what Sensor for a laser line

I have been thinking on a sensor setup to trigger a stop event on a bicycle on board Arduino, what I have envisioned is a laser line like those used by laser levels. this would be aimed and aligned at the finish line of a race. The on board sensor (I don't know what would be best) would pick up the laser as it crosses it. The finish line would be about 4 meters wide.

Here are my problems I am asking for help with:

1) is there a laser like this I can get that is safe for eyes and invisible? This is in case of reflection to keep participants safe. 2) What sensor would work for this? 3) can I set this up so that there are perhaps three laser lines at different points on the course so the on board computer could record timing per section? the onboard sensor would have to distinguish between each color or wavelength.

Thanx in advance :)

found these: http://dx.com/p/2-5mw-650nm-line-laser-module-3v-46389

Class 2 spread out should be eye safe but not invisible.

still no idea how to reliably sense the line as it is crossed.

You are proposing a laser transmission beam, stationary, and a sensor mounted on a bike (in motion)? That will be very difficult to get that laser beam (1/10 cm or less) to be detected by a sensor on the moving bike. Maybe switch from laser, to just IR (it would be easier to detect). Won't be as accurate tho.

Its really just a timing system, I am trying to come up with a way to tell the onboard Arduino that it has crossed the finish line so the timer stops. Has to be pretty accurate but not in the 1000 of a sec, 10ths would be ok.

I was looking at IR sensors and thinking if I could adapt them somehow that the laser 650nm light had hit it and go from there. The larger dome of the sensor would make for less of a problem to ensure it gets touched by the light.

http://hobbyking.com/hobbyking/store/__26932__Arduino_Infrared_Sensor_small_.html

but I don't think these will detect the right frequency and distinguish it from the mix of light in full daylight.

it needs to be a laser line of course so the rider can be anywhere on the finish line, unless someone has another idea on that?

Any ideas?

Use a conventional fixed laser/mirror/detector setup to detect the bike crossing the line, and a radio transmitter from there to tell the Arduino that it has crossed the line?

You might need a timing blade on the front of the bike to achieve consistent detection.

I still don't see this being accurate (all the time). How about if two bikes cross at the same time. Only one bike would see the light, and the other one would be shaded from the laser. At the distance from the laser to the bike, how large of a spot light do you expect to be present? How high off the ground would that be?

PeterH: Use a conventional fixed laser/mirror/detector setup to detect the bike crossing the line, and a radio transmitter from there to tell the Arduino that it has crossed the line?

You might need a timing blade on the front of the bike to achieve consistent detection.

Something like this was the original idea, but there was no way to distinguish what bike crossed the line, if more than one was crossing at close to the same time, all bikes would get the signal and stop timer right....

[quote author=jack wp link=topic=188776.msg1396934#msg1396934 date=1379547531] I still don't see this being accurate (all the time). How about if two bikes cross at the same time. Only one bike would see the light, and the other one would be shaded from the laser. At the distance from the laser to the bike, how large of a spot light do you expect to be present? How high off the ground would that be?

[/quote]

I was thinking of placing the laser above the finish line so it would have a clear unobstructed view, the sensor would be on the bars on the steering stem likely. so laser straight down and sensor straight up.

laser would have to be setup on a bar say 3.5 meters off the ground, two or three lasers could be used to ensure coverage.

At the distance from the laser to the bike, how large of a spot light do you expect to be present?

Normally a laser beam is less than half the size of your little finger nail. If the bike moves left or right by a hair, it would not hit the sensor.

Is this system supposed to declare the winner of the race? I don't think so. You still need judges, and maybe video with replay to do that.

actually its a training device, and the laser line is across the path of the bike so left or right would not matter. the only way the laser would not have a clear line to the sensor is if the rider was over the bars.

Is the laser line horizontal, or vertical ?

What size of light do you expect to be hitting the bikes sensor? The size of a green pea, or less?

I'd normally suggest modulating the laser at 38KHz and using an IR receiver as the target (use TSSP4038 or TSSP58038). But with a bike and the narrow pieces of the frame/tire it might make the detection a little funny -- you'd have to do the math to figure out what distance a ~40mph (?) bike travels in 1/38000th of a second to see if the width of the tire or frame would be enough to break a 1/38000th of a second pulse. Otherwise it might only break when the cyclist's body breaks the beam and make your timing less precise.

See http://www.righto.com/2010/03/detecting-ir-beam-break-with-arduino-ir.html for details. TSSP4038/TSSP58038 are designed for continuous signals and the IR receivers of the type used with remote controls will not work. Vishay's products are better quality and certainly worth their slightly higher cost.

What you're doing here is replacing the IR led with a laser module, and it will work fine and over very long (10s of meters) distances. There won't be any problem modulating common laser modules at 38KHz; you might need a transistor if it's over the 20ma arduino pin limit. I never took current draw as a consideration and I'm not sure what they draw for current. I tested with a red dot laser yanked from a laser pointer and it worked fine; I can't say how much distance you'd get with a "line" laser. Make sure you get a focusable module though so you can widen the dot and make it less difficult to target on your IR receiver.

As an aside, the Hobbyking sensor ("PIR sensor") is the wrong type of sensor.

[quote author=jack wp link=topic=188776.msg1396960#msg1396960 date=1379549387] What size of light do you expect to be hitting the bikes sensor? The size of a green pea, or less? [/quote]

Ahem.

It's a "line" laser module - contains a diffraction grating which spreads the light into a line rather than a spot of the same width. You do substantially lose intensity in daylight. I gather his intent is to fire it from above - in the finish line "arch" which may also provide some shade.

There is still a problem of ambient light which would normally be solved by modulation but the bike's movement could be a problem here.

It doesn't seem to be much easier than when we considered it twenty years ago for the "Ironman" events. I think RFID on the ankles was used but the (commercial) equipment was expensive.

An interesting alternative - use an IR laser (200 mW) and a scanning mirror (think: laser printer) to generate the "line" with intrinsic (and predictable) modulation. You are then looking for a few "blips" from the sensor with very precise and steep attack and decay.

DaleE: Something like this was the original idea, but there was no way to distinguish what bike crossed the line, if more than one was crossing at close to the same time, all bikes would get the signal and stop timer right....

You haven't mentioned that requirement until now. That makes the problem massively harder and I can't think of any feasible way to achieve that.

actually its a training device

Then it's not critical. If two bikes crossing the line nearly the same time, both got a signal when the first one crossed. Use the suggestion from PeterH.

First I want to thank everyone for your help, I am clearly not getting what is needed communicated here however :)

This is for a training device and more than one rider may or may not use one at the track, I am wanting the entire device to be on board the bike, the only thing that is at the finish line is some kind to marker that the onboard system can reliably detect to stop the timer and say the run is complete.

So I envisioned a laser line, like those used by laser levels, projected across the finish line from above, I don't know why it would need any kind of modulation, it either touches the sensor for a split second or it does not. On the bike the sensor that simply has a wire that is at 0 unless the laser light touches the sensor then a pulse is sent out, the on board Arduno nano would use this pulse to set a condition in the program.

there is little chance of the sensor not receiving the laser physically, yes I am worried about daylight interfering and causing false signals or failing to sense the laser. I will have to work out this in proto.

using another Arduino fixed at the finish line to send out a wireless 'stop' signal is not out of the question. but this unit would not be able to distinguish between one bike and another, since the system I am working on is for a BMX track the problem at hand would be that all bikes are within radio range, so the first bike across the line would stop the timer on every bike on the track, this would make the entire system pointless as logging performance related to the training would be corrupted.

my understanding of RFID is that it is proximity based very short range, so I don't see how the RFID tag on the bike would get close enough to the sensor to trigger, and if the sensor was sensitive enough and the tag signal strong enough then instead of a finish line we would have a finish zone :) this is why the idea was born to have everything on board the bike and simply have something (anything does not have to be laser) that could be sensed at the finish line with some accuracy, 1/10 sec is good.

Thanks for the detailed clarification. The main concern about having the sensors pointer up, is that if the sun shines on them, I think they will be unusable. Maybe if you could position the lasers on the ground, and the sensors were aimed down it may be able to work with the sun shining.

Another option: use a short range transmitter (bluetooth is about 6 foot range I think). Maybe run the antenna along the ground at the finish line, with about 2 foot range. Then any bike that comes within range to pick up the transmission, can stop their clock.

looks like I might have found part of the solution, hopefully :) I am from the look of things going to be designing my own circuit to a point and that's a bit outside my comfort zone but wth :) There are photodiodes that are tuned to 650nm I found this one that is not crazy $$

http://www.digikey.ca/product-detail/en/OPT101PG4/296-36066-5-ND/1899803

I already ordered the 60 degree 650nm laser line generators, @ less than 3$ I can play with those. I can get this for about 10$ so that's not too bad to do some experiments with.

[quote author=jack wp link=topic=188776.msg1397725#msg1397725 date=1379606775] Thanks for the detailed clarification. The main concern about having the sensors pointer up, is that if the sun shines on them, I think they will be unusable. Maybe if you could position the lasers on the ground, and the sensors were aimed down it may be able to work with the sun shining.

Another option: use a short range transmitter (bluetooth is about 6 foot range I think). Maybe run the antenna along the ground at the finish line, with about 2 foot range. Then any bike that comes within range to pick up the transmission, can stop their clock. [/quote]

Daylight.... I agree, and I am worried about it, but I still think its worth a try. lousy thing is the time of year, I cant get a proto together to test in mid summer daylight, B.C. Canada is a bit overcast starting soon.

Bluetooth gives another issue IMO, the "sensor" comes into range of the finish line and the two BT units have to handshake b4 a signal can be sent to the Ardunio, I don't think its possible to get a reliable finish on that.

DaleE: I don't know why it would need any kind of modulation, it either touches the sensor for a split second or it does not.

The need for modulation critically relates to the matter of ambient light which is always going to be your major problem. While you will get a transient from entering the laser line, you will get a similar transient from any other sharp shadow passing - such as an overhead powerline just as one example. Using a modulation of known frequency - such as my scanning mirror suggestion - allows you to discriminate on that frequency rather than a transient quite dependent on cycle speed and scattering of light.

DaleE: There are photodiodes that are tuned to 650nm http://www.digikey.ca/product-detail/en/OPT101PG4/296-36066-5-ND/1899803

Well, in fact no. The "tuning" in this respect is rather worse than using a crystal set to tune the AM broadcast band! Do take a look at the datasheet. The device is actually catalogued as an "Ambient Light Sensor".