Hall Effect Tachometer

[Commanderbob]

I am trying to make a tachometer with an AVR, a magnet, and a hall effect sensor. The hall effect sensor I have is this one. When there is no magnetic field it outputs around 2V that goes up or down if there is a positive or negative field applied.

Originally I though of using an opamp and the AVR ADC to check when the magnet went by.  The problem with this is I will be polling the hall effect sensor. Do you think it would be better to use a comparator and a pin change interrupt? If I used a comparator then I would have to use two and a OR gate to detect both + and - fields so I don't have to worry about which way I install the magnet. The magnet will not pass the sensor more then 450 times per second (~27,000 RPM), usually under 250.

Is it better to use one opamp and the AVR ADC, comparators and a OR gate, or something I have not mentioned?

Thanks,
Justin

[hudbrog]

Imho AVR internal comporator is the way to go

[Commanderbob]

Can you set the voltage to compare it with to anything you want? I read that it uses the same reference as the BOD. Does that mean you can only use the voltages the BOD can be set to?

[omkar]

A bit late to reply to this post but to anyone who finds it on google search ... here is the link to something I did and worked:
http://www.societyofrobots.com/robotforum/index.php?topic=14973.0

There are links to the Hall effect sensor I used and the arduino program related to it.

Hope it helps.