The TimeCatcher is a 4-channel timing device with 100 second range and 0.1 millisecond resolution. I built it at the request of my brother-in-law for use in his high school science class. A typical use would be to have a series of optical interrupter switches (commonly called "photogates" in this context) along the path of a falling or rolling object, to demonstrate and measure accelleration due to gravity.
Here is the spec/instruction sheet that was supplied with the prototype unit, in Acrobat .PDF format. Get Acrobat Reader if you don't have it already.
I am considering commercial production of this device, as I'm pretty sure it could be built for less than the $245-$1200 price of existing photogate timers (which are generally much less capable than mine). I would appreciate hearing from educators (or anyone else who could use a timing device like this) concerning the features it should have. Here are some of the main design decisions:
- How many input channels are needed? None of the existing timers seem to support more than two. My brother-in-law specifically requested three, so that accelleration could be demonstrated with a single measurement. 20 channels (at the current 0.1 ms resolution) would present no problems other than the cost of all the photogates...
- What resolution is needed? If the number of channels is limited to about 5, I could measure in 0.1 microsecond increments without too much difficulty, although I probably wouldn't bother to go below 1 microsecond since even that is somewhat less than the response time of the photogates.
- What accuracy is needed? 100 parts per million (0.01%) is easy, I'm not sure why the prototype ended up with 5 times that much error. Better than 5 ppm is achievable at perhaps $20-30 extra cost.
- What level of student-proofing is needed on the external connections? The current design should be immune to any misconnection that doesn't involve an external power source. However, plugging the inputs into an AC outlet is going to completely fry the device - does it need to be able to withstand that level of user stupidity?
- Does the display need to be large enough to read from the back of a classroom? That would add quite a bit to the cost, a video output for use with a TV would probably be the cheapest way to accomplish this.
- Does the display need a backlight so that it is readable in dim lighting?
- What other capabilities should the device have? Much of the cost of building it is in the processor, display, power supply, case, etc. that are not specific to any function: additional functions could be added for far less cost than a separate device to perform them, so it may make sense to throw in features even if they are completely unrelated to the basic task of timing. In particular, the microcontroller chips that I'd probably choose for a mass-produced version have 5 to 12 analog inputs that might as well be used for something. For example, a multichannel voltmeter with 0-5 volt range could be added for little cost other than the input connectors.
Any questions or comments about the TimeCatcher can be directed to me at JasonHarper@pobox.com.