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.|
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.|