Quote Originally Posted by Hairy Animal View Post
If it helps, I recently got 10 off SS49E magnetic sensors for 94p (inc. P&P) from a Chinese supplier via a well known auction site. As they're in a TO92 case, they're very small but do need added circuitry to do anything useful.

Basically, they provide a mid-level voltage output (typically 2.5V on a 5V supply) which varies up or down depending on the field strength and polarity, i.e. a north pole will deviate the signal one way, and a south pole the other way.

Coupled to a small CMOS op-amp configured as an amplifier, you could make quite a cheap but sensitive basic magnetometer.
I've had a little bit of a play with Hall Effect sensors like the SS49E mentioned and Op-Amps and initially had good results however, When I left the system running for a while a get drift in the zero point. At first I thought it was problems with the amp circuit, but I narrowed it down to being the null drift as a function of temperature. In other words, if the sensor itself gets warmer or cooler, the voltage output for zero Gauss moves. When using the sensor in its intended range (+/-670 Gauss,) this drift is tiny and can be mostly ignored. But when you amplify that drift by 100x, it becomes fairly significant. I'm seeing drift of 2-3 Gauss, which means being able to reliably detect accurate Gauss fields is probably not realistic hence the thought of trying a magnetometer.