includes USB, ethernet, audio & video outputs, storage card (SD, MMC & SDIO) plus many GPIOs. Since the Raspberry Pi board only has these basic features, you may not able to use it as a standalone board if you intend to drive relays, motors or LEDs for example. When it comes to capturing inputs from switches and analogue/digital sensors, some form of signal conditioning may be necessary before the devices can be connected to the Raspberry Pi board.
Having said that, to further extend its functional capability, a secondary expansion board for Raspberry Pi to interface with other peripherals will be required. To fulfil this requirement, you will need a custom made PCB interface board with the necessary interface circuitry to tailor to your application. Jaavin Electronic Solution has the capablity to design according to your requirements. Picture above shows one such example.
This custom made expansion board has a relay driver to switch a 3A load, 12V lamp indicator outputs, sensor switch inputs and 1W amplified audio out.