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Jetpack Hardware Design

Overview

The Jetpack tracker is relatively straightforward, with a few key underlying goals and principles dictating its specific design decisions.

You can download the schematic to see the electrical design at a high level.

Design goals

  • Easy for end-users to use
  • Cheap to fab / assemble
  • Reliable

Some specific decisions are elaborated on in more detail below.

Easy for end-users to use

Jetpack aims to be easy to learn and use to a low-skilled beginner end-user.

To this end, it relies on easily-acquired hardware, the RPi Pico board, as a base, to allow:

  • Easy USB access to "flash" the software to run the tracker
    • Being able to drag/drop the firmware makes the hardware accessible to nearly everyone
    • The RP2040 and related chips' ability to do this is a "killer feature," -- you must have this
  • Simple to solder to
    • Easy pin breakouts to access

Cheap to fab / assemble

JLCPCB assembly is how Jetpack accomplishes maximally low-cost production.

The low cost is accomplished by:

  • Designing the PCB to stay within standard PCB fabrication capabilities
    • As in, don't do things like micro-vias if you don't need them
  • Using "basic" components (eg common resistors, caps, etc) instead of "extended" parts which incur extra assembly fees

Cost savings example

In the schmeatic of Jetpack, you can see a bank of capacitors in parallel, achieving a large total capacitence. A few smaller-capacitence "basic" caps were joined together instead of a larger "extended" single capacitor, for cost savings.

RPi Pico as a Component

The RPi Pico board is a mass-produced and extremely well-tested piece of hardware.

The board was incorporated into the design of the tracker for those reasons.

Don't make what you can buy for cheaper! It's a great building block!

Not to mention, learn from the pros how to lay out components and fab, they're literally doing it at a global scale.

Reliable

Hardware can be hard to use sometimes. It pays to study the components, characterize them, and learn how to work around their rough spots.

Jetpack had to work around 3 notable hardware issues:

  • RPi Pico SMPS using Solar
  • Inrush Current Limiting
  • GPS Module Integration

RPi Pico SMPS using Solar

The SMPS (Switched-Mode Power Supply) on the RPi Pico can seize up in a scenario where you feed a solar-panel power supply to it under conditions where power availability slowly increases during sunrise (ie every day).

This was identified early in tracker development. It was not possible to swap out the SMPS on the RPi Pico board, so a workaround was put in place -- the Power Monitor chip.

There is a document observing this issue and ultimately resolving it via a power montior. It's long and not very pretty, but it's how I worked through the issue, so if it's any value to you, enjoy!

Inrush Current Limiting

To limit power consumption, Jetpack "turns off" the GPS module and the radio transmitter at various times during the lifecycle of the tracker.

However, this introduces a design challenge -- how to handle the sudden "turn on" inrush of power consumption of (say) the GPS module when enabled (leading to voltage sag and possibly undercurrent/voltage reset).

The Jetpack design considers how to slow inrush to avoid power supply voltage sag

The document linked considers various high-side and low-side switching tradeoffs, as well as simulates how to slow the inrush current itself.

The final design incorporates these insights.

GPS Module Integration

The cheapest suitable GPS module available was the ATGM336H, a competent module, but has quirks both in software and hardware.

This document captures the research that went in to learning how to incorporate it.

Parts

On the Jetpack tracker page is the downloads section including the BOM (bill of materials), which specifies the part numbers of the components on the assembled PCB.

Using Basic components, such as the multiple capacitors (part number C16780), reduces the cost of the production of the board.

JLCPCB lets you search parts on their search page, such as the C16780 search and product page.

Wherever possible, Jetpack uses the cheapest and most in-stock parts possible

This maximizes cost savings and keeps the tracker available to purchase in the future with no new hardware design changes.

KiCad

KiCad was used to put the board together and also is the source of several of the screenshots of the tracker used on the website.

The specific KiCad design files are not available for download right now.