Making a Hard Drive Laser Oscilloscope

You might remember the Hard Drive Laser Oscilloscope from a few weeks ago. I have had many inquiries about how to build one. This page should give you enough details so that you can make your own.

Find an old hard drive, I used an old Maxtor 300MB drive. Very old drives will not work for this project since the drive needs to have a voice coil actuated read write arm. Most drives around the 30MB size use stepper motors to move the read write arm.

Flip the drive over and remove any circuit board mounting screws. Many manufactures use special screws such as Torx head screws. You can purchase a set of specialty screwdriver bits at most hardware stores. If you don’t have the correct bit you can usually use a set of Allen wrenches.

Carefully remove the circuit board and look to see how signals are transferred from the circuit board to the drive. In this case there are two thin strips (lower left on circuit board) for low current signals, and some contacts (upper center of circuit board) for motor power.

Flip the drive over and remove the screws that are holding the drive cover on. Inspect the read write arm actuator to make sure it looks like the one in the picture. The curved metal in the lower left of the picture houses two powerful magnets and a voice coil in the center of them.

Next you need to find how the voice coil is connected to the drive. Look for two small wires coming out of the voice coil. Look closely to see what they are connected to. Since the voice coil requires more power than the signals going to the hard drive heads the traces are usually a bit larger. In my case the connection comes over on the two lower large traces of the flexible connector. If you have a Ohm meter it would be a good idea to meter the voice coil to make sure your stereo can power is. You should be fine with any value of 8 Ohms or greater.

Now trace out where the voice coil wires terminate within the hard drive assemble. In my case the traces were visible on the board that the flexible cable plugged into.

In the lower left you can see the two slots where the low lever signals and voice coil power is connected. At this point it would be easiest if you have a ohm meter to true out the wires you are after. In my case it is quite obvious since the pads for the voice coil is larger than the others.

We now need a cheap mirror for the project. I uses a small two sided one that I got from the dollar store.

Remove the glass from the holder, hopefully better than I did 🙂

Trace out where the voice coil is actually connected to the main circuit board (control board) and solder a few wires to them. If you have an Ohm meter you should meter your newly soldered on wires and if all is good you should read your coil resistance. It is very important to make sure that you don’t have a dead short which is easy to do by accident when soldering wires that are so close together. A dead short could cause permanent damage to the stereo that will be powering this unit.

The designers must have had a bit of extra time on their hands, look in the upper left corner of the circuit board.

It is a good idea to disconnect the control board voice coil drive circuit from the voice coil. Do this by tracing the voice coil traces to find a convenient place to cut one of them. In my case I cut the trace where it exited the drive chip. Look between R615 and R622 (below the large chip). The mark that looks like a scratch was made with an exacto knife and cuts right through the trace.

You will need two mirrors for the project. Since mine was broken already I just smacked it with a hammer to but out some pieces that I could use. I have a glass cutter but after breaking the glass when trying to remove it carefully the hammer felt much more satisfying…

Mount the mirrors, one goes in the center of the platters. The second needs to be hung from a pivot point. I hinged mine from some bent 10 gauge wire I had laying around (large shiny wire), the green wire is 24 gauge telephone wire. Make sure you mount it in a place that will work. The laser that will be mounted has to be able to bounce a beam from the hinged mirror to the mirror in the center of the platters. Some hot glue holds it all together. Once this step is done the center mirror should turn with the drive platter and the side mirror should swing freely.

Connect the read write arm to the first mirror, I used some more light gauge wire to do this. Whatever you use should have some play in it. Something that is very rigid would not perform well.

The last step is to mount and align the laser. I used a tie strap around the laser body to keep the button pressed. The laser was hot glued to the hard drive. While the glue was drying I did the fine tuning, bending the swinging mirror mounting wires to have it bounce the laser light off the center mirror etc.