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Powering a LED watch
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alank2:
Hi Everyone,

I'm thinking again about a project that is an AVR based watch maybe with a HCMS-3906 display.  I've recently discovered by playing around with these displays that some take crazy amounts of current to look good, but some like the 3906 can give a bright display on much smaller current down to even a few mA which is impressive.

The question though is how to power such a thing.

Q#1 - Is a switcher required for any sort of accuracy if using a 32kHz crystal?  or can I get by driving it directly from a rechargeable lithium battery like:

RJD2450
https://www.mouser.com/ProductDetail/Illinois-Capacitor-CDE/RJD2450?qs=sGAEpiMZZ

Q#2 - Does anyone know/recommend a battery charging IC for this battery that allows charging it from a 5V USB micro jack.

Goals for trying a project like this would be trying to keep it from getting too thick and simplicity.  I was thinking of maybe 3 PCB's with the center one being the active one meaning it has the display soldered directly and the underside of it a coin holder.  The top pcb would be just above the display surface to offer it some protection and the bottom pcb would be to give something between your wrist and the battery holder.

Thoughts/ideas?
DaJMasta:
A switcher?  Switching power supply?  If you can power your micro from the source, the crystal certainly doesn't need any excessive voltage or anything.

For batteries, the simplest for that sort of form factor is probably a small LiPo cell and a charger IC - since charging from 5V is very common, you should be able to find a plethora of LiPo charger circuits in very small packages that will run from a USB supply.  Standard LiPo voltage is 3.7V, too, so you're in a good range for a lot of micros without extra power circuitry.
alank2:
Yes, a SMPS.  My question about it is - will having or not having a SMPS cause any serious issues with accuracy on the RTC crystal.  Will it run slightly faster at 4.2V vs 3.6V, etc.
mariush:
The display runs at 3.3v but datasheet says it can run up to 5.5v (and min. 3.1v) so you don't need any ldo (regulator) for it. It can run straight from battery.
Whatever micro you choose will also run at wide voltage range.
So you just need a tiny batterry charger ic to reduce 5v to max 4.2v ...see https://www.digikey.com/short/p7wj8t  (assuming lithium battery)

A micro running at <1 mhz will use maybe 1-2 mA of current but you could set a wake-up / interrupt on button press to enable display and show time, and other than that sleep and wake up every 100ms or something like that (set a timer, interrupt on overflow etc)

Audioguru:
My father-in-law had an LED watch 40 years ago. Every time he looked it was not showing anything. Useless. Why do you want to make such an old thing?
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