I'm looking to build a lap timer (for pedal prix bikes) by using a pair of nRF24L01+s. Unfortunately IR is rather unreliable as there can be up to 3 bikes between track side and our team's bike. Communication will be both ways for basic messages (and obviously lap time display for both the pits and the rider).
The track is approximately 5m wide and the bike will be travelling around 50km/h as it passes. Communication is fine. The problem I have is finding a somewhat consistent trigger point. I'm hoping for accuracy of 0.1s but within 0.5s will be fine.
What I was thinking is logging the delay between the bike and pits as it comes into range, once this hits a local minimum as the bike is at its nearest (before rising beyond a certain threshold to eliminate random errors) having this as the trigger point and then sending the lap time between nodes before going out of range again.
If anyone has any thoughts on how well this will work in practice or alternative suggestions for a trigger then I'd love to hear them!
I don't have an answer but I don't think your title expresses your problem.
It seems to me your difficulty is with detecting the time when a bike passes the finish line - especially if a few bikes are very close together. That doesn't seem to have anything to do with 2.4GHz transceiving.
A better title would attract interest from people who know something about your problem.
I think this has come up before and I'm not sure there is a simple solution.
...R
By the way, if you modify your original post you can edit the title.
Put the arduino on the bike and the marker on the track. This would solve the problem of multiple bikes at the same time.
Albeit some additional weight on the bike, I.e perhaps a few ounces.
I'll have two Arduinos. One on the bike and one mounted on our pit wall. Unfortunately I can't put anything physically on or across the track and nothing can protrude from the bike.
Wireless is not directional without a large antenna - even then I doubt if it would be precise enough. Light is the usual mode for determining if a line is crossed. But that is useless if two or more bikes can cross the line at the same time.
How do they do it for grand-prix motor racing? There may be detectors set in the track?
I wonder if some doppler system would work - the moment when the doppler signal transmitted from a bikechanges from coming closer to going away would be the moment when it was closest to the detector. I have no idea how you make doppler work with an Arduino and it may be possible to trick it by riding towards or away from the detector.
Could you have some system in which a detector on the bike is triggered by a narrow light beam at the finish line arranged so the beam would be visible even if there were other bikes (how?) and which would transmit its ID number when it detects the beam.
Not sure about the doppler effect itself but that's sort of what I'm thinking about trying. As the bike gets closer to the receiver the delay should get noticeably smaller? I want to know if this minimum delay (closest point) can be reliably and repeatedly found.
Race control use some sort of magnetic strip over the start/finish line to detect the transponders fitted to bikes as they cross the line. However with only 4 race days a year where this is in place using that as a trigger makes testing an absolute nightmare. Whereas I can test/adjust it weekly at training if it's a standalone system.
