Time stopping with light barrier

Hello Arduino Community,

I want to create a stopwatch with a light barrier for skiing. The start is no problem but the only thing i worry about is the finish.
On one side I think I will use IR light (where to get something like an IR laser pointer?) and an photo sensor (do i need a special IR one?) with an IR filter in front of to block ambient light. On the other side I will use a retroreflector to bounce the light back to the station. The photo sensor should now be connected to an arduino uno. Is the arduino fast enough to recognize the photo sensor change? (Racers speed can reach 100km/h so the light beam is only for a very short time interrupted). Also which clock module is capable of accurate time measurement (in 5 hours the difference should not be longer than 1-5ms) in very cold conditions (-20°). In the end the finish station should be able to recognize the racer and the accurate time, up to milliseconds.
Also it would be very nice if it would work when it snows.

Do you have any additional ideas or suggestions where i can get the parts? (The price range for the finish station is about 50-100€).

Hi perotom and welcome.

No-one makes IR laser pointers. Can you think why?

An alternative would be an ordinary torch, preferably one with a focussing beam like a maglite, at one side of the finish and a light detector at the other, mounted inside one end of a small dark tube. The tube would exclude most daylight and you would align the torch to point down the tube to the sensor.

Temps of -20C could be a problem for Arduno parts, they are not designed for that but maybe someone from a cold country can advise.

Arduino will be fast enough. The standard millis() function is accurate to 1ms. But if two skiers are only 5ms apart, won't they be both breaking the beam at the same time?

Paul

PaulRB:
Hi perotom and welcome.

No-one makes IR laser pointers. Can you think why?

An alternative would be an ordinary torch, preferably one with a focussing beam like a maglite, at one side of the finish and a light detector at the other, mounted inside one end of a small dark tube. The tube would exclude most daylight and you would align the torch to point down the tube to the sensor.

Temps of -20C could be a problem for Arduno parts, they are not designed for that but maybe someone from a cold country can advise.

Arduino will be fast enough. The standard millis() function is accurate to 1ms. But if two skiers are only 5ms apart, won't they be both breaking the beam at the same time?

Paul

Maybe because you can not see them? :roll_eyes:
The two finish stations are about 5-10m apart so the racer don´t has to worry about crashing into it. Is a torch (IR tourch?) strong enough? would it be a good idea to use IR light? I only want to have one station with electronic devices because of battery and cases and the size. if i have on both ends electronics i need twice the compontents (arduino, battery).

I think you understand something wrong. I save at the start station the exact time when the racers leaves and in the finish i want to do the same so i need two arduinos on the start station and in the finish which are accurate for about 5 hours. The difference between those two stations after 5 hours should not be bigger than 1-5ms because each racer should have a really precise timing. Hopefully you understand what i mean?

perotom

Yes, I must be confused. You say the finish lines for the two competitors will be 5-10m apart but you only want one Arduino & battery, but you want 2 timers that remain accurate to 1ms of each other over 5hrs.

With one arduino you have only one time reference but you could time two separate intervals and there would no drift.

Is the start 5 x 100Km away or is the race over a circuit?

The advantages of IR are that is invisible to the eye (more of a disadvantage than an advantage for your project) and that it penetrates fog better (so maybe works better when snowing, but I doubt it is significantly better).

Maybe it is hard to understand because my native language isn´t english. So i made a sketch right now. There will only be one racer at the time on the track. The only thing thats important is that the start station and the finish station must be in time sync for 5 hours and they only should differ 1-5ms. The start and the finish are not connected to each other.

Real life example:

I create a course on the slope for 3 racers for training. First starts racer 1 than 2 than 3 than they will go up and race again. after the training is done i want to read out the start station (timestamps) and also the finish station (timestamps). Now i can calculate the time the racer needed to get the track done. Now you maybe imagine why the start and finish have to be very accurate.

To the IR beam: Can i simply build an IR beam with an IR led? is it string enough for 10-20m (way to the reflector and back again)

I don't think the problem is with your understanding of english, but rather with the way you have described the project.

Is all of this correct
The finish line is between two marker posts about 5m to 10m apart.
You are thinking of shining a light from one marker post to be be reflected back from the other marker post.
The reflected light will be detected by a sensor.
When the skier breaks the beam of light the Arduino will record the time.
There is no possibility of two skiers crossing the line at the same time.

Then there seem to be some obvious questions...
How will you know which time measurement belongs to which skier?
How many different times need to be recorded before the Arduino can be reset - how many must it be able to save in its memory?
How much time will elapse between the start of a run by one skier and the finish?
Will the finish-line Arduino be in electrical contact with the start signal?

I don't immediately see that an IR beam has any advantage over a visible light beam that is properly shielded from ambient light. If a laser could be used safely it might be useful.

If the start and finish signals are not electrically connected you will probably need a Real Time Clock (RTC) at each end to ensure the times relate to each other.

...R

Yes thats exactly it! No there is no possibility of two skiers crossing at the same time.
I know which time belongs to which racer because they will start over and over again at the same position. Racer 1 always as first and Racer 2 as second and so on. I think about 20 times are enough. The runtime of a racer is about 1-2min. No the start and finish don´t have contact.

but why do most of the systems use IR beams?