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

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

shabaz — Wed, 26 Feb 2025 21:57:13 GMT

The continuity tester mentioned in the comments is now documented here:

[mention:788ae571cfda4d1a9a71fa0737e53c3a:f7d226abd59f475c9d224a79e3f0ec07]

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

shabaz — Thu, 13 Feb 2025 10:11:22 GMT

A through-hole version idea.. more as a beginner project.

This is still very rough, needs to be worked on at some point. It needs test pin holes added. The layout closely follows the schematic, to make it easy to see what's going on.

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

shabaz — Tue, 14 Jan 2025 14:47:02 GMT

Here's the current circuit diagram, with the op-amp inputs flipped back to match the monochrome fuzzy diagram. I didn't change anything else, apart from correcting the tone generator capacitor value (any other value discrepancies can be tweaked during test if needed), and also added an LED.

For the enclosure, I decided on a Multicomp G404, which is very similar to the one I used on an earlier project ( [mention:d6fd8841fe064bf6b25be4ebfd935833:f7d226abd59f475c9d224a79e3f0ec07] ) but is smaller. The battery holder is for 2 x AAA, which should hopefully last ages. The awkward positioning of the LED is to try to get it into a flat part of the case molding, which happens to be there. There's just enough room for the two 4 mm banana sockets, on either side of the power switch. The speaker will need to be stuck to the inside of the case (above the battery holder), or glued somewhere onto the PCB if it is small. I have not decided on that, I figured that can be done later.

The underside of the board contains all the SMD components, since the top is taken up mostly by the battery holder.

Underside PCB layout:

I won't do anything with this for a day or so, in case anyone has any further ideas. After that I'll generate the Gerber files and upload to GitHub, and order some boards.

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

shabaz — Wed, 08 Jan 2025 09:07:22 GMT

Just seen that this sort of thing used to exist (it's not manufactured any more), called a 'short finder'.

It's basically a probe and all the electronics of a continuity tester all collapsed into one item. Nice that it had the pointy probe too, so the user can flip it over. Plus a cap for the brush.

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

obones — Tue, 07 Jan 2025 14:27:16 GMT

Nice trick indeed.

What I've always been wondering about, though, when doing this kind of analysis are what risks are there of frying something on the board?

I mean, we're applying a voltage across components that may not like whatever voltage the probe is sending.

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

DAB — Mon, 06 Jan 2025 19:36:31 GMT

Great idea and solution.

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

Jan Cumps — Mon, 06 Jan 2025 12:35:38 GMT

For this application (and for general continuity probing) it's good to have a meter with a good latching continuity mode.

The type that beeps clearly when you briefly flick two normal probes against each other.

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

shabaz — Mon, 06 Jan 2025 12:11:29 GMT

I've been using the brush probe (I have yet to use the pad probe), and it's providing tons of insight as to which signals go where. Super-useful because even when you think you've found the end of a track, you can still sweep around the board to see if the signal split anywhere else too.

The brush probe isn't any good on the top (component side) of the board, but I suspect the edge of the pad probe could work as a cushion against ICs. Perhaps a narrower pad probe would be better though.

In terms of desired improvements, I find myself needing to frequently swap between brush and normal pointy probe. Also, I would like to have an even wider brush probe, perhaps an inch width.

If I was any good at 3D, I'd try to make a probe holder that allows one to dock or store the unused probe ends. I think I'll just find a box and glue a steel strip inside, to clip the unused probes magnetically.

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

genebren — Sun, 05 Jan 2025 21:21:04 GMT

Very clever work here [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] ! In the early 1980's I worked with an in-circuit board tester manufacturer, which a group that was responsible your developing board specified test programs for use on these testers. Companies could hire the group to provide a turn-key solution for clients so they could have a quick start on integrating the testers into their production lines.

The group did attempt some similar approaches to help understand the customers boards (where adequate information was not available). They used brushes, meshes, and a roller device with embedded wires to determine which components were to which circuit nodes. While not a complete solution, it was helpful to get enough information to start generating a board file to build their test programs from.

Later, we used another similar approach to 'scratch' around an IC, with a 'listening' probe, while the tester would query the test (pogo) pins on the bottom of the board associated with the device, to verify if the test pins were in fact connected to the board. We would display a pin outline on the screen, and the pins would change color as the test pins detected a probe touch.

It will be interesting to see how these techniques work for you. I really like your brush construction, and I think you mesh block is a very clever design.

Thanks for sharing!

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

kmikemoo — Sun, 05 Jan 2025 17:54:16 GMT

[mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] Nice! I like the brush adaptor. I can see a use for this in making sure that I don't have any inadvertent grounds or bridges.