timing

smanderson

As a final engineering project, we are making a track that will time and calculate the velocity of mousetrap race cars. The track has a series of lasers hitting photoresistors, which are tied into a BOE board. Every time a laser is blocked by a car, we should be able to measure a voltage change (and we can). Since the velocity measurements will be feet/second, we need a way to accurately measure the time between voltage changes in seconds (or some other fairly absolute measure of time). Since we have to mark the time exactly when the voltage changes, pause and count do not seem to be viable alternatives. Does anyone have an alternate way of measuring the time between two events in absolute terms using the BOE? (Oh, and using count and dividing the total time by the number of changes will not work, since we want the max velocity, not an average). Thanks in advance for any help.

Martin Hebel

My students have done the same to measure velocity of a payload leaving a barrel (see my signature image). As long as the transit time is less then 130mSec, you can use the PULSIN instruction to measure down to 2uS. One issue that comes into play is that the instruction only wait 130mSec before moving on, making a decent probablility it may miss the pulse from the sensors. A 1st pair would be an option to trigger the code to measure the pulse of the 2nd pair.

On another note, my brother adjusts camera (35mm SLR's) and needed a device to measure shutter speed. Using a IR Transistor, a pointer laser, and an LCD I developed him a nice little device that can measure very accurately, though it does 'miss' now and then due to the timeout and looping involved.

Good luck!

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Martin Hebel
Southern Illinois University Carbondale - Electronic Systems Technologies

Personal Links with plenty of BASIC Stamp info
StampPlot - Graphical Data Acquisition and Control

Martin Hebel

Oh, photo resistors have a failry SLOW response time. Hope they work out for you.
-MH

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Martin Hebel
Southern Illinois University Carbondale - Electronic Systems Technologies

Personal Links with plenty of BASIC Stamp info
StampPlot - Graphical Data Acquisition and Control

smanderson

Thanks for the response, but these cars are moving fairly slowly (<5 ft./s), and the sensors are spaced 1-1.5 ft. apart.. Thus, 130 is probably too fast, but the photoresistors are fast enough. Does anyone have suggestions for timing slower events (though timing cannon shots sounds cool)?

Martin Hebel

Another possibility would be to use an AND gate to trigger an external 100Hz (?) oscillator with the sensor and use the COUNT instruction to measure the number of pulses.

-Martin

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Martin Hebel
Southern Illinois University Carbondale - Electronic Systems Technologies

Personal Links with plenty of BASIC Stamp info
StampPlot - Graphical Data Acquisition and Control