# transistor switching question

#### Garry71

Joined Aug 4, 2007
2
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

#### lightingman

Joined Apr 19, 2007
374
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.

#### hgmjr

Joined Jan 28, 2005
9,029
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

#### Garry71

Joined Aug 4, 2007
2
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

#### hgmjr

Joined Jan 28, 2005
9,029
Decided to run a few numbers and here is what I estimated.

Assume the odometer is set for 30 inch diameter wheels.

Wheel Circumference = $$30\pi$$inches = 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