Building Block Boards#

As part of improving my design process, I’ve decided to create a set of repeatable circuit designs for common functions that I expect to use again and again.

These designs — or “Building Block Boards” for now, until I come up with a better name — are intended to be small, validated modules of functionality. Each board will have its own documentation, schematic, PCB layout, and bill of materials, giving me a known-good starting point for future projects.

The aim is simple: reduce the risk of designing the same circuit badly more than once, speed up prototype development, and build up a library of practical hardware blocks that I can reuse with confidence.

There is also a practical testing reason for doing this. Rather than designing one large board with every subsystem on it and then trying to debug everything at once, smaller building block boards allow me to validate sections of a design independently. If something is wrong with the regulator, USB-C input, RS-485 interface, or GPIO expansion, I would rather discover that on a small, inexpensive module than on a larger board where multiple untested circuits are interacting with each other.

This approach should make failures easier to isolate, easier to understand, and cheaper to fix.

The first set of building block boards will include:

  • A 3.3 V regulator module with protection against common bench mistakes, including resettable over-current protection, reverse-polarity protection, and basic input transient clamping.
  • A USB-C breakout board exposing power and data lines, with resettable over-current protection and basic backfeed protection.
  • An RS-485 interface board.
  • A 74HC165 parallel-in, serial-out input board with added ESD protection.
  • A 74HC595 serial-in, parallel-out output board with driver stages for open-collector-style outputs.

Another intention is to design these boards using production-friendly component sizes from the start, such as 0402 and 0603 where appropriate, rather than only using larger prototype-friendly parts. I’ll admit I still feel slightly unsure about that, as it makes hand assembly in my own workshop less practical.

However, once a board design is validated and locked in, the aim is for that repeatable circuit to stop being a source of risk. I want to spend more of my time working on the unique parts of a project, rather than repeatedly rebuilding the same supporting circuits and hoping I have not introduced a new mistake.

Using production-friendly parts should also make the designs more realistic for PCB assembly.

That means choosing components and footprints that are suitable for assembly, while also paying attention to availability. Ideally, key parts should be available from more than one supplier, such as Mouser, RS, or LCSC, rather than relying on a single distributor.

This should make the boards more practical to manufacture. PCBWay and JLCPCB can often achieve faster turnaround when parts are available through LCSC, while UK-based PCB assembly services are more likely to work comfortably with parts sourced from distributors such as Mouser or RS.

The aim is to avoid parts that lock the design to a single supplier, keeping the boards more flexible if I need to change manufacturer, assembly service, or sourcing route later.

I’m also considering standardising these designs around four-layer PCBs rather than two-layer boards. There is a cost trade-off, but for small boards the difference is often modest enough to justify the benefits.

A four-layer stack-up would allow me to use a dedicated ground plane, improve return paths, simplify routing, and generally make the boards less sensitive to layout-related noise and EMC issues. That does not magically solve every EMC or signal-integrity problem, but it should give the designs a better foundation than trying to squeeze everything onto two layers.

For boards involving USB, RS-485, microcontrollers, shift registers, protection components, and external connectors, that feels like a sensible compromise. The aim is not to over-engineer simple circuits, but to make the repeatable designs robust enough that I can trust them in future projects.

Next Steps#

For this week, I’m focusing on the USB-C breakout board and the 3.3 V regulator module.

The 3.3 V Regulator Board#

The first version of the 3.3 V regulator board is based around an AP2114 3.3 V LDO (as previously used within the weather station project). The input side includes a resettable PPTC fuse, transient protection using a 5 V TVS diode, a basic 5.6 V clamp, and P-channel MOSFET reverse-polarity protection before the regulator input.

This is not intended to be a fully industrial power-protection design, but it gives me a much safer and more repeatable starting point than dropping a bare regulator into every new prototype. It also gives me a useful test case for validating footprints, protection choices, component sourcing, and PCB assembly assumptions.

The intention is not to make the board indestructible. The protection circuitry is there to handle common prototype mistakes: brief shorts, minor input transients, reverse-polarity connection, and the kind of mishaps that happen on the bench when moving wires around.

If someone feeds 30 V into a small 3.3 V regulator board, the realistic outcome is probably a damaged regulator board. That is acceptable for this design. The goal is not to survive every possible fault condition, but to make the board more forgiving when used in the sort of 5 V prototype environment I actually expect it to live in.

The USB-C Breakout Board#

The first version of the USB-C breakout board is intended to provide a simple 5 V input from a common USB-C wall supply. It will use a 16-pin USB-C connector, with 5.1 kΩ pull-down resistors on the CC1 and CC2 lines so that a USB-C source recognises the board as a sink device.

On the VBUS line, I’m adding a resettable PPTC fuse rated around 500 mA hold current and 1 A trip current, along with a Schottky diode for simple backfeed protection. As with the regulator board, this is not intended to be a fully featured USB power-management design. It is simply intended to make the breakout board safer and more predictable when used in small 5 V prototype circuits.

It will also expose the D+ and D− lines, allowing the board to be used with a USB-to-UART device such as an FT232 or CH340 when microcontroller programming or serial access is required.

AliExpress#

So why not just use AliExpress breakout boards?

I will still use AliExpress boards for quick experiments, and I expect that will continue. This is not intended as a criticism of those boards or the people making them. I genuinely like them, and they are often very useful little modules.

However, the goal of these building block boards is different. This is about validating my own designs, documenting them properly, and creating repeatable circuits that I can trust.

I am not just trying to get something working once. I want circuits that I understand, have documented, have validated, and can confidently reuse inside larger designs.

With generic breakout boards, there is always a risk that the design changes without notice. A module bought today may not be electrically identical to one bought six months from now, even if the listing looks the same. Components may be substituted, PCB layouts may change, and protection circuitry may be absent, simplified, or implemented differently from what I expect.

For quick bench testing, that may be acceptable. For reusable design blocks, it is less ideal.

By designing my own boards, I can control the schematic, component choices, PCB layout, documentation, and bill of materials. That makes the design more repeatable and gives me a known baseline when I incorporate the circuit into a larger project later.

Closing Thoughts#

This is still early work, but it feels like a useful direction. By turning common circuits into documented, validated building blocks, I should be able to move faster without treating every new project as a blank sheet of paper.

The real test will be whether these boards save time in future designs. If they do, then the effort spent creating them should pay for itself quickly.

If you are looking for reusable open hardware modules to speed up your bench testing, or need support designing custom breakout circuits, check out our catalog at Protoworks (NI).