Hall effect sensor testing

Thread Starter


Joined May 5, 2009
Hi -

This is my first post - after reading some of the information provided on this site I am somewhat intimidated...!

My problem: I have a Ford GT40 replica that uses a Smiths Electronic speedometer....the speedo has never worked, I have substituted another speedo head and have removed the speedo and the Hall effect senor from the vehicle and set up a test bench.

I am getting 0.01 v as an output from the sensor - I suspect that this is the cause of the problem but want to make sure that I am testing the sensor correctly....

Do I have to have a pull up resister in order to test it correctly?

Thanks for your help!


Joined May 6, 2009
Yes this is likely an open collector device and would require a pull up resistor, typically limiting current to max of 20mA. Also you will need a magnet to excite the sensor into changing state. Some speed sensor use the Hall sensor as a "geartooth" sensor, in which case there is already a bias magnet behind the sensor (molded into the plastic). You can check for this with any ferrous metal. If this is the case you will need to rotate a suitable gear at a fixed gap (2-3mm, not more) away from the active face of the sensor. If there is no magnet molded into the sensor then it most likely needs a south pole of a magnet or alternating south and north poles from a "ring" magnet. most speed sensing inputs look for a uniform 50% duty cycle signal.

for more detailed info look at http://www.melexis.com/Assets/Hall_Applications_Guide__3715.aspx it should help.