Brush and Large-Area Multi-Meter Probes for PCB Reverse-Engineering: A DIY Approach!

This blog post contains some ideas for creating a probe to aid with reverse-engineering PCB layouts. You'd use this probe to temporarily replace one of the normal pointy probes on your digital multi-meter (DMM).

Often, you may want to run a continuity test across various parts of the circuit board in order to figure out which PCB trace goes where. This can get tedious with dense boards, or multi-layer boards where the trace is not feasible to follow by visual inspection alone.

Time can be saved by using a brush-like or pad-like probe with your multimeter. The probe would provide the ability to contact large areas of the circuit board quickly or simultaneously, and then the user could revert back to the usual pointy probe to identify the traces further.

I had some random parts lying around at home that I decided to use. I’ll describe it all below, but I wouldn't suggest it's the best way of doing this. You may have far better methods. Also, you may have other spare bits at home to implement the same thing in different ways. A 3D-printed version would be quite interesting too.

Hopefully, the 60-second video explains things:

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Brush and Large-Area Multi-Meter Probes for PCB Reverse-Engineering: A DIY Approach!

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To start, I cut some Perspex (acrylic) sheets, one piece approximately 1 inch square and another slightly larger, to experiment with making different-sized pads. I drilled holes to fit 10 mm diameter Magnetic Pogo Pin Connectors, which were a random purchase from AliExpress a while ago because they looked interesting.

For the probe wand, I used an eyeliner bottle; the brush tip can be pulled out with pliers, but since the handle is smooth and not easy to hold, I wrapped a sheet of silicone rubber around it to obtain a better grip.

The bottle also has a 10 mm hole drilled at one end, and the stalk end has a 3 mm hole drilled for passing the probe wire through.

The assembly of the probe wand is self-explanatory from the photos:

Here’s the finished result of the gluing. Loctite “All Plastics” superglue worked well with the Perspex/acrylic. For the wand, I used 90-second fast-set epoxy glue. Take care while gluing to avoid it spreading onto the magnetic connector mating or soldering surfaces!

Next, I cut some EVA foam sheet (it is also known as craft foam) to the same dimensions as the plastic sheet.

I had some fine-woven copper mesh sheet. I cut it into a length that would wrap around the EVA foam like a hoop. I also cut some sticky take to apply to the EVA foam at 90 degrees to the copper mesh hoop.

The idea is to tape the EVA foam to the plastic sheet, with the ends of the copper mesh wedged between the foam and the plastic sheet. The mesh makes contact with the connector, and doesn't need to be soldered. It would be easy to disassemble and replace if the copper mesh ever gets damaged, by just peeling off the tape.

Here's the finished result:

View from the other side, showing the sticky tape attached to the plastic:

I also decided to create a brush. For that, I obtained some finely stranded wire. I put it in ferrules and crimped them, then snipped off the plastic, leaving just the metal part. See here to learn about ferrules. 

I soldered all of them together by first aligning them in a clamp/vise.

Then, I directly soldered the magnetic connector to it and then covered it all with black PolyDoh (moldable plastic that is low-cost and extremely useful; there are many engineering uses for PolyDoh!). You can heat it up to soften it, and then press it into place by hand, and then use a metal object to flatten it into a better shape. The wire bristles were straightened and then given a haircut.

They appear to work!

When using such probes, some precautions need to be taken:

• The board must be unpowered
• Ensure capacitors are discharged, and batteries are removed prior to use
• After use, ensure the board is free from accidental wire strand remnants!

I hope the blog was useful or that it can spark further ideas. If you have ideas/suggestions or reverse-engineering stories, it would be great to hear them!

Thanks for reading.