Solution ideas for interfacing automotive sensors to PC for digital gauge cluster

I'm interested in a project to help me get into programming and I think this is a great way to stay interested since it deals with something I'm already passionate with. What I'm looking to do is interface all of my automotive sensors (Speed, RPM, voltage, oil pressure, coolant temp, etc) with a small Linux based micro PC (such as Raspberry Pi | An ARM GNU/Linux box for $25. Take a byte!). I know I'll need a RS232 (or USB->232) compatible device to receive the signals but there are so many interface boards I really don't know where to start with picking the correct one. All help appreciated!

If you are willing to learn enormous amount of new things, you can develop your own board for that purpose instead of using some off-the-shelf solution. General way to do this is communication interface + logic + analog front-end.

Simplest communication interface would be RS232 using MAX232 chip. The problem with RS232 is that modern devices rarely contain such port and you will need a converter to USB. Otherwise you can use some flavor of FT232 from FTDI to get USB<->UART converter. Using MCU with USB module is an option, but software side of this is kind of complex.

MCU for such thing can be pretty much any MCU with some processing power and enough I/O pins. You may want something with large memory, because converting sensor output to understandable form (i.e. converting voltage from pressure sensor into PSI value) often requires lookup tables which take lots of memory.

As for front-end this depends on what type of output do your sensors give. If it's digital (SPI, I2C) then it is quite straightforward. You probably need to devise some kind of protection for MCU (some logic buffer, voltage clamping, etc) but other than that it's more of a software thing. In case of CAN bus some input circuitry is required but that can be fund in specifications and application notes. special kind of sensors are RPM sensors, which give impulse signal and you need to measure frequency (which can be done using MCU input capture features). More evil (and more widely represented) are sensors with analog output which is voltage in most cases but can also be resistance or current. In such case you need analog front-end consisting of signal conditioning part and adc. Signal conditioning is most important because it converts whatever the sensor outputs to voltage range convertable by ADC with best precision possible. You can consider using integrated solutions like LMP90xxx family from NSc/TI. They contain analog multiplexer, programmable gain amplifier and a 24-bit sigma-delta ADC. In most cases this takes least space on the board and has superior signal quality compared to probably any circuit you can design and assemble at home without some time-intensive research and many hardware revisions. Most problems you will face when interfacing thermocouples (best example would be Exhaust Gas Tempareture (EGT) sensor) because thermocouples give voltage variation in order of tens of microvolts per degree celsius.