sensor selection

[parts-man73]

My son asked me to build a project for him. A speedometer for his bike. I plan on using a Picaxe for the brains of the project. Also 2-3 7 segment LEDs for the display. I wanted to display the speed in whole numbers and also tenths, but haven't decided if it'd be necessary to display speeds over 9.9 MPH.

The real reason for my post is for ideas to use for a sensor. I have thought about a simple reed switch and placing a magnet on the wheel, or several magnets spaced at even intervals. Or perhaps some type of hall effect sensor?

Any ideas or comments are welcomed and appreiated!
Brian

 

[hippy]

I know I commented on something similar a few months ago, but can't track it down using Search.

Reed relay and multiple magnets would give more timing pulses which would make for better low-speed accuracy, but at higher speeds only a single pulse per revolution could be good enough. The code could choose between various algorithms to use depending upon the speed last determined.

 

[eclectic]

Brian. Try

http://www.rev-ed.co.uk/picaxe/forum/Topic.asp?topic_id=4733&forum_id=26&Topic_Title=bicycle%2Bspeedometer&forum_title=No+new+posts+please%21+18

<A href='http://www.rev-ed.co.uk/picaxe/forum/Topic.asp?topic_id=4733&amp;forum_id=26&amp;Topic_Title=bicycle%2Bspeedometer&amp;forum_title=No+new+posts+please%21+18' Target=_Blank>External Web Link</a>
e.

 

[Rickharris]

As always when designing something a good start is to look at comercial products that do similiar things. Most cycle speedoes use a reed switch and magnet. <A href='http://picaxe.orcon.net.nz/bikeodo.bas' Target=_Blank>External Web Link</a> stans input <A href='http://us1.webpublications.com.au/static/downloads/sc/pdf/bicycle.pdf ' Target=_Blank>External Web Link</a>

<A href='http://us1.webpublications.com.au/static/downloads/articles/104804_Projects.txt' Target=_Blank>External Web Link</a>
<A href='http://www.siliconchip.com.au/cms/A_30709/article.html' Target=_Blank>External Web Link</a>

lots on google!

 

[parts-man73]

Thank you for all your input. I think that I will use a magnetic reed switch for sensing the speed. I didn't think about what to do with the input until you guys brought that up. I was planning on counting pulses per second. But with a 29&quot; diameter wheel, that's less than 1 pulse per second at speeds of less than 5 mph. Not a great resolution, as I wanted to show speeds right down to tenths of a MPH.
I do like the idea of measuring time between pulses. That would give me a better resolution at the speeds he travels at. This is a kids bike, and at 9 years old, he's not going to be going any faster than 10-15 mph.



Thanks again!
Brian

 

[Dippy]

I think that might be why people have mentioned multiple magnets.

Edited by - Dippy on 16/08/2006 17:05:36