# Driving a DVD-ROM lens actuator

I have a spare laser diode lying around not being used. The other day I had an idea that it would be cool to have it draw the time on the wall for me as a laser wall clock. In order to do that, I'd have to bend the laser beam very quickly and precisely.

A thing that does just that is a DVD-lens actuator, except that I'd substitute the lens with a small mirror. That's the easy part, the hard part is controlling the actuator. I tried to Google around a bit but I couldn't find any information, not even a description of the wires. The drive logic has to have a way of controlling it, but I haven't been able to decipher it yet.

Can anyone tell me something more about these actuators? Is there documentation or maybe someone could at least point me in the right direction?

The lens is moved by what is essentially a voice coil. There will be several, oriented perpendicularly, in order to orient the lens in multiple dimensions.

Essentially, they are electromagnets, and a reasonable electrical model is an inductance in series with a resistance:

simulate this circuit – Schematic created using CircuitLab

The current through the inductor creates a magnetic field that attracts or repels from another magnetic field. This force works against a spring or some tension, so as a first approximation, the deflection is proportional to current.

Measure the coils with an ohmmeter to figure out which connections are the corresponding ends of a coil.

The simplest way to drive these is probably with PWM. Limit the voltage and duty cycle to avoid overheating the coils. You probably won't find a datasheet, so determine the limits by experimentation. If you smell something funny, or notice they are hot, try less power :) Most likely, the power you will have to apply to get them to move over their useful range will be well under the power that destroys them.

The frequency response of your actuator (and thus, the sharpness you can get in the corners of your laser pattern) will be improved if you can drive the coils with a current source. This is because the inductance will resist changes in current, so you will get to your target current fastest if you apply a high voltage, then back off the voltage as the current approaches the target.

An even better system also takes into account the mechanical impedances of the masses attached to the actuator. Of course it's getting pretty complicated at this point. Depending on what you are trying to do, a simple open-loop PWM control may be good enough.

• I'd suggest tracking the circuitry back and rescuing the existing driver circuit, or at least the driver chip, if possible. That's likely to give you some good clues on how to drive it, current limits, etc. and the device data sheet may contain useful info. Jul 3, 2013 at 9:07
• @JohnU I'm using and old ('98) cd drive, I traced the circuitry back to an BA5972FP IC, which is a OpAmp cascade of sorts, it just transforms one voltage to another with seperate power sourcing on the output (excuse me my lousy terminology). From this IC, the lines go directly to a large smart chip, which also does all sorts of stuff including pcm decoding and whatnot. But at least it gives me some general idea about voltage and current... Jul 7, 2013 at 11:20