electronic stopwatch "speedometer"

[Smitty Two]

I'd like to make a relatively inexpensive stand-alone gizmo to measure
the speed of a steel ball on a track as an accessory for a kid science
experiment. So I'm thinking that an emitter-detector straddles the
track, and the ball interrupts the beam.

The associated circuit sends a start pulse to an electronic timer when
the detector goes dark, and a stop pulse to the timer when the detector
sees light again. The little rug rats can then calculate the velocity of
the ball based on how long the pulse was, knowing the ball's diameter.

But, I don't know what to look for in terms of a little digital panel
meter that hopefully has some sort of timer function built-in. Any leads
would be greatly appreciated. Either the source of such a device, or at
least a buzzword to google for.

 

[Smitty Two]

Open up a real stopwatch and tie into the switches. That actually
works pretty well.

And maybe use foil contacts in the track, instead of photoelectrics.

John

All right then. Cruising eBay I see a lot of cheap stopwatches, but most
of them are 1/100 second. I'll likely need 1/1000, but I found a couple
of those in the $25 range which isn't a deal breaker. Thanks for the
suggestion!

 

[Rich Grise]

Here is a link to a DIY kit that provides the stopwatch function and a
LED panel that displays the elapsed time. It should be possible to
interface with it to start and stop the clock via an external circuit
such as the detector you describe.

http://www.hobbyengineering.com/H1736.html

With something like that, it might be a good idea to get the more advanced
kids (what age range?) involved in building the kit, and trying to
understand the circuit.

Cheers!
Rich

 

[Rich Grise]

On Tue, 20 Oct 2009 17:22:34 -0700, Smitty Two


Open up a real stopwatch and tie into the switches. That actually
works pretty well.

And maybe use foil contacts in the track, instead of photoelectrics.

http://www.pbresource.com/rollobut.htm

Cheers!
Rich

 

[Rich Grise]

All right then. Cruising eBay I see a lot of cheap stopwatches, but most
of them are 1/100 second. I'll likely need 1/1000, but I found a couple
of those in the $25 range which isn't a deal breaker. Thanks for the
suggestion!

Or use a digital oscilloscope and get the kids interested in that,
too.

When I was a kid I measured the velocity of a .22 rifle, breaking two
wires and using an analog scope. It's tricky to aim the rifle while
looking at the scope. I used a stack of old Tom Swift books to catch
the bullet.
[/QUOTE]

"There's more than one way to skin a cat," said Tom categorically.

;-)
Rich

 

[Smitty Two]

A ball that drops 1m in .45s due to gravity will have a
velocity of about 4.4 m/s and if its diameter is 1cm it
will eclipse the sensor only for 2.3ms which would give
you only 2 counts on a 1-ms-resolution stopwatch. What
sort of accuracy and resolution are you looking for? It
looks like you will need a high count rate or two sensors
perhaps 10 or 20 cm apart.

Agreed. I'm not sure what the velocity of the ball is. In the range of
1m/s I would guess, and the diameter is 1/2", so it's true that a
millisecond timer isn't going to give me as much resolution as I'd like,
but hopefully it's close enough. The dual-sensor approach is good in
that regard but of course it adds to the parts count.

 

[Smitty Two]

Or use a digital oscilloscope and get the kids interested in that,
too.

Well, this thing* started out as a fun way to pass an hour in the
evening, and then the kids took it to their sixth grade science class,
where it was a hit. So I'm going to donate it to the school for future
classes. The speedometer is just an enhancement, but I don't want to
spring for an o'scope.


* http://scitoys.com/scitoys/scitoys/magnets/gauss.html

 

[Smitty Two]

Here is a link to a DIY kit that provides the stopwatch function and a
LED panel that displays the elapsed time. It should be possible to
interface with it to start and stop the clock via an external circuit
such as the detector you describe.

http://www.hobbyengineering.com/H1736.html

Thanks Doug. Good thing someone else replied to you or I wouldn't have
seen your post, since I block googlers by default. But anyway, I think I
had found that thing in my searching, and ruled it out as too expensive.
Looks like it'd be $25 for the main kit and another $15 for the
stopwatch module. Now that I look into it further I see it only has
resolution of 0.01 second, also.

 

[Smitty Two]

Why not two hall effect sensors, or magnetize the steel ball and use reed
switches say, six inches apart under the track. Start a fast counter with
the first switch/sensor and stop it with the second one. You could
"calibrate" a digital counter by using a proper selection of clock
frequency.

Jim

Thanks, Jim. Not sure about the hall effect sensors, I've never used
them and my understanding of them is weak. Can't magnetize the balls,
it'd be counterproductive for this application (magnetically driven
linear accelerator)