[linux-audio-dev] Setting the Beat of a Clock

Fred Gleason fredg at salemradiolabs.com
Wed Sep 28 16:34:45 UTC 2005

On Tuesday 27 September 2005 18:18, James Courtier-Dutton wrote:
> I am thinking that it might be better to use an optical instrument to
> measure the timing.
> Just place the optical sensor in the center (or a pair of sensors if you
> also need to know direction) and measure the interruption of the beam.
> This could potentially measure the position of the pendulum accurately.

I wrote a small Linux application that did something similar to this a few 
years back.  It was for a race car track used in the local Cub Scout Pack's 
"Pinewood Derby" program.  These are small wooden cars (about 20 cm long) 
that are rolled down an inclined wooden track.  Gravity is the sole motive 
force.  A switch at the top of the incline detected when the starting gate 
was lowered, while an infrared LED/detector pair over/under each lane 
detected when a car passed over the finish line.  Input into the computer was 
done by means of the parallel port, and the various line states polled by the 
program.  The hardware was very simple -- a few resistors and transistors in 
addition to the LEDs, detectors and switch.  I was able to measure intervals 
to a precision of around 0.001 sec with this setup. 

The bigger challenge here I suspect will be the issue that was initially 
brought up:  finding an accurate and portable timebase standard.  This was a 
problem I didn't have with my race track project, as all I cared about was 
the *relative* accuracy of the results.


| Frederick F. Gleason, Jr. | Director of Broadcast Software Development  |
|                           |             Salem Radio Labs                |
| Like the ski resort of girls looking for husbands and husbands looking  |
| for girls, the situation is not as symmetrical as it might seem.        |
|                                        -- Alan McKay                    |

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