Voil Coil Whisker Striker

When building electromechanical sound pieces, I sometimes find solenoids to be too loud for piezo-amplified and coil pickup applications. The voice coil from an old hard drive allows for precise control of a small striker, specifically the thin carbon fiber rod used in this example.

A voice coils act somewhat like a reverse solenoid. Instead of a stationary coil moving a solid metal or magnetic shaft with an applied current, a freely moving coil is propelled through the magnetic field of a stationary magnet. Because the mass of the moving coil in a voice coil is much lower than the shaft in a solenoid, it can move at a much higher frequency and because of that it is the basis for most loudspeakers and electrical dial meters.

In this example, I am using a 9 volt battery to demonstrate the movement of the voice coil, but this can easily be controlled using a switch, relay or transistor. It can also be controlled using a highly amplified audio signal, working like a simple VU meter.

Tools and Parts used in this tutorial:
Hard drive
Carbon fiber rod (.033" is a good diameter)
Thin flexible 2-strand cable (the thinnest ones I have found are those for earbud headphones)

Thin solid wire (bus wire or gardening wire, just to hold things in place)
Precision screwdriver set (most hard drives use torx screws)
Hacksaw
Small angle cutters
Needle nose pliers
Wire strippers

Soldering iron
Helping hands alligator clip holder
Two-part 5-minute epoxy
Mixing sticks and paper to mix epoxy
Rosin core solder
Super sticky tape (gaffer tape or electrical tape)
9 volt battery
Silver permanent marker

Not shown:
Flat head screwdriver
Hammer and Punch.

With a precision screwdriver, remove all screws holding the hard drive cover plate from the housing. There will usually be a hidden screw under a warranty-void sticker connecting directly to the voice coil bearing.

Open the hard drive. It is usually sealed with a weak adhesive, but sometimes a butterknife or a a flat screwdriver is needed to pry it off.

The hard drive's disk is usually held in place with a retaining piece screwed onto the shaft of the motor. Some drives have more than one disk stacked on top of each other, so you might need to remove the motor assembly or the voice coil arm to remove the disk. Push the voice coil arm out of the way to slide out the disk.

The drive controller board is usually exposed on the underside of the unit so that it can be easily replaced. It is usually screwed in place and may have a bit of vibration-dampening foam adhering it loosely to the underside of the housing.

The drive motor is often screwed to the housing beneath the disk or through the underside of the housing. This one is press-fit instead and needs to be pushed out using a hammer or an arbor press. On this drive, it wasn't really necessary to remove it, but often times the mounting screws can get in the way of the cuts needed to expose the voice coil arm.

Remove the top stationary magnet. This is sometimes screwed into place, but in this drive it is just held in place with it's own magnetism and a couple indexing slots.

We'll need to temporarily remove the voice coil arm to remove some of the external electrical connections and expose the solder points for powering the voice coil. This is usually a hex or flat headed screw passing through the middle of a circular bearing.

Most hard drive voice coils have a return magnet setup to bring the arm back to a home position. The arm is usually made from aluminum to prevent it from interacting with the magnets and on this one there is a small steel clip which draws it towards the small rare earth magnet on the housing. In many hard drives, the arm has a small cylindrical or spherical magnet that pulls the arm back to home position rather than a clip.

I removed this clip in this example because I can get quicker action if it isn't always pulled completely back and because the hard return can make a loud click. Removing this depends on the situation, usually the springiness of the cable I use to power the coil is enough to bring back the arm, but in some situations you might want to keep the magnetic return.

I usually cut the electrical connections from the voice coil arm to prevent it from getting in the way of the cable to power the coil. There are usually a few chips mounted to a ribbon cable or a small PCB screwed to the backside of the arm. This one just has a thin mylar strip. Be careful not to remove any solder points that connect strictly to the coil because without them its difficult to get a good connection.

The tip of the the arm has the read / write heads used for accessing the disk. They are usually attached to a tiny strip of steel glued or crimped to the aluminum arm. They are easy to clip off to give a clean gluing surface.

I measure a square roughly the size of the radius of the arm to keep it from protruding when activated but open to allow the striker element to swing without any obstacles. With a metallic marker I trace the square and then with the drive housing clamped to a workbench I carefully cut the lines with a high tension hacksaw. This could also be done with a bandsaw, but sometimes there are steel components or screws and its easier to avoid damaging the blade if it is done manually.

I try to find the thinnest 2 or more strand cable with accessible wires for controlling the arm. I've found that the cables used in headphones, especially the in-the-ear earbud variety are the best easy to find variety. The cable I'm using as an example came from headphones and has 4 wires, but I'll only be using just two of them. Remove the unused wires and strip and coat the ends with solder (tin them) to make them easier to attach to the voice coil leads. Many thinner cables use epoxy varnished multistrand wires, which don't get stripped, but instead you burn the epoxy coating away and coerce the solder to stick with a bit of extra rosin flux. Its important to tin the wires because the irregular shape of the voice coil arm in conjunction with tiny leads makes it hard to do this in one step.

Position the voice coil arm so that the solder points connecting to the coil are accessible using the helping hands. Place the leads of the cable that was just tinned onto the solder points, one at a time, and using the soldering iron, quickly melt them together. These joins are very weak so be careful. If you accidentally tear the solder joint away, it is possible to pull on the thin coil wires and break them, which would be very difficult to repair.

Mix a small amount of two-part epoxy and dab it onto the solder points on the voice coil arm and curve the cable up the side of the arm to reach a good location to secure it. Using a piece of thin wire, secure the cable in place and apply another dab of epoxy to keep it from sliding. Take care not to use too much epoxy as it might restrict movement, especially between the stationary magnets. Sit back and wait for the epoxy to cure.

Reinstall the voice coil arm, tightening the screw until snug. Replace the top magnet. Bend the cable back towards the housing and position it to a location where it can be secured using a small piece of wire or a ziptie. Ensure that there is enough slack to allow for the arm to move to its full extents. Sometimes it might be necessary to drill a mounting hole if there isn't anything available. In this example I attached it to a small hole in the top magnet assembly.

Now position a length of carbon fiber rod on the top of the voice coil arm and tape it to position. A longer piece tends to whip its target more than percussively strike it, whereas a very short length can snap on impact. A longer piece can be especially interesting when attached to an AC power source, creating undulating and regularly spaced vibrational patterns down its length.

Prepare and apply 2-part epoxy to the strand, taking care to encapsulate it and coat the top of the voice coil arm. Let it dry and you are ready to test it.

Strip the other end of the cable and find the leads connecting to the voice coil. Touch these wires to the 9 volt battery and note the polarity. Connecting it backward will pull the arm in the home direction.

Mount into your contraption and have fun!