Skip to main content
Topic: DaTajm - A sneak preview of my new homemade wristwatch (Read 40054 times) previous topic - next topic

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #15
Hi !
Nice job, very inspiring.
I was looking for a cool project combining small ICs, RTC and power consumption optimization, and I think I found one !
As someone said, I was thinking about soldering the LEDS on the top of the lowest PCB. Holes on the middle PCB, and on the top PCB, "holes" of copper.
I have some questions about your design : the screws are threaded in the lowest PCB only ? Is it robust enough ?
What are those pads next to the battery positive terminal ?


I made some PCBs on Eagle with MSP430G2452 16QFN package. Thinking about a capacitive touch button as TI MSP430G2xxx has CapTouch IO feature.

Regards,
Val'

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #16
[quote author="Montspy"]I have some questions about your design : the screws are threaded in the lowest PCB only ? Is it robust enough ?
What are those pads next to the battery positive terminal ? [/quote]
The holes are 1.3mm in the bottom and middle pcbs and 1.5 mm in the top,  I'm using M1.4 x 4 mm screws bought at ebay  http://r.ebay.com/bAaohV

I'd say that they hold the pcbs together good enough.  I even put in a CR1220 battery (0.4 mm higher than the size the watch was made for) and could screw it together without any issues.

I've opened and closed the watch at least ten times and it still feels rigid and stable.  But of course I've been careful not to over-tighten the screws since I think the threads in the FR4 would strip if you're a bit heavy-handed with the screwdriver.

The holes are non-plated,  maybe they would be even more robust if I had them plated...

On the top pcb there's a row of five pads that are the standard PIC ICSP progamming connector at 0.1" distance that I can connect via pogopins to my Pickit3 for (re-)programming.  But during development I just soldered a standard 0.1" header to it so I could have it plugged into the Pickit all the time.

[attachment=0]

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #17
And yes....  I glued the bottom and middle pcbs together with a few dabs of epoxy.  I'n not sure that was necessary but it makes life things easier.

Originally I planned to have two large plated holes in the middle PCB and some exposed copper on the bottom pcb so I could solder them together, but I realized that the plating of the hole might come in contact with the iscp pads on the top board so I took them away.

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #18
Thank you very much for the link to the screws, I was looking for some small screws and those are perfect.
The design is ready for prototyping I think. I'm just waiting for my Launchpad which is out of stock (no luck!). And for the answers from Seeedstudio since the cutouts are a lot more complicated.
I tried to use all the good advices I got here.
It's sad we can't use blue, green, pink, white LEDs since their voltage is >3.0V...
Plus I need to use current limiting resistors as the MSP430 don't have any built-in.

Val'

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #19
Most green and yellow leds have Vf well below 3 volts so they should work pretty well.

I used two current limiting resistors in my watch. The less are in a 6 by 2 grid and the anodes connects directly to 6 port pins and the two cathode groups are connected to two pins via resistors.  Since I only drive one single led at a time I don't need to worry about getting different light levels depending on how many less I light up.

My tip is don't ask Seeed,  just send the files and look happy. I've exceeded the max 5 subboards rule and had 25 or so tiny subboards on a pcb.  Didn't ask first and it was accepted. Sometimes they even do v-scoring.  And another guy did panelizing with both long slots and drillines.

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #20
I'm gonna use red and orange LEDs. As I have less room to put the µC, I used a 16QFN, and I have to drive 16 LEDs (12 for hours + 4 for 1-minute precision). Using charlieplexing gives 5 pins, and 5 resistors.
Seeedstudio order is passed, I need to buy the crystal, and wait ;)

My Launchpad is sent too, so I'll be able to program before the boards arrival, which is cool.

We'll see how it goes.

Val'

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #21
Hi !
I built my watch :)

I got one major problem : the contacts to drive the LEDs from the top to the bottom were not good enough, I had to use magnet wire to get good connections. Well, this is a working prototype. I have only unpopulated picture for the moment.
[attachment=3]
Here is the back of the bottom PCB : MSP430G2452 in 16QFN, 32768Hz crystal, few passives and contact grid (bottom left)
[attachment=2]
The middle PCB with hole for the LEDs, coin cell, and 5 current limiting resistors (soldered in the rectangular castellated holes).
[attachment=1]
And the bottom PCB with LEDs and positive coin cell pad.
[attachment=0]

For a video, click here. I was under a huge light to be able to record with my phone. External LEDs are red, center ones are orange. They are brighter than they appear.

I paste the Youtube description here as it contains more technical info :
Quote
This is the demonstration of my prototype of LED wristwatch.
It has 12 red LEDs to display hours and slices of 5 minutes. And 4 orange LEDs to obtain a minute-precision display.
How to read it :
Press. First is the hour. It is displayed like on a clock dial.
Then it changes after 1 second to minutes display. Each red LED is 5 minutes.
Then you add the orange ones : none is +0, the top one is +1, the right one is +2, the bottom one is +3 and the left one is +4.
It uses a capacitive button. It is powered by a standard CR1216 coin cell (~25mAh).
The MCU is a MSP430G2452 for Texas Instruments (the same one that come with the Launchpad). The capacitive button is handled by the built-in PinOsc in the MCU. I added a 37.768kHz crystal to keep time as accurately as possible.
It is composed of 3 PCBs :
- The bottom one holds the LEDs.
- The middle one has 5 current limiting resistors and a hole for the coin cell.
- The top one has the MCU, crystal, capacitive button and a few passives.

Thanks to tle Low Power Modes of the MSP430, I was able to lower the current draw in sleep mode to less than 35µA (Includes wake up every 250ms to keep track of time and to check if the capacitive button is pressed).
The 16 LEDs are controlled by only 5 pins by using charlieplexing (http://en.wikipedia.org/wiki/Charlieplexing).

PCBs were produced by Seeedstudio Fusion PCB service (really cheap and high quality boards).

The QRCode links to my LinkedIn account : http://www.linkedin.com/in/valentintrimaille

The idea came from matseng (viewtopic.php?f=56&t=5417).

Regards,
Val'

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #22
Ooooohhhhh....  Really nice!  I'm glad that my first prototype inspired someone to take it to the next level.

The holes for in the top pcb where the outer and inner rings coincide - are they drilled as vias/holed or milled (Eagle laye6 46)?  I thought that Seeed don't like to make two holes that are too close due to the risk of breaking the drill bits.

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #23
Thanks !
About the "holes". They are not holes. I just removed copper and coating. It is translucid enough to see the leds shine through.

Val'

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #24
Ah, I see.  That's smart.

The wall here at the milled half-moon seems very thin - do you have the actual size of it? And did Seeed complain about it?  I think the spec is minimum 1 mm thickness of "walls".

[attachment=0]

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #25
Looks great Val!  How long do you think the watch will last one one battery?

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #26
The battery will be completely drained in 25mAH/35uA hours = 25000/35=714 hours = 29 days.  So I guess 2-3 weeks if looking at the time a few times a day.

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #27
[quote author="matseng"]Ah, I see.  That's smart.

The wall here at the milled half-moon seems very thin - do you have the actual size of it? And did Seeed complain about it?  I think the spec is minimum 1 mm thickness of "walls".

[attachment=1][/quote]
Yeah, they are 0.6mm (24 mils).
Seeedstudio did not sent me anything about that, and the milling is very clean. I think 0.5mm (20mils) is OK.
They were worried about the castellation of the middle PCB because the milling cut more than half of the via. Some boards are OK, not each board.


[quote author="matseng"]The battery will be completely drained in 25mAH/35uA hours = 25000/35=714 hours = 29 days. So I guess 2-3 weeks if looking at the time a few times a day.[/quote]
Yes, exactly. I managed to get the MCU to sleep even more, but I haven't been able to measure the current draw with the last firmware. It's sad rechargeable CR1216 (like CR2032 -> Lir2032) doesn't exist.

2-3 weeks is too short for me, but with the capacitive touch button, the MCU has to wake up every 250ms to monitor the sensor.
Maybe using a specialized IC, but they all draw more than 15µA, which is too high to justify the addition of an new IC (less room, more expensive, more complex, etc. ...).

Here are the Eagle files of this v1 : [attachment=0]

Val'

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #28
I totally love both of these! I'm going to have to cook one up, I think. Did you guys both use fusion PCB? What layer do internal routes go on, dimensions just like OSHPark?

Re: DaTajm - A sneak preview of my new homemade wristwatch

Reply #29
Yes, I used Seeedstudio Fusion - if they still are called that, but I think the name is only "PCB Service" today.

I only used 2-layer board so no internal routes.