light gate stopwatch

[BANE]

Hello all,
I have a friend who is trying to measure the time it takes for a little wooden car to go down a ramp.  My idea was (trying to keep it simple since he does't have a microcontroller or a big budget)  getting two light gates (one at the top of the ramp and the other at the bottom) and hook it up to the trigger on a standard stop watch.  When the first gate is breached the timer starts and the second stops it. 
This is just a rough idea, does any one see any problems or an easier way?

Thanks in advance,
Bane 

[corrado33]

Pinewood derby?

I loved pinewood derby.  The boxes/wedges always won.  hahahah It's all about friction.  On another site I frequent (car site) we had a whole thread on how to... uh.... beat the system.  

The thing is it's all down to luck.  If you accidentally put your wheels SLIGHTLY "wrong" i.e. not flat, you probably had a better chance at winning than someone who's entire wheel was touching the ground.  Also, a LARGE wedge cut out in the front of the car made it sit further up on the starting peg.  You were already ahead before the race even started!  Ok I'll stop telling you how to cheat.  

So on to the topic at hand.  Are you trying to specifically time it, or just see who wins?  I've never heard of a "light gate/bridge" but if was able to act as a switch I don't see why it wouldn't work?

Oh here's a link to do something similar to what you're doing.
http://www.planet-scicast.com/experiment.cfm?cit_id=2771

[Billy]

I have a friend who is trying to measure the time it takes for a little wooden car to go down a ramp...... does any one see any problems or an easier way?

If all he wants to do is measure how long it takes, and not to build a machine that takes measurements, use a video camera to record the car go down the ramp. Put a clock with decimal point in the shot and watch the clock in the video in slow motion...or even easier, use a stop watch 10 times while playing back the video and take average of the 10 records.

If he wants to build a machine to keep track of time, then your through beam idea will work although it would be easier to have the car release a mechanical switch as it leaves the start and hit a mechanical switch at the bottom. A through beam sensor will be harder than you think.

[Soeren]

Hi,

[...] getting two light gates (one at the top of the ramp and the other at the bottom) and hook it up to the trigger on a standard stop watch.  When the first gate is breached the timer starts and the second stops it. 
This is just a rough idea, does any one see any problems or an easier way?

The easiest way I can think of (and have actually done a number of times) is to use a cheap LASER pointer and two small pieces of eg. 1mm polycarbonate (or similar clear plastic that's somewhat scratch resistant) for beam splitters for the light to breach and two photo transistors, two regular BjT's, a few resistors and caps to interface the stop watch. You may even get away with using LDRs instead of photo transistors, if you can find two that exhibits identical delays (it's OK that they're slow here, as long as they are equally slow, as that will give the same net result as with photo transistors).

The timing is be started on the Start/Stop button, but for the "stop", either the same button can be used or the "Lap time" button.
Dissecting  a stop watch from a dime store, you'll usually find very primitive contacts, with the button pressing a bit of nickel flashed steel towards another similar bit. With care, it's fairly easy so solder wires to the contacts. If you wire up all 3 buttons (usually Start/Stop, Lap and Mode), you can mount the watch in a box with switches to make it easy to zero the count etc. and if triggered by a transistor (that's itself driven through a cap with a pull up resistor on the basis), manual buttons won't damage the electronic interface.

If/when you get your paws on the watch, do take some sharp photos of it, both initially and when opened - with focus on the contacts - and post the best, if you need help identifying where to solder or something similar.


I have modded a number of very cheap stop watches for similar purposes and as long as you can solder wires to the contacts without burning the plastic they're seated in (a pair of pliers can help keeping the temperature of the metal down so it doesn't melt its seating), the rest is easy.

[Soeren]

Hi,

After I wrote the above (and shut down my PC), it struck me, that if you are contend with using just the Start/Stop switch, it can be simplified a bit.

A single receiver will do, and 2 or 3 small mirrors (depending on mounting method), plus the LASER pointer of course and you need to mod it for supply from beefier cells.
The +3V mentioned could be 4.5V, if that's what your LASER pointer takes, to simplify the supply.

Here's the receiver/Stop Watch interface:

This way you don't need to match the LDRs, as there's just the same one, so it will exhibit the same response time at first and second break.

If the LASER and mirrors are mounted in square aluminum tubes/profiles, it should be very easy to line the two sides up - or you could make a large square tapering U-shape to put the track into.