electronic stopwatch "speedometer"

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

In high school we just used a real stopwatch, by hand. That also taught us about measurement uncertainty, consistency, getting the stop and start delays to cancel, detecting bad runs, etc. And then we compared our measured 'g' with what was expected.

The measurement aspect was almost more educational than the experiment. More useful too, even all these years later.

-- Cheers, James Arthur

We did a high-school physics experiment where a falling weight pulled a paper strip through an electric doorbell, between the hammer and the bell. One kid waved a piece of carbon paper in the gap, so we got black dots on the paper tape, more or less periodically. We then measured the spacing and concluded stuff.

John

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.

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!

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

Cheers! Rich

Pasco might already make something like that.

Or are you wanting to build it yourself?

George H.

...

...

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.

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.

John

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

;-) Rich

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.

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.

• formatting link

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.

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)