Step 1: A Magnetic Speed Sensor Is...
This sensor only detects movement of ferrus objects near the sensor so is typically used for speed sensing (for example a wheel sensor in an ABS system) usually in combination with a toothed steel wheel.
They are very rugged sensors and are not affected by dirt and have a very high signal output making them less sensitive to noise (ideal for automotive applications)
For example the sensor I have made will generate a sine wave of 50Vp-p into a 1Kohm load (that is a peak of 50mV)
Step 2: You Will Need the Following...
2. A relay (I used a miniature 12V relay, but others should work)
3. A broken mobile phone
4. A bit of 2 core wire (twisted pair will help with noise immunity, but is not necessary)
Step 3: Get the Magnet
Step 4: Get the Coil
Step 5: Attach the Magnet
Make sure the magnet is at the same end as the connections for the coil
Step 6: Attach the Wire
Step 7: Case the Sensor
You should end up with the very tip of the iron core of the coil showing at one end as in the photo.
Make sure the connections inside don't touch the case, prehaps a peice of plastic tube might be usefull as an insulator.
Step 8: Test the Sensor
The sensor is then connected to my scope.
Step 9: Result!
Here the scope is set to 5V per division so it is outputting about 40Vp-p (Although this is with no load as I was in too much of a rush this morning and forgot to put on a load resistor - If I had used a 1K resistor the output would have probably dropped to about 5V)
If you can be very patient I will post some circuits in the future to use this in real life, I.e. attached to my Landrover engine.