Mini CD drive tray mini-project

This was a little project: tearing into a laptop's CD/DVD drive to see how it worked.  I probably should have, but didn't, salvage the laser LED.

After taking out all the innards, I was left with this cool little assembly.  It would drive the "sled" (the thing that moves the laser in and out from the center to the edge of the disc) using a worm gear drive.

The little square chip is a BH6546KV.  I didn't think I'd do much with this, but for fun I did some brute force testing, sticking a power line somewhere, and dragging a ground line across pins to see what might happen.  I found that on the bank of lines closest to the wheel, I could get the motor wheel to turn a 3-step motion.  Only then did I actually look up the datasheet to see where the pins were.  The chip is an impressively capable thing that handles the sled, focus, and motor drive, so it's a nice all-in-one package.

I wasn't sure if I'd go ahead with this but eventually got up the nerve to try to solder to the tiny pins of the chip.  The fortunate thing with this chip is that the motor output pins for each of the three steps uses a shared pair of pins.  That is, there are two pins that are the same for each step point, yielding a total of six output pins, but wired as three pairs.  That gave me a prayer at soldering to two of them at once, i.e., a solder bridge wouldn't hurt.  Result:

I had also connected a red wire to the resistor shown, because I'd found that connecting a battery to it, and touching one of the yellow/white connection points to ground would move the motor.  Later, though, I found that that red wire connection was shorting something, so it isn't used.  But, it was useful to have for testing the solder connections of the white/yellow wires before moving on.

After several soldering attempts, I got a good enough connection, so I wrapped the wires around the board and taped them down to avoid straining the connections.  I then checked the motor for its current draw, and found that it draws 0.74 amps.

After that, I wired things up in a dodgy system as seen here:

I'm using the same L298N motor controller board as was used for the WBMD but only three of its inputs and outputs.  By driving the lines high/low in different patterns, I could get the wheel to spin at various speeds, and effectively get half-stepping.  With acceleration code, I could get it to spin at a pretty high speed (aka low delay, high pulse rate).

When I added an actual disc to the system, I found that it wouldn't work all that well.  There isn't much friction holding the disc to the motor, so you have to ramp the speed up slowly.  Also, there's a lot of springiness to the motor, so after doing a single step, there's a lot of backlash.  I'm sure there's quite a bit of science that could be applied such that through clever and quick sequencing of motor turns, it can get up to speed smoothly and quickly.  What I did instead was start slowly, and ramp slowly up to a point where the disc was spinning, and then I kicked in rapid acceleration code.  It reminded me of a World War I-era biplane, where someone shouts "Contact!" and it sputters to life, sometimes dying out, sometimes revving the propeller up to full speed.

I had hopes that maybe I could make some kind of robot assembly with this, using the CD/DVD as a wheel, but with so little friction, that wouldn't work.  Or, I'd need some other way to increase the friction in the connection between the disc and motor.  I'd have better luck with the DVD assembly that I pulled out of a purple boom box -- but that's for another blog page.

The system makes some very fun motor sounds, and the L298N motor driver gets fairly hot, so I'm probably over-driving the current or something.

I'm chalking this one up as a "Why?  Because it's there." project.

Merit badges:
- Solder wires to a tiny chip
- Make the CD/DVD mini drive do tricks