[dorkbotpdx-blabber] Making a stationary racing machine

[Paul Stoffregen]

A few years ago I used one of those little sensors.  It was already 
installed on the bike.  The trip computer part had long since been 
removed, but the magnet on the wheel and sensor on the frame were still 
there.  I had absolutely no information about it.

It had 2 wires, which would short together when the magnet passed under 
the sensor.  I believe it was a tiny reed relay.  I used a 10K pullup 
resistor to 3 volts.  It did not matter which polarity I used, and I did 
try reversing the wires and it worked either way, 3 volts without the 
magnet, almost exactly 0 volts with it.  I did not see ~0.2 volts as a 
bipolar transistor would have, or something higher like a diode drop.  
When shorted, it was almost exactly zero.  It remained a short 
indefinitely if I held the magnet in place, so I do not believe it was a 
hall effect or other active sensor, as it was only 2 wires and had no 
way to get power during that time.


-Paul



Dread Pirate Kermit wrote:
> On Wed, Jun 18, 2008 at 10:22 PM, dan p <gunterhausfrau at gmail.com> wrote:
>   
>> sure, lots.
>>
>> a typical bike computer uses a magnet and a "sensor" I believe usually a
>> coil and counts the number of times the magnet passes the coil (pulses from
>> the coil, magnet on the wheel).
>>     
>
> I think they typically use a Hall effect sensor, but close enough.
> Whenever the magnet passes the sensor, a pulse get's sent to the
> 'brain'. X pulses/sec = Y MPH. Most modern motorcycle spedometers work
> almost exactally the same.
>
>