I will be going on a journey of creating my first PCB and who knows perhaps it can evolve into a product. This page will keep track of development and specifics. The general idea is creating a small form-factor MFM that allows optional interchangeable sensors. Due to the form-factor it has the code-name Micro.

Table of Contents

Requirements / Orientation

For simplicity the Micro PCB will be manufactured and (mostly) assembled at JLCPCB. This means that there are limitations to what can be assembled on the PCB.

Form factor

The Micro should be able to hide in plain sight, this should improve robustness and decrease probability of vandalism. The closest standardized form-factor currently is an Adafruit Feather. However the Feather adheres to imperial dimensions and the Micro will adhere to metric dimensions.

Open

Power

1 month plan

The first iteration of the Micro should be able to go a minimum of 1 month on a battery. At 400μA we require at least a 300mAh battery. The biggest advantage is that the enclosure can be very small with such a battery.

Possible candidate is a Lithium-Ion-Polymer (LiPo) battery. There are quite a few small (35mm x 45mm) variants that provide up to 800mAh (2 months at 400μA). Besides the size, LiPo batteries are also rechargable. Embedding a charging circuit into our circuitry is feasible. (TP4056)

Open

Because LiPo batteries have a distinct natural discharge, the requirement must be at least 300 / 0.80 = 375 mAh (assuming 20% drop in 1 month).

With a LiPo battery, using a 60mm x 65mm enclosure is an option.

1 year plan

For the first versions the Micro should be able to go 1+ years on a battery. Assuming the use of an AA battery - which stores around 2000mAh - we would have to use less than 200μA on average. The amount of batteries depends on the enclosure.

Possible candidate is a Lithium Thionly Chloride battery. The battery has a relatively high capacity with 2700mAh, but most importantly delivers at 3V. The downside is that it only delivers a continuous 50mA. For wireless communication this means a (super)capacitor will be required.

Battery location

JLCPCB does not provide battery holders for assembly. This means a separate battery holder will most likely be required in the case. Ideally the holder connects to the circuitry with a JST 2-PH connector. This allows the circuitry to be disconnected from the battery holder when replacing batteries.

A replaceable battery also allows for easier maintenance and less waste.

Processor

The processor must be able to enter a low power state to avoid unnecessary power consumption. It should provide USB support as well.

STM32F070CBTx

Peripherals

The Micro’s firmware must be easily updated in case of new version or when more external peripherals become supported. To ease this process the circuitry will provide a USB port which allows firmware upgrades.

In case of the 1 month power plan the USB doubles as charger for the internal LiPo battery.

External peripheral

Single external peripheral connector

Final list

References