Re: [Gumstix-users] gumstix tachometer

[Keith O. <kso...@gm...>]

It's kind of ironic - two problems in the same day that can be solved 
with the same technology. (i.e. using a opto-interrupter module (like 
from a ball mouse)).  She could mount a comb-disk on the hub and it 
would be immune to bounce, though she would want to protect it from 
getting dirt/mud/etc. in it.  Check my previous post for more details.

Dave Hylands wrote:

>I got an email from Liz though the wiki. Unfortunately, I got an error
>saying ecegal (her wiki id) can't receive messages, and no email was
>provided. So I'm posting my response to the list in the hopes that
>she'll see it.
>
>Hi Liz,
>
>What's the max RPM that you need to measure?
>
>I'll guess that the bike tire is about 2 ft in diam, which is a
>circumference of about 6 ft. Max speed is probably around 30 mph =
>158,400 ft/hr = 2640 ft/min = 440 RPM.
>
>By putting some conditioning/filtering on the reed switch (to get rid
>of switch bounce) you should be able to do it by using the interrupt
>feature of the GPIO lines (having the GPIO lines generate an interrupt
>whenever a change occurs).
>
>I'd HIGHLY recommend using a hall effect sensor rather than a reed
>switch. The reed switch will have lots of bounce which may make
>decoding the signal much more difficult.
>
>You really need to take your magent/reed switch put it on a bike and
>put it on an oscilliscope and look at the signal. If it's a nice clean
>signal, then you'll be fine. If it has lots of spikes, then you won't.
>So before you even get to hooking it up to anything, you need to be
>able to generate a nice clean pulse for every revolution of the wheel.
>
>Dave Hylands
>
>
>>Hi,
>>
>>I wanted to build a tachometer using a gumstix product and bumped
>>
>into your page that details a robostix tachometer.
>
>>My project entails obtaining revs/min of a rotating bike wheel and
>>
>will be using a magnetic reed sensor/switch.
>
>>Is it possible to obtain the RPM data using the gumstix 200xm with an
>>
>additional waysmall RS232 platform?
>
>>Any comments or suggestions will be highly appreciated.
>>
>>Thank you for your time,
>>
>>Liz
>>
>--
>Dave Hylands
>Vancouver, BC, Canada
>http://www.DaveHylands.com/
>


-- 
Keith Olson
K-Soft Consulting


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