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29 December 2010
Author: Giorgos Lazaridis
Reverse-Engineering the Linear Guide of a DVD Head


Some of my trash bins. Full of parts to be resurrected

Most of the times, research comes from the need. This time, i needed a linear guide with a good precision, for a project that i plan to make. I began searching my scrap boxes and my museum, and did found some solutions. But the most simple and effective, was hidden in my -very- old CD (or DVD) player (or recorder). All DVD recorders have their head (laser diode, receiver and some other gizmos) mounted on a carriage, able to travel linear on rails with great precision, radially in respect to the CD. One motor is used to move this carriage. This system is exactly what i need for my coil winding project (not ready yet).








Guts out

So, lets begin. First i need the victim. In my museum i have ten or fifteen old DVD players. So i chose one (randomly) and opened it. This is what i found inside (most probably many of you have done this before, don't you???):


This is the victim... Within 5 minutes, it was broken into pieces. This is the piece that i am looking for But first, i have to lighten it from some very interesting parts...


From now on, i use super small screw drivers, philips and torx type. The head hides a small treasure:


Here is a nice lens mounted on an electromagnetically movable mechanism The flexible PCB was (brutally) removed (it was glued on the body) And the laser diode was revealed! And of-course i took it out and saved it for my Phaser-Plasma satellite destroyer
Underneath there is this metallic cover which was removed... And more interesting parts were revealed! The laser receiver... Another tiny lens and some more prismatic glasses.


After an hour or so, i finally got in my hands the lightened mechanism that i am about to use.


It is nice isn't it? 35mm stroke, not bad for a coil... And here we are: the moving mechanism




The moving mechanism

The mechanical part of the mechanism is straight-forward. A motor moves a gear with approximately 2.14 reduction ratio, and then it moves another gear with another 3.2 reduction ratio, and then moves the last gear with 1.14 increment. The final gear has 21mm diameter, and it converts the rotational move into linear. The calculations are now easy to make. Reverse engineering the PCB was kinda harder though, because i had absolutely nothing in my hands, not a part number, no nothing.





Reverse engineering the PCB

In a first glance, i only could recognize the motor type. Only two wires comes out of it and goes to a + and - connector. I bet that this is a brushed DC motor. Then i noticed something strange on the first gear, the one that is directly mounted on the motor. A black disk that looked like rubber was on the perimeter. But hey, what does a rubber on the gear??? Maybe it is not a rubber. It could be a magnet...:


Identifying the motor was not hard. It is a brushed DC motor What is this black thing around the gear? Is it rubber? Is it magnet? So i put a screw and... Voila! It is a magnet!


I was not surprised to find out that this is a magnet, and i was not going to be surprised, if there were Hall sensors around there. After all, the magnet-Hall sensor pair is frequently used in many applications, like in this liquid flow sensor, in which a Hall sensor is used to detect two magnets. So, i looked closer and... hmmmmmmmm, what are these under the disk?





At that point, i had to remove the motor and the PCB form the case to work easier. And so i did...


The motor and the PCB had to be dismounted I took some very close images of the PCB And another with a backlit to reveal the traces And i put them side by side on my PC for analysis


It looks easy, but it was not that easy at all. There were tracks going under the SMDs, and also, as you notice, none of the SMDs -except the resistors and the capacitors- had clear markings to identify them. The two big SMDs under the disk, could be Hall sensors, yet i was not sure. I only suspected it. Also, the name of these parts encouraged me to this thought (H50 and H51). Maybe H comes from Hall... So, slowly and carefully, i made a schematic on paper with these photos. This is a technique that i have not used before, but i plan to use it again in the future. I do not know how good it will work with double sided PCBs, works sweet for single sided like this. Here is the schematic.





And now comes another wild guess. If these parts are indeed Hall sensors, then they should have a positive and a negative supply. So, two of their pads should be connected together. And indeed, if you notice the schematic, there are 2 pads connected together. Actually, these are the only pads of these SMDs that are connected together. Then, i had to identify which of these two is the positive. Luckily, near by, a capacitor was to save the day. It had the name C50, which means capacitor, and it had also a mark (like the ones on the diodes) which marks the positive lead, probably is a tantalum capacitor. Following this line, i found the positive trace, and thus the other is the negative! That simple.

It was time to identify all the connector pads. I had the positive which was pin #8, the negative is pin #3, the motor is connected to pin #1 and #2. There are 4 more pins waiting to be identified. From the schematic, these goes through 470 Ohms resistors to the 2 SMDs that i suspect they are Hall sensors. So, i consider them as outputs...




The oscilloscope will reveal the bare truth

Before i connect the PCB to the oscilloscope, i first had to solder wires to the pads. That was really not easy. The pads were too close one each other, and i had to be super careful to avoid short circuits


I put back the motor and the PCB, and i removed the second gear for the motor to run freely Then i soldered wires to the pads. The wires are connected to a connector, and from there to the breadboard I put 3 volts to the motor and 5 to the other circuit. The other 4 pads goes to the oscilloscope..


Isn't it great when things just work? The 4 channels of the oscilloscope shows the outputs of 4 Halls sensors, they ARE Hall sensors after all. Take a closer look:





The yellow and green channel comes from the first Hall sensor, and the other two from the second.


The motor in free-run Blocking the gear with my finger reduces the frequency of the sensors And of course, the world outside the DVD case is more brutal, so a few hot-melt glue will keep the wires in place





The End

So, now, i do know what type of motor is. I do know the reduction ration for all gears. I do know what each pin of the PCB does. I will make a PWM controller for the motor, and a couple of Schmidt Triggers for the Hall sensors, and the control of this cool linear guide will be in my hands. I suggest you save your old DVD drives, because one day you may need them!









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  • At 25 February 2015, 4:40:58 user Michel Champagne wrote:   [reply @ Michel Champagne]
    • Thank you for a great article about a n interesting and informative
      reverse engineering project. Great work !!! Giorgios. The passion and expertise you have for all your projects and probably in all your life motiveates all your fans! Many thanks again, keep up the great work you are becoming better known every day on the web . Looking forward to your future and present successes


  • At 26 January 2014, 22:48:32 user supergru wrote:   [reply @ supergru]
    • A suggestion for making a schematic from images. Once you have the bottom and top side images, flip one of them (do the adjustments you need -i usually crop and resize them-), then in Photoshop (i use this soft), create layers for each imagen and overlap them using transparency. So you can see both the top and bottom side overlaped and you can easily draw the tracks. It saves lots of time!


  • At 12 December 2012, 17:19:21 user Maks wrote:   [reply @ Maks]
    • This is cool! I got a bunch of old DVD drives that I broke apart and extracted the linear guides and the lasers. I think I prefer the stepper motor drivers because I can make it move very precisely. My next project with them - making a Laser engraver with just two stepper motor drive mechanisms and one RED laser DVD writer diode. Should be really fun!


  • At 8 June 2012, 2:43:39 user Dherreiros wrote:   [reply @ Dherreiros]
    • XCELLENT!!! Very interesting, my friend. thank you for doing this videos that open doors to the knowledge, thanks for the passion you put on every project and many thanks for sharing them with others...Many thanks because instead of sharing pornography and unmoral material you spend your time doing this kind of educational videos.


  • At 10 April 2011, 6:02:07 user Kammenos wrote:   [reply @ Kammenos]
    • @Adam Frederick i made this hack because i will use this mechanism as a stacker mechanism for winding small coils with thin wire. I've seen another that used it to make 3-axis CNC for plexiglass jewels.


  • At 10 April 2011, 2:32:11 user Adam Frederick wrote:   [reply @ Adam Frederick]
    • I have a few of this leanear guides saved too but what to do with them? Nothing comes to mind. What have other people used them for?


  • At 23 March 2011, 8:59:05 user aaron wrote:   [reply @ aaron]
    • hmm...i think we have the same passion, only, u have vast skills and me, still in the first stage in exploring this wonderful world of electronics.
      I find this site very interesting. /from Philippines


  • At 8 March 2011, 21:05:08 user Arif Junaedi wrote:   [reply @ Arif Junaedi]
    • very good articel, n i very happy read N read againt,thanks


  • At 4 February 2011, 21:55:10 user Anonymous wrote:   [reply @ Anonymous]
    • Thank you,
      I always wondered how these precice movements are done in a DVD-Player.
      Fascinating Project!


  • At 30 December 2010, 9:11:45 user Nick Raymon wrote:   [reply @ Nick Raymon]
    • I feel like Iím constantly looking for interesting things to read about a variety of subjects, but I manage to include your blog among my reads every day because you have honest entries that I look forward to. Hereís hoping thereís a lot more great material coming!



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