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go kart lap timer

Discussion in 'General Electronics Discussion' started by vvizard, Apr 16, 2012.

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  1. vvizard


    Apr 16, 2012
    Hi. I'd just bought a go kart, and find the price of a decent lap timer very high, so I'd like to try making one myself. I've had electronics in (high) school (10 years ago), so my education in the field is not great, and I'm rusty of what I once knew. I do know though, how to program MCU's (ATMEL AVR's).

    Go kart tracks - Lap timing system
    AFAIK the system that makes commercial lap timers work, is that the tracks have a magnetic bar installed under the tarmac at (the bare minimum) the start/finish line. They also usually have some other bars installed to divide the track into sectors, making it possible to analyze lap times in 2-3 sectors around the track.

    My idea
    Put a hall effect sensor on my cart. Connect it to an AVR MCU, along with an LCD panel, and some sort of buttons to select the amount of magnetic bars for the track. Use the Hall effect to detect start/finish line and other sector-bars, and display the time of last lap on the LCD.

    I ordered a basic Hall effect sensor to start with. I connected it to the ADC of an ATMEGA device. Without influence of any direct magnets, the ADC gave an input of ~ "140". With a (quite strong household magnet) at 3mm distance, it changed to "139", and then the value dropped almost to zero as the magnet traveled the last 3mm towards the sensor. The problem is the sensor doesn't register a thing until the magnet is 3mm away.

    I need to detect a magnetic field that is probably 1" beyond the tarmac. Add to that, 1-2" of kart ride height. How can I accomplish this? I've not been to the track to measure the field of these bars. For all I know they might be "enormous" but I somehow doubt it (the distance is quite far).

    So I was wondering if there is better sensors than the hall-effect sensor, or maybe other kind of it you would recommend me to use? The one I have now is a quite small sensor made for small PCB's. I reckon there exists something "industrial" that might be bigger, and able to sense at further distances.

    Any other ideas (amplification etc) is also very welcome.

    AFAIK, the commercial lap timers also use Hall effect sensors. That's why I pursued one in the first place.

    Any help highly appreciated, to start brushing off some old electronic skills :)
  2. jackorocko


    Apr 4, 2010
    I am no expert so take my advice with a grain of salt.

    But couldn't you use a coil of wire to bring the magnetic field , in a sense, closer to the sensor? Isn't this how a current measuring device works. a magnetic field effects a coil of wire and the hall sensor is connected to the coil to measure the change in magnetic field strength
  3. vvizard


    Apr 16, 2012
    Tanks for the reply, but how exactly would I accomplish this? I mean how should I connect the coil to the sensor?
  4. vvizard


    Apr 16, 2012
    I found some specs for the system embedded in the kart tracks. Maybe someone would care to take a look, and point me in the right direction to how I could detect passing over such a system. Maybe Hall effect isn't the way to go at all?

    Any help that could point me in the right direction is highly appreciated. I'd like to detect the loop somehow, so that the detection of such a loop can be registered on a micro controller.
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    That's an interesting problem. As you've already found out, a static magnetic field and a hall effect sensor won't work, because they need to be much too close for that application, where the go kart is bouncing around and needs reasonable ground clearance.
    The magnetic loop idea, I assume, transmits a particular frequency, probably a continuous sinewave signal, which can be detected and isolated with a filter in the receiver. This has two advantages over a static magnetic field: the alternating field can be detected by a large loop in the receiver, giving much more sensitivity, and filtering can be used in the receiver to reject noise.
    This approach is probably needed, but I think there might be other options apart from an alternating magnetic field.
    Ultrasonic sound might be a winner. You would have an ultrasonic transmitter, probably mounted beside the track and facing sideways (putting it below the track facing upwards would probably invite problems from dust and dirt). Also because of the Doppler effect, it might be possible to detect the exact point when the receiver passes the transmitter with quite high accuracy.
    Infra-red light might also work, though the Doppler effect doesn't apply and you would have to detect the precise passing of the transmitter by looking at signal strength, or perhaps have two adjacent transmitters with different carrier frequencies and detect when the predominant frequency at the receiver changes from the first to the second carrier frequency.
    Perhaps others can suggest other approaches.
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