This is a supporting frame for three perpendicular electromagnetic coils as a handheld probe. This device may be useful for

• detecting/measuring electromagnetic interference (EMI/RFI)
• electroacoustic music

The different orientations of the coils mean that a field of any polarization will be picked up by at least one of the coils. (Note that this is no longer the case if you then sum/mix the output signals because there can then be phase cancellations.) For sound-related purposes, they also allow for collecting multiple timbres and stereo effects.

Quick demo: picking up noise from a cordless screwdriver.

Required materials

• Magnet wire (I used 38 gauge)
• 3 × 3.5 mm jacks (I used Kycon STPX-3501-3C-1)
• Solder
• Glue to join the sphere halves (I used cyanoacrylate/superglue)
• Glue to secure the wires exiting the coil (I used hot glue)

The handle shaft does not require glue as it is a tight fit. Too tight, really.

Caveats and notes

This is the first version. If I were revising it, I would consider:

• Slightly looser fit of the handle.
• Increased depth of the coil slots, allowing more wire to be used. (The crossings would need special features since their double thickness would become more significant.)
• Slightly chamfered slot edges to prevent the formation of sharp corners which can catch the wire as it is wound on.
• An alignment jig for gluing the sphere, separate from the shaft, which can be removed after gluing and before the coil is wound. This would allow more easily handling the wires.
• A jig for rotating the sphere for coil winding.

Assembly

1. The sphere half without the extra grooves between the coil slots and the center hole is the "bottom" half. Place this on the handle shaft.

2. Ensure the second half will fit on and lines up, then glue the two halves together.

3. Remove the handle if you can, to prepare for adding the wire. (I didn't, so it is possible to install the wire with it fully assembled, but it was inconvenient.)

4. Pass the end of your magnet wire through the center hole, leaving a generous tail end allowing for the handle length. Mark this end of the wire if you want to keep the coil orientation/phase consistent.

5. Wind the coil on, however much wire the size of the slot and your patience allows.

6. Pass the other end of the wire through the hole, ensuring the wire is snugly caught on the corner lays flat against the plastic, and add glue to secure the coil.

7. Twist the wire pair together. (A drill pin vise, or other rotary gripping device may help.)

8. Repeat for the other two coils.

9. Pass the 3 wires through the handle and the corresponding hole in the junction box, then push all 3 parts together firmly (the holes are tapered for a friction fit).

10. Solder the wire pairs to the jacks.

    I used TRS ('stereo') 3.5mm jacks and wired the coils to the tip and ring contacts. This pinout is appropriate if you are expecting to connect to a balanced microphone input, and will also work with an unbalanced TS cable. If you might be using a TRS stereo plug or similar, it would be more appropriate to wire to tip and sleeve.

11. Press on the cover. If it is not a snug fit, glue it or wrap tape (add a comfy grip!) around the entire box.

License

Copyright 2019 Kevin Reid kpreid@switchb.org.

This work is licensed under a Creative Commons Attribution 3.0 International License.