Remotely activated stopwatch

[kahrez]

Hi everyone

I am looking to build a device which will start and stop a stopwatch when it is passed. The situation is on a bike track, to automatically start a watch attached to the bike as you begin the track and stop it as you exit, thus giving a consistant lap time.
The device must not require physical contact and must be able to work at a distance of 1-2m (the more the better). The track in question is not a loop so would need two separate triggers at the top and bottom. Response time is not too much of a concern, quick is good but so long as its consistant its ok. Compact and lightweight is important too.
At the moment i have a button mounted on the bar to trigger the stopwatch which works but requires attention to push and obviously you'll hit it at a different time each lap.

I imagine some kind of radio activated relay would do the trick but i've no experience with radio triggering and wouldn't know where to start.

Thanks for any help!

 

[Boncuk]

Remotely controlled stopwatch

You could use an IR-barrier. I guess there will be no pedestrians in the way of start and finish. Otherwise this area has to be restricted to public.

Make a transmitter of an NE555 circuit working at 38KHz and duty cycle ratio of 30 to 40%. That way you can increase the max. current of the IR-TX diode which results in better range. Focussing the IR-beam the range will be 10m without problems.

Using an IR diode TSAL6100 it can stand a continuous forward current of 100mA (pulsed you might apply 120mA to be safe) and emits a narrow beam of +/- 5 degrees.

For a receiver select a TSOP1738 which works best at 38KHz. As long as the IR-beam hits it, the output of the receiver is logical high (+VC). If the beam is interrupted it will jump to logical low (0V) which can be used to trigger the watch.

Mounting the barrier at average chest height of the drivers there will be no inaccuracy in time measurement.

Boncuk

 

[umagrama]

Infrared would do the job -- maybe you could find a motion sensor where you can adjust the range of the sensor. That would simplify your project.

A mechanical system would work too -- have a line across start/end of the track where the bike tire would push it down and push a trigger.

That's what i got thinking about it for a couple minutes. Good luck.

 

[kahrez]

Pedestrians are not an issue. The start and finish are several kilometres apart so cannot be connected - this is why i want to trigger a watch mounted on the bike as opposed to triggering a unit by the side of the track.

Thanks for your help!

 

[Boncuk]

Pedestrians are not an issue. The start and finish are several kilometres apart so cannot be connected - this is why i want to trigger a watch mounted on the bike as opposed to triggering a unit by the side of the track.

Thanks for your help!

Pedestrians will become an issue if they pass the IR-barrier simultaneously in any case. (The circuit won't trigger)

Mounting the watch on the bike the logic I described doesn't apply exactly any more. The receiver must be mounted on the bike and in that case it will cause a positive going output it's hit by the IR-beam.

Boncuk

 

[gauravraina]

Thankyou !

Thankyou Boncuk !
i had the same problem, will try your suggestion