Re: Timing accuracy. I haven't any experience myself of timing critical projects, but from what I have read, this might be borderline if you need +/- 10mS accuracy, assuming a Raspi running Linux and without any additional circuitry. Hopefully somebody with more experience will chime in on this. If it turns out it is not accurate enough, helper circuitry in the form of a PIC or AVR chip should do the trick.
Re: Real time clock. The Raspi does
have a RTC; what it lacks is battery backed
RTC. This means that, if the clock is not set after each boot, either manually or with the help of an NTC server, the date and time will be incorrect. It is still perfectly capable of measuring elapsed time; it just won't be able to tell you if this period occurred in the Jurassic or today.
Re: Sensors. I would be disinclined to use distance sensors for this, not only because of the limited sensing distance, but also because of the diffuse sensing zone; it would be difficult to define a sharp and repeatable finish line. A visible-wavelength laser has the advantage of being
a line. I agree that the standard LDR (light dependent resistor) photocell has too slow response time, but you can also get semiconductor light sensors that are much faster. There is one such at the bottom of this page, http://www.futurlec.com/Photocells.shtml
, but other might be better suited for this application; this is just an example I remembered coming across before.
Re: Distance. 100m should be quite doable via twisted pair cable, such as telephone or network cable. The problems with long wire runs are, on the one hand signal attenuation due to wire resistance, and on the other hand susceptibility to interference. By using moderate current (in the range of 2 to 20 mA) over twisted pair wire, there should be no problem. Radio might make for easier setup, but the delay inherent in WiFi communications might be too high. Simpler radio modules like some of these, http://www.futurlec.com/Radio.shtml
, should have lower propagation delays. (And no, I don't have any connection with Futurlec except as a satisfied customer; they're just my first port of call when looking for electronic components