Project of watch with a RP2040 and a e-ink screen

[aarstt]

Hi everyone,

For years, I’m thinking about coding and making a small electronic project. So today I take the plunge! I’m a beginner, I've never code and I have never really solder. A few days ago, I didn’t know what’s a microcontroller…

So, my project it’s to make a watch as thinner possible with which I can read the hour and why not the date.

Currently, I can devise my reflection around four points:

– After some research I think that use a RP2040 microcontroller. I think it’s a bit complicate to starting with a RP2040 like that and it will be easier to start with something like the ¹Adafruit QT Py RP2040. Thus I can have already a board with a USB-C connector and some GPIO pins.

– For the screen I was thinking it will be interesting to have an e-ink display about power consumption, I was thinking that the ²Adafruit 1.54" Monochrome eInk / ePaper Display with SRAM - 200x200 with SSD1681 could be fine. I’m asking me about the board who accompanies this screen. Do I have really need it if I’m using a Adafruit QT Py? Because if I don’t really need it will be good news to connect the screen directly to the Adafruit QT Py. Thereby, I can save place.

– Regarding the power, given that my project is mobile I need a battery. I don’t know the power consumption of this watch project but I hope it will be low than I will be able to use the smaller battery possible. The ³Lithium Ion Polymer Battery - 3.7v 150mAh could be fine. In this case I’m asking about how to connect this battery to the Adafruit QT so as to have the alimentation of the watch and the possibility recharge it with the USB-C connector. Otherwise, if possible, why not using a coin cell battery.

– About the code, I've never really code, I’ve played a little bit with Swift Playground but nothing else. From what I understood, micropython is adapted at my project.

I will be very happy to read some comments or questions about my reflection. All your ideas will be very important to move forward my project.

Thanks a lot!

¹Adafruit QT Py RP2040
https://www.adafruit.com/product/4900

²Adafruit 1.54" Monochrome eInk / ePaper Display with SRAM - 200x200 with SSD1681
https://www.adafruit.com/product/4196

³Lithium Ion Polymer Battery - 3.7v 150mAh
https://www.adafruit.com/product/1317

[dastels]

I'd say you're on a good path (maybe the right path... but there are so many options it's hard to make that claim).

For battery power I would go with a LiPo. The biggest that will fit. 500 is probably going to be it. You can use a LiPo BFF https://www.adafruit.com/product/5397 to connect the battery and provide an easy (i.e. without having to remove the battery) way to charge it.

And I advise you use the display and board as is. Given your coming into this fresh, that's the best way. Yes, you'll end up with a somewhat chunky watch, but it the hardware will be quick and relatively easy to put together and get running.

Being new to soldering, I suggest you do a few practice projects first.

Dave

[aarstt]

Thank you very much, Dave. Thanks to your answer, my project takes shape!

The LiPoly Charger BFF looks exactly what I need. Unfortunately, it is out of stock but anyway, I think I will start my project without. I’m so impatient to begin and I think I have already a lot of stuff to do so this part can wait a bit. About the battery, do you think that’s 500mAh is really essential to power the watch? That’s not too much (and too big) for my small watch?

For the screen I was learning how to wiring it at the QT Py RP2040 and I fell on this message:

¹ "Do not update more than once every 180 seconds or you may permanently damage the display"

Obviously, I was imaging to refresh the display every 60 seconds. So maybe it’s not adapted to my project. I found the ² SHARP Memory Display Breakout - 1.3" 96x96 Silver Monochrome and according to the description "[…] is a cross between an eInk (e-paper) display and an LCD. It has the ultra-low power usage of eInk and the fast-refresh rates of an LCD". However, I had read is "write only […] and the downside of a write-only display is that the entire 96x96 bits (1,152 bytes) must be buffered by the microcontroller driver". I don’t understand if the QT Py RP2040 is compatible with it. Do you think, it is?

Though my project is becoming clearer some news questions appear:

For example, I wonder how to adjust the hour on the watch? I maybe also need buttons to adjust it, no? And if my watch switch off, should I to readjust the hour after each reboot? Maybe I can integrate two buttons on my project but do I have enough GPIO on the QT Py RP2040 for that if I’m using the LiPoly Charger BFF and the screen? On the pictures (https://cdn-shop.adafruit.com/970x728/5397-09.jpg) I saw all the GPIO of the QT Py RP2040 are soldered with the LiPoly Charger BFF…

Regarding the development I understood I need headers, Breadboard, Jumper Wires in addition to my components. Something else maybe?

Thanks a lot for your help!

¹ https://learn.adafruit.com/adafruit-1-5 ... pectations
² https://www.adafruit.com/product/1393

[dastels]

You can try to go smaller. They are available in the shop down to 150 (and typically the smaller capacity means smaller size). It comes down to power consumption of the circuit, and how long you want between charging. My approach would be to get a selection and test the final project with each to figure out where the sweet spot is for you and your usage. Too much isn't an issue, but too big (physically) is. Again, I'd experiment and find the battery that gives the best (or at least reasonable) combination of size and capacity.

Yes, you can use an external charger while working on the project. A big advantage of the BFF is that it allows you to recharge the battery without having to remove it from the circuit. I really like to have a built in charger like that. It means a lot less hassle to charge and it means less wear and tear on the battery connections since the wires won't be moved/flexed each time. The connection of the wires to the battery are a weak point.

That update rate limitation is a drawback of using an eInk display. For a watch you really need a 60 second update cycle if not a 1 second rate. Or even faster.. think about if you add a stopwatch feature. The RP2040 has loads of RAM so buffering it won't be a problem.

The 96x96 SHARP display is discontinued, but https://www.adafruit.com/product/3502 is the same size (and technology), but with 168x144 resolution.
The LiPo charger doesn't take any GPIO unless you want to monitor the battery/power - which you probably do - in which case it uses 1 GPIO (A2). Only

The screen will take 3 (MOSI, SCK, and one digital output for an select). If you want to use something on the I2C bus you can use the STEMMA-QT connector which doesn't use the SDA & SCL edge pins. That would leave you with 7 GPIO for buttons or whatever. Only 5v, groundm, and optionally A2 need to be connected. All the pins on the BFF are soldered to provide a solid mechanical structure.

It sounds like you have a handle on what you need. I approach would be to get the board, solder the header pin on and use them with the breadboard to develop the hardware design, and the software. Once everything is finalized, buy another set of parts and assemble it without using header pins as much as possible. The goal then is small size and solid connections (i.e. everything soldered) Wiring things together directly will give you that. I recommend the stranded silicon coated wire for that. It's flexible and the insulation doesn't melt when soldering it. E.g. https://www.adafruit.com/product/3164.

Dave

[aarstt]

Thanks a lot Dave,

I will start with a 150mAh battery and I will maybe buy an other battery later when I will have some step back.

The BFF is really perfect. I can’t imagine my watch project without the functionality which offer it.

OK that’s perfect I will go with the 168x144 resolution screen SHARP. I will also buy some small buttons given I have enough GPIO!

Everything sounds like clear except one thing :

I’ve start to learn about the software that I've to develop and I found some exemples of projects that use RTC module (Real Clock Time). Do I need it with the Adafruit QT Py RP2040?

Thank you very much to your answers. They are really helpful and realize my project!

[dastels]

The RP2040 does have something it calls an RTC, but there's no indication that it's persistance across power cycles. It appears to be mainly for support real time (i.e. time on the clock) alarms, interrupts, wake-up, etc. If you want time to be kept regardless of power you'll need a battery backed RTC module.

Dave

[aarstt]

Ok. Thank you