Introduction: DIY Variable Spacing Multimeter Probe From Drafting Compass

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About: Talents too many, not enough of any.

While looking at multimeter probes to make working with small components like SMDs easier, I was intrigued by a set of variable-spacing test probes made by E-Z Hook, which allowed more precise spacing of the probe tips via a thumbwheel. Then I saw those prices...! I decided to see if I could make my own for a more reasonable price.

Step 1: Parts List

  • Drafting compass - I used the Helix T79012 model compass, which is reasonably compact at roughly 4" in length. It can be had on Amazon.com anywhere from $7.75 to $14.78 new. The only drawback of this model is that it only has one needle point. I got mine at American Science & Surplus, which is generally a great site for any maker.
  • Replacement needle points similar to the one that comes with the Helix cost about $6-9; I found it much cheaper to buy two low-end Westcott compasses at Michael's for roughly $1-2. As a bonus, you get a couple pencils!
  • 24 AWG solid core hookup wire - you might want it in two different colors to keep track of polarity.
  • Heat shrink tubing - you will definitely want this in two different colors! The pieces I used were roughly 3/8" diameter.
  • Wire cutter/stripper
  • Heat gun for tubing
  • Soldering iron + solder
  • Superglue
  • Needle file to clean any superglue residue

The picture here shows the needle tip from the Westcott compass (top) compared to the tip from the Helix compass (bottom). Given the different lengths and point tapers, I decided to buy two Westcott compasses rather than try to trim the longer point down. This does result in much longer probe points, which you may want to sheathe with heat shrink tubing to avoid shorts.

Step 2: Take Apart Your Compass

You do not need to take the entire thing apart as I did here; I did so simply to satisfy my curiosity as to how it was put together. In fact, I recommend leaving the top half alone as it's a bit annoying to put back together. Notice that this model of compass has knee joints, which help in making precise drawings and dimensions. For our purposes, what this means is that the lower half of the compass is made of nonconductive material (the upper half is too, but that is not always the case with some models) plus we have anchor points for our wires.

Step 3: Immobilize the Compass Knee Joints.

This is easily done with superglue. However you'll want to check that glue has not seeped into the holes where the thumbscrews originally went, as it will narrow the hole and make it difficult to put the wire through (easily checked by trying to thread the wire). If this happens, simply ream out the hole with a needle file.

Step 4: Cut Your Wire

The two lengths here are about 8", with 1/2" of insulation stripped from each end.

Step 5: Joining the Wire to the Needle Tips

All the following steps will be shown using the black (ground wire) only. You don't want a wrapped splice as that will widen the diameter of the needle tip too much. Just do an inline splice and keep the solder to a minimum.

Step 6: Putting the Tip Back in the Compass.

Reinsert the needle tip into the compass leg (I put ground on the right leg), making sure the splice and solder are on the inside of the leg for a closer fit. The wire is bent upward for the next step...

Step 7:

Thread the wire up through the hole that held the thumbscrew which secured the compass leads and points, then across the leg, back down through the knee joint hole, and out the back of the compass.

Step 8: Apply Heat Shrink Tubing.

How long a piece you need is mostly done by eyeballing; I used a piece roughly 1-1/2" long.

Step 9: Repeat Steps 5-8 for the Other Leg.

No pictures here, since the steps are repeated, but I'll take a moment to note that you have the option here to add an activation switch which is depressed when you want to take readings. I ultimately decided against doing it, but if you wish to, you will want a surface mount tactile switch. Good ones can be had at Radio Shack in a set of 4 for $1.49; look for part #275-0004, or #275-0002 if you want a higher-profile switch. The switch will be soldered inline into the red (hot/center) wire before applying the heat-shrink tubing (step 8). Perhaps in a future Instructable I'll do a Mark II version of the probe with such a switch.

Step 10: Done!

The probe is shown here successfully measuring voltage on a battery. For now, I used alligator clips to connect the bare wire ends extending from the back of the compass to the multimeter, but this is not ideal. There are several options for finishing the ends:

  1. Solder the wires to small sockets that can be slipped over the 2mm pin tips of a standard multimeter probe. Happily, the cheap Westcott compasses can have their pin sockets clipped off the main compass body to provide the material!
  2. Use standard 4mm banana plugs to end the probes. Most will not take wire as thin as 24AWG, though, so you will need to butt-splice the wires to heavier cords, either by soldering, crimp connectors, or an inline ADC connector such as this one.
  3. E-Z Hook's probes offer the option of terminating in a male BNC coaxial connector, though I think I'd rather use an adapter.

I plan to terminate my probe in 4mm banana jacks as part of an ongoing project to make modular test probes.

This is my first Instructable, and I hope you find it useful or at least interesting! Feedback is appreciated.