DIY hard drive clock (part 3)

Index sensor

Purpose of  index sensor is to detect beginning of disk revolution to synchronize image being generated.
There are many kinds of remote sensors with identical logic output which are suitable for this task. The only difference between them is how sensors  interact with indexing disk.
- IR photo interrupters. Require cut slot or hole in disk.
- IR photoreflective sensors. Require high contrast mark (black line on a white background) placed on a disc surface.
- Hall effect sensor or reed switch. Require magnet attached to disk.

I've searched through my parts cabinet and found among my stock few SS49E analog Hall sensors.
Output  of SS49 is proportional to the strength of the magnetic field.
Normally  output is 2.5V but it rise up to 5V or drop to 0V when sensor is facing corresponding pole of a magnet.



Output of Hall sensor is connected to the gate of 2N7000 MOSFET  switch.  When magnet travels within close proximity of hall sensor,  MOSFET connects load resistor to ground therefore generating negative  pulse which is detected by external interrupt circuit of  microcontroller.

Hall effect sensor connection.

Hall sensor, 2N7000 MOSFET and load resistor are assembled on a tiny PCB which  is mounted in level with bottom surface of indexing  platter.

Hall effect sensor, MOSFET and load resistor mounted on PCB.

Tiny neodymium  magnet for index Hall sensor has came from old hard drive too.
Originally it was used to hold magnetic heads actuator at parking position.
Magnet is superglued to bottom surface of indexing platter.

Hall sensor is facing magnet. Sensing distance 2-3 mm. 

Hall sensor PCB embedded into system.

(part 2)<--- DIY hard drive clock ---> (part 4)