Very small RTC and battery for RP2040 MCU based, 1.28inch Round LCD

So, I’ve got one of these RP2040 MCU based, 1.28inch Round LCD (onboard sensors) | Waveshare | Core Electronics Australia, I’ve got it working with micropython easily enough. It’s pretty cool in what it can do.

To make it work as a watch (I’m thinking of turning it into a pocket watch sort of thing) I need an RTC. The restriction being I want it to be as small as possible, and fit within the formfactor of the main board.

The Makerverse Real Time Clock (RTC) with Supercapacitor Backup RV-3028 | Core Electronics Australia looks pretty great, but I’m not sure about how tall it is with the capacitor.

The Gravity: I2C SD2405 RTC Module | DFRobot DFR0469 | Core Electronics Australia also looks pretty great, but I’m not sure of the height. It also looks like I could trim the board down to remove the mounting holes and the gravity connector, but again I’m not sure on the height.

This looks to be the smallest battery 3-pin LiPo Battery for PiJuice Zero 500mAh | Core Electronics Australia, would it be safe to just connect earth and gnd and ignore the 3rd pin?

One last question: Could I use one of the battery based RTC, but remove the battery holder and just connect it to the same lipo? The whole thing is going to be in sleep mode most of the time, RP2040 in deep sleep so, drawing very little power, and the RTC could draw from the battery to keep time.

1 Like

Hi Doug
I don’t know anything about what you are attempting BUT

Not a good idea to interfere at all with these boards. They might be multi layer and you could do irreparable damage to other circuits. Opening out mounting holes is a no no as they might have bias connecting the top and bottom layers or anything else.
Cheers Bob

1 Like

Hey Doug,

This sounds like an awesome project! I think the form factor may be a bit of a problem with it which is why I usually suggest the Bangle.js as a great way to have a DIY smartwatch as it is already in a sizeable form factor.

Bob is absolutely right here, you never know the magic behind those layers of a PCB and where a tracer might be to get cut and ruin your component. I do not recommend cutting, drilling or carving at PCBs unless you are willing to potentially sacrifice the board.


1 Like

The core electronics ones have the kicad files on the github repo, so I’d check that before trimming. The DFRobot is just 2 capacitors and a couple of pull up resistors and then the main component, I think it’s pretty safe. Actually with the DF Robot I’m (almost) tempted to take the SD2405ALPI-G off the board so I can set things up more compactly.

The bangle.js is significantly more expensive, also I’m more familiar with the RP2040 programming.

Though you did prompt me to look around. This T-Watch 2021 – LILYGO® would have been a much better choice, ESP32 based and has everything except the battery. That said, the board I bought was (more or less) and impulse buy. The goal is more to make a fun project than to have a watch.

1 Like

For power you could use one of the following and they have only + / - leads.
I used the 120mAH for a device similar in size to what you are using, it is very small.
Just a suggestion.

For the RTC you don’t need much circuity and any manufactured one will always be too large.
This might be better custom made using surface mount components, although you need specialised gear to make it. The use or not of a button cell is dependant on the particular RTC chip, probably you can set it up to not use it. It might require a configuration change to the RTC chip.

Anyway all the best, kool project
Cheers Jim

EDIT: The PR2040 has an accurate clock built in and should keep time ok as long as the device has some power. I think it can be setup to run in deep sleep mode. The designers probably didn’t see the need for a RTC, the only problem being when the battery dies the time is lost.


Hi Jim,

Thanks for that! I didn’t realise the rp2040 had that good a clock. I had a look at the spec and it oes use more power than an RTC (mili A vs micro A), but it’s probably good enough to have a play. My thought is to put it to sleep when a GPIO is in one state (probably low). It’s probably good enough to give it a try, see how I go. I tend to get caught in analysis paralysis and the the idea/project stagnates.

So, I might just mock it up on a bread board and give it a try :slight_smile: I’ll see if I can find a battery connector locally


1 Like