transistor switching question

Discussion in 'General Electronics Chat' started by Garry71, Aug 4, 2007.

  1. Garry71

    Thread Starter New Member

    Aug 4, 2007
    2
    0
    Hello everyone.
    I had to change the battery in my bicycle computer/speedo, so now I've lost the total milage recorded. I'd like to "spin" it back up to the milage I had, so I need a transistor switch which will produce open and short circuits at the output, controlled by a R/C timer to trigger the transistor on and off.
    The speedo is triggered using a reed switch fixed to the fork with a magnet on the wheel which spins past the reed switch.
    Anyone know of a suitable circuit which i could use? I've got some BC109's left which I can put into service.

    Thanks a lot
    Garry
     
  2. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Why not just glue a small magnet to a small motor, place it near the reed switch, and leave it running for a while ?????....Or a small home wound electro magnet and a signal generator.....Or NE555.Daniel.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Further to lightingman's suggestion you could attach a small magnet to a cordless drill and with patience you could get the reading back.

    If I remember correctly some of those odometers have a setting that allows you to adjust them for different sized wheels. If your odometer has this feature then I would suggest you set it for the largest wheel diameter/circumference. This will cut back on the number of revolutions of the magnet required to get the reading back to the value you are looking for.

    hgmjr
     
  4. Garry71

    Thread Starter New Member

    Aug 4, 2007
    2
    0
    I know about the wheel size trick, but I didn't think about it untill after tapping the contacts with a coin for a while(yes i know i'm sad!)

    I thought about using a small relay wired up so it turns itself on and off, but I don't have one to hand. It's a good laugh watching a mains power one walking along the bench!

    I never thought about the motor/ magnet thing, so I might as well try that with a battery drill.

    Thanks a lot to both of you.
    Garry
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Decided to run a few numbers and here is what I estimated.

    Assume the odometer is set for 30 inch diameter wheels.

    Wheel Circumference = 30\piinches = 94.25 inches = 7.85 feet.

    1 mile = 5280 feet

    revs/mile = \frac{5280}{7.85} = 672.6

    Assuming you can achieve a comfortable 500 RPM from the cordless drill that would yield the accumulation of 0.743 miles per minute or 44.6 miles per hour.

    It would take approximately 2 hours and 15 minutes for every 100 miles on the odometer.

    I don't recall you mentioning what your mileage was when the reading was lost.

    I hope you have a lot of coffee as well a few "NoDoze" tablets.

    Happy Virtual Biking....
    hgmjr
     
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