How crazy is this MCU wristwatch idea ? - Page 1
Forum section: electronics / projects

author: opablo
author_rank: Regular Contributor
timestamp: December 07, 2012, 09:05:14 pm
content: I was surfing the intertubes when I bumped into this:http://www.ebay.com/itm/Micro-7-Segment-Display-4-Digits-DIP12-3PCS-/320974305751?pt=LH_DefaultDomain_0&hash=item4abb8f39d7That display looks soooo cool... So then I immediately though of this:Things that I do NOT care about- Only way to set the time is to re-flash the damn thing jaja, no problem- Only way to see the time is by pressing the button that enables the display.- To reduce wiring the button should enable the display through MCU code.- PriceThings that I DO care about.- Battery life- Sexy appearance-- hence the use of to-39 transistors; I love how they look-- no SMD. only through-hole technology-- standard prototyping boardMy question is... what does your guts say about it's battery life ?Maybe if this have a 1 week battery life it could be stored without the battery and only used for special occasions

author: opablo
author_rank: Regular Contributor
timestamp: December 07, 2012, 09:09:39 pm
content: I just remembered the existence of transparent heat shrink... maybe the entire thing could be heat shrinked each time you replace the battery (I don't care if I have to expend 1 US$ each time I want to use it.... it's not an everyday watch...

author: c4757p
author_rank: Super Contributor
timestamp: December 07, 2012, 09:10:26 pm
content: That MCU can shut down into a very, very low-power mode, so if the display is off, it all comes down to the idle power used by the boost converter. What is that?Edit: Remove one "very" from "very, very low-power" as the clock oscillator is still running.

author: mariush
author_rank: Super Contributor
timestamp: December 08, 2012, 12:27:16 am
content: You can further simplify it by going with something like apic16lf1503or something similar, which allows you to run it directly from a cr2032 battery instead of having to use dc-dc converters.The chip will run from 2.3v to 5v and you can lower it down to as low 32khz while idle (but do check the notes in the datasheet, the lowest frequency it can handle may depend on the voltage used)You can then use aPCF8563(or something similar) along with an 32khz oscillator to keep track of everything time related and you basically just wake up the processor when buttons are pushed, to retrieve time/date/etc from the time chip and show it on screen or set up the time/date.The timer chip uses around 0.3 mA, the pic less than 1 mA... a CR2032 has about 220 mAh so the whole thing should last more than a weekWith DIP chips and nothing smd, the whole watch would already be tall, so you might as well go with a rechargeable battery, something like this:http://www.hobbyking.com/hobbyking/store/__11874__ZIPPY_138mAh_20C_Single_Cell_.htmlhttp://www.hobbyking.com/hobbyking/store/__18790__Turnigy_nano_tech_130mah_1S_25C_Lipo_Pack_Nine_Eagles_style_T1_Single_Rail_for_2010_.html

author: opablo
author_rank: Regular Contributor
timestamp: December 08, 2012, 12:58:48 pm
content: mariush: those are all great ideas !!! all of them !Now I think the project is doable and I will start doing the maths... thanks

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 08, 2012, 01:26:11 pm
content: T0-39 transistors? what is this, the 70's..?QuoteWhy would that be....?You have a button. Use some imagination....

author: mariush
author_rank: Super Contributor
timestamp: December 08, 2012, 04:51:17 pm
content: Agreed.Keep button pressed for 5 seconds ->ask if user wants to enter set mode (show on screen SET?)no reply in 5 seconds = cancelpressed button within 5 seconds = entered set moderepeat four times starting with the left most digitblink the current digitwhen button pressed, increase digit by oneno press for 5-10 seconds jump to next digitwhen last digit manipulated, exit set mode.super easy to do with a microcontrollerI'd lay the npn transistors by the sides of the display, 2 on left, 2 on right ... two to-92 cases should make a nice round circle on the sides, if you're into round things.For a one off project you could go for some pre-biased npn so that you won't have to use a resistor between the base and the chip pin so you'd keep the layout nice .. something like UNR421 comes to mind:http://www.digikey.com/product-detail/en/UNR421D00A/UNR421D00ACT-ND/972577

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 08, 2012, 09:47:31 pm
content: Some PICs have built in RTC, which should simplify your design.

author: c4757p
author_rank: Super Contributor
timestamp: December 08, 2012, 11:34:56 pm
content: I know this doesn't contribute anything, but I felt I'd add: Even if I'm the only one here who agrees with you, I think TO-39 (and TO-18) are sexy too. There's a kind of dignified "old-style industrial" look to them. I have a few 2N2222 (the real ones, not PN2222) on hand; I'd use them more often, but lately I've been coincidentally doing a lot of circuits with complementary pairs and the only PNPs I have are TO-92 (or SOT-23). I only got the 2N2222s by accident and I just don't see justification for the cost of picking up some 2N2907s too.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 08, 2012, 11:48:11 pm
content: Quote from: c4757p on December 08, 2012, 11:34:56 pmMaybe, but not in a watch, which you want to be small. SOT23 is not significantly harder to solder than through-hole.

author: c4757p
author_rank: Super Contributor
timestamp: December 08, 2012, 11:57:01 pm
content: Quote from: mikeselectricstuff on December 08, 2012, 11:48:11 pmThat ship sailed with the choice of microcontroller. It would need to be a small QFP at largest.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 09, 2012, 12:43:49 am
content: Quote from: c4757p on December 08, 2012, 11:57:01 pmPlenty of 18 and 20-pin devices that could do the job - even a DIP18/20 isn't that big, especially if you have SM transistors/passives underneath it.

author: Everett_Tom
author_rank: Contributor
timestamp: December 09, 2012, 01:01:07 am
content: Texas Instruments has a development kit turned watch:http://processors.wiki.ti.com/index.php/EZ430-ChronosYou could probably get one of those, and then mod it to use your new display. I'll bet there's info on the current watch's battery life, you could then adjust by estimating the power consumption of your new display.Assuming the voltage levels of the TI part were high enough to drive the segments.

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 09, 2012, 09:09:33 am
content: These small displays probably need no transistors at all and be driven my the MCU directly with only 8 current limiting resistors. Even if you need transistors, there are dual transistors in SOT-363 packages (that slightly smaller than SOT23). Ditto for resistors, you can get arrays that pack four of them in a 0603. Use a MCU in QFN package and the whole thing probably does not need to be any bigger then a regular watch.

author: opablo
author_rank: Regular Contributor
timestamp: January 19, 2013, 08:59:20 pm
content: Hi guys... I'm the OP; sharing with you an update on my project.Why I started thinking about this? Because I liked that 4x7seg display and ended up buying it.Why it evolved into a real project ? Because I wanted to learn to use an ATtiny (And program it using an Arduino nano; because I don't have a chip programmer)(I'm only an electronics hobbyist not a professional like many of you)Power supply: Defined as a CR2016 lithium cell. Will be glued and hidden between the board wiring and the wristband.Voltage booster: Discarded when I learned that there are attinys that work on 1.8->5.5vTime tracking: I started to want a real/useful clock so I learned that the only way to accomplish this is to use a dedicated RTC chip and completely disable the MCU. I chose a DS1302 RTC that works on 2.0->5.5v with it's own crystal and 3-wire serial. It claims to use less than 300nA at 2v.Microcontroller: Needed 7 lines for segments + 4 lines for digits + 3 lines for communication with the RTC + 3 unused lines for reprogramming = 17 lines. I choose an ATtiny2313V running at 1Mhz, works on 1.8->5.5V, have 18 io lines, 2KB or program memory and comes in DIP.Transistors: I used old and rusty TO-18 NPN ones circa 1987. Just because I love how they look. In the original design I said TO-39 but that is just a mistake; I was referring to TO-18. The base resistors are tiny 1/8w ones hidden behind in-between the wiring.Switch: Picked a small but usable one and picked one that has a metal top and black button so it matches the general style/colors of the rest of the board. A lot of attention and time was dedicated on how it looks.Theoretical calculations about power consumption and battery life:Power supply: CR2016 assuming a 90mAh down to 2.0vIdle state: (can't measure) 300nA -> 34 years !?!? what !? Is this calculation done right !?Time reading: (measured) 35mA -> 2.6 hours -> 2.5 years assuming 10 seconds of reading per dayTrying to measure the idle power my best DMM (UT61D) said 0.0uAI need Dave's uCurrent but can't afford one.Obviously the schematic, code and everything is available for any of you. Just ask for it.How it looks so far:Missing work: Solder thick copper bridges where the fabric(surfer style) wristband will be attached and to submerge the back wiring with what I think you call hot melt glue.

author: EEVblog
author_rank: Unknown
timestamp: January 19, 2013, 09:16:49 pm
content: Major bragging rights for anyone who gets through a US police state checkpoint wearing one!And please have a camera rollingDave.

author: marshallh
author_rank: Unknown
timestamp: January 19, 2013, 10:13:52 pm
content: I know you said you like through hole for some reason... but you could really shrink the size of that going to surface mount.I have a HP 4-char LED display a bit like that maybe I will do something with.

author: svofski
author_rank: Regular Contributor
timestamp: January 19, 2013, 11:57:29 pm
content: I think it looks greatCongratulations!

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 20, 2013, 12:23:17 am
content: It would look even nicer with gold topped white ceramic ICs

author: cwalex
author_rank: Frequent Contributor
timestamp: January 20, 2013, 03:52:26 am
content: Looks greatI love the look of the old led displays and the metal can transistors. Nice choice and great project.