From: "Richard Sim"
Subject: Detecting moment when a fast moving object passes over a line
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sat, 23 Nov 2002 14:42:29 -0500
NNTP-Posting-Date: Sat, 23 Nov 2002 14:46:17 EST
Organization: Cogeco Cable
Hi, I'm working on a project currently that's for automatic lap scoring in
radio controlled car races, where the cars may be travelling in excess of
40-50 MPH at the point where they cross over the start/finish line, and
several cars may pass over the line within close proximity of eachother.
The cars will have "transponders" in them, which detect when they cross over
the start/finish line, and send a signal to a reciever/decoder that's
connected to a PC.
I'm having trouble finding a way to implement the start/finish line
reliably; the biggest problems are that the transponder may be anything up
to 15cm (6") above the ground with upto 3mm of graphite, plastic or
aluminium between it and the ground as they will be inside the cars, and
must be as small and light as possible (1.5"x1.5" is pretty much stretching
Anything that works on line-of-sight (such as IR) is out of the question
unfortunately, as the cars may very well be completely covered in mud, dust,
So far I'm thinking of using 2 linear hall effect sensors connected to an
op-amp to create a very sensitive magnetic sensor, with some kind of wire
loop or electro-magnet under the start/finish line.
I'm not very confidant that this will work however, as firstly I think the
wire loop/electro-magnet wouldn't have a strong enough magnetic field to
trigger the hall effect sensors, and secondly the hall effect sensors may be
triggered by the car's motor as depending on the type of R/C car, the motor
could be very close to the transponder (and the motors have very powerful
magnets as they can go in excess of 50,000 RPM).
The tranponders have to transmit their unique ID to the reciever/decoder
when they pass over the start/finish line, and to do this I'm probably going
to be using the Linx ES series of RF modules (but the LC series for testing
however as they're much cheaper).
I'm definately a beginner at electronics, and this is the first complex
circuit I've worked on, so any help in finding a reliable way to implement
the start/finish line & transponders would be greatly appreciated (and
please be detailed as, like I say, I'm a beginner).