How to make a magnetic speed sensor similar to those used to sense wheel speed in ABS systems
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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
Step 7: Case the Sensor
Now drill the centre out of the bolt and fit the sensor inside using plenty of hotmelt glue (or epoxy).
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
Here I have a test rig lashed up on my bench - a gear from my lathe mounted in the chuck of an electric drill with a 'G' clamp on the trigger.
The sensor is then connected to my scope.
Step 9: Result!
Here you see the sensor working, the closer you can get the sensor to the moving teeth, the larger the output.
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.