cheap photogate hacked stopwatch

[gromf]

I'm looking to create a cheap photogated timing circuit to measure the time it takes model cars to go down a short ramp.  It's based on an idea we saw at Legoland, where you can build different cars and test them down a ramp to see which design is fastest. 

Technical photogating equipment is very expensive.  For this application, something imprecise would be fine.  I thought about hacking a stopwatch (as in this project), but using infrared beams (cheaper/safer) instead of lasers.  I didn't fully understand the circuit in that project, and wondered if the first beam needs to latch or shutoff once it's broken to prevent a false second signal that would stop the watch -- for example, from a second axle.

Does anyone have a simple idea on how to do this?  Maybe there is a kit that does this already?

I'm a novice with circuits but do understand soldering, switches, and basic components. 

Thanks!

[Recyclojunk64]

An easier way to do it might be to use a soundcard (line input) and simply record the pulses that come through the photodiodes with it. Audacity or a simmilar audio recording program would suffice. Of course this would take longer to analyse the data (maybe 30 seconds or so), but it would probably be much easier than mucking around with relays and stopwatches.

[kfitch42]

I assume you are looking for something like this: http://www.hmpg.net/pinewood/

[gromf]

Both the sound card idea and the derby racer project are great ideas and get at the basic concept.

I'd really like to make a standalone project that doesn't rely on computer processing.  I think a couple photo triggers and a simple latching circuit connected to a stopwatch would work.  I can conceptualize it, I suppose, but don't know the schematic or exact hardware requirements.

I only need a single lane, by the way.  Just a simple start/stop.  Probably 3 seconds apart on the ramp.

Thanks for all the ideas!

[codeboy2k]

How are you at PIC programming??

I just whipped this up, based on a Kicad sample.  The following needs software written for it, but the schematic is a simple single chip solution.
You can use the start button to start the timer and the phototransistor can stop it.  You can change it to have 2 phototransistors to start and stop it.

You need to write software to reset the count to 00.00 and count up by 0.01 seconds until the stop signal is triggered.

With 4 digits as shown, you can have 99.99 seconds and get 1/10 second resolution.  You can use 3 digits instead of 4 digits if all you want is 9.99 seconds max.

[Recyclojunk64]

Here's another idea that would probably be the simplest way to do it. You could drop the idea of the photo gates (and all the circuitry needed for it) and just wire up two micro-switches to the stopwatch. Hold the car up against the first one, release it (the first microswitch is wired normally closed and connected to START/STOP) and it hits the second one with enough momentum to click the relatively small microswich( which is wired up to LAP/SPLIT as normally open). Then just hit the first one once before holding it down and then pressing the second one at the same time to reset it. Needless to say, a third microswitch should probably be wired up in parallel with the second one unless you are able to reach from the start to the end of the track. It *should* work unless the START/STOP button being normally closed interferes with the LAP/SPLIT function. It worked on my watch when I tested it just then.

Parts: 2 or 3 micro-swiches and a stopwatch

[gromf]

This is a good idea -- and I think I'll borrow from it for at least the "start" switch.

I'm still stuck on making this super easy for a 5 year old.  I really wanted it to be as easy as him rolling it down the ramp and hitting reset.

I'm also still stuck on avoiding a computer interface.

I'm sure I'm being very naive about this, but I thought that something like a photoswitch (even repurposed from the ones that ding when you enter an old dry cleaners) would work well.  Those ding, so there's got to be a circuit that's taking a trigger and doing something once.  I figured that could be the "start."  It would be the same for the finish.  I'm sure it could be done at a component level (rather than hacking a door chime).  The part I'm stuck on is how to get a momentarily closed circuit without a false double "close" from two axles.  That's why I thought that each of the photoswitches would need to latch (at least temporarily).

Thanks everyone for the ideas.