How to make a magnetic speed sensor similar to those used to sense wheel speed in ABS systems
Step 1: A magnetic speed sensor is...
The sensor works using a iron cored coil with a magnet attached to one end. When a peice of ferrous metal is moved towards the end of the sensor it changes the shape of the magnetic field in the coil, this changing magnetic field then induces current to flow in the windings of the coil resulting in a small amount of electricity being generated.
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...
1. A large nut and bolt
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
Rip apart the mobile phone and recover the earpiece, rip this apart and you will be left with a small but strong magnet.
Step 4: Get the coil
Now dissasemble the relay very carfully (it is very easy to break the coil wires) until you are left with just the coil and it's iron core.
Step 5: Attach the magnet
Place the magnet on the end of the core of the coil, there is usually a 'head' to the core a bit like a nail - stick the magnet on this end.
Make sure the magnet is at the same end as the connections for the coil
Step 6: Attach the wire
Solder the wire onto the coil contacts