Author Topic: Powering a LED watch  (Read 2453 times)

0 Members and 1 Guest are viewing this topic.

Offline alank2Topic starter

  • Super Contributor
  • ***
  • Posts: 2200
Powering a LED watch
« on: January 16, 2019, 01:56:41 am »
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?
 

Offline DaJMasta

  • Super Contributor
  • ***
  • Posts: 2392
  • Country: us
    • medpants.com
Re: Powering a LED watch
« Reply #1 on: January 16, 2019, 02:53:25 am »
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.
 

Offline alank2Topic starter

  • Super Contributor
  • ***
  • Posts: 2200
Re: Powering a LED watch
« Reply #2 on: January 16, 2019, 02:57:51 am »
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.
 

Offline mariush

  • Super Contributor
  • ***
  • Posts: 5170
  • Country: ro
  • .
Re: Powering a LED watch
« Reply #3 on: January 16, 2019, 03:13:10 am »
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)

« Last Edit: January 16, 2019, 03:18:19 am by mariush »
 

Offline Audioguru

  • Super Contributor
  • ***
  • Posts: 1507
  • Country: ca
Re: Powering a LED watch
« Reply #4 on: January 16, 2019, 04:30:43 am »
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?
 

Offline ArthurDent

  • Super Contributor
  • ***
  • Posts: 1193
  • Country: us
Re: Powering a LED watch
« Reply #5 on: January 16, 2019, 06:18:27 pm »
Forty plus years Texas Instruments sold one of the first digital watches using a rather small (by today's standards) 4-digit LED display. Red was about the only color LED available back then. Because the LEDs were power hungry T.I. had a button to turn the display on when you wanted to know the time but remain unpowered otherwise. I recall being thrilled to get the watch on sale for $35 ($225 in 2018 dollars) and the same quality watch today would probably sell for around $1. Here's a short video of the same type of watch I had.

 

Offline mariush

  • Super Contributor
  • ***
  • Posts: 5170
  • Country: ro
  • .
Re: Powering a LED watch
« Reply #6 on: January 16, 2019, 06:25:16 pm »
Here's some more inspiration:







 

Offline alank2Topic starter

  • Super Contributor
  • ***
  • Posts: 2200
Re: Powering a LED watch
« Reply #7 on: January 16, 2019, 08:03:57 pm »


Some of these are power hungry, but some are quite bright with little current.  This model is very nice at only 5mA.
 

Offline alank2Topic starter

  • Super Contributor
  • ***
  • Posts: 2200
Re: Powering a LED watch
« Reply #8 on: January 17, 2019, 12:30:44 am »
I found the MCP73831 series charge controller IC.  It comes in 4 varieties (picture enclosed).  Does anyone know which is better/ideal for the coin cell I mentioned in post #1?
 

Offline mariush

  • Super Contributor
  • ***
  • Posts: 5170
  • Country: ro
  • .
Re: Powering a LED watch
« Reply #9 on: January 17, 2019, 05:31:14 am »
Look in datasheet for that battery: https://media.digikey.com/pdf/Data%20Sheets/Illinois%20Cap/RJD_Series_Rev2016.pdf

Charging Voltage 4.2VDC + 0.03V
Charging current 0.5CA
Charging Time < 3.0 hours
Charging method Constant Current/ Constant Voltage (CCCV)

1C is 200mA for that battery, so ideally you'd want to limit charge current to 0.5C or <= 100mA

That battery is nice, but I'm not sure it has protection circuitry built in (for auto cut-off below 3v etc)
Also, consider you'd need a battery holder.

You may want to consider batteries like this one: https://www.digikey.com/product-detail/en/adafruit-industries-llc/1317/1528-1839-ND/5054547
It's 25mm x 20mm x 4mm  , smaller than your 24.5mm dia. x 5.4mm (+ holder) and you can desolder wires from the protection pcb.
or 290mAh at 24x24x5.8: https://www.digikey.com/product-detail/en/tinycircuits/ASR00007/1832-1032-ND/7404517
    
MCP73831-32 can be configured from 15mA to 500mA by using a specific resistor value : https://www.digikey.com/product-detail/en/microchip-technology/MCP73831T-2ACI-OT/MCP73831T-2ACI-OTCT-ND/1979802

Ireg (mA) = 1000 / Rprog (kOhm), so with a 10kOhm resistor Ireg = 1000/ 10 = 100mA

You could also check LTC4054LES5-4.2 in thin package: https://www.digikey.com/product-detail/en/linear-technology-analog-devices/LTC4054LES5-4.2-TRMPBF/LTC4054LES5-4.2-TRMPBFCT-ND/1888451
 

Offline alank2Topic starter

  • Super Contributor
  • ***
  • Posts: 2200
Re: Powering a LED watch
« Reply #10 on: January 17, 2019, 01:00:53 pm »
Thanks Mariush - I was looking at many of those rectangular batteries last night and I agree that they might be better suited.
 

Offline netdudeuk

  • Frequent Contributor
  • **
  • Posts: 462
  • Country: gb
Re: Powering a LED watch
« Reply #11 on: January 19, 2019, 08:27:51 am »


Some of these are power hungry, but some are quite bright with little current.  This model is very nice at only 5mA.

I like that. Who sells it ?
 

Offline alank2Topic starter

  • Super Contributor
  • ***
  • Posts: 2200
Re: Powering a LED watch
« Reply #12 on: January 19, 2019, 05:42:10 pm »
Digikey, Mouser, others:

There are a ton of them in various sizes, 4 char, 8 char, even a 2 row 16 char (it is just two 8 chars mounted to each other).

The best ones are:

hcms-2905
hcms-2915
hcms-2925
hcms-2965
hcms-2976

Don't let the 3.3V ones dissuade you, they still can handle 5V and are even brighter than the 5V versions.

hcms-3906
hcms-3966
hcms-3916
hcms-3976
hcms-3907
hcms-3967
hcms-3917
hcms-3977

There are a lot more, but these are the best in terms of brightness vs. current used.  They have a ton of traditional orange, yellow, green, and red models that consume much more current for the same level of brightness.

FWIW, the smaller 3.7mm makes smoother looking characters in my opinion, but the larger 4.6mm are bigger though the pixels stand out more.
« Last Edit: January 19, 2019, 05:43:45 pm by alank2 »
 

Offline vk6zgo

  • Super Contributor
  • ***
  • Posts: 7852
  • Country: au
Re: Powering a LED watch
« Reply #13 on: January 20, 2019, 11:37:05 am »
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?
Was it the type where you were supposed to push a button to make it display, like the one I had?
After a while, the switch for that button would fail.

Mine didn't get a chance to fail, though.
I lost it while I was laying pavers, so I think it finished its life with a big slab of concrete sitting on top of it!
 

Offline Kilrah

  • Supporter
  • ****
  • Posts: 1852
  • Country: ch
Re: Powering a LED watch
« Reply #14 on: January 20, 2019, 01:41:27 pm »
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?
You can have an accelerometer turn it on for you nowadays. And rechargeable batteries mean it's a lot less of a problem if it's empty in a week... heck that's a lot compared to most smartwatches of today.
 

Offline alank2Topic starter

  • Super Contributor
  • ***
  • Posts: 2200
Re: Powering a LED watch
« Reply #15 on: January 20, 2019, 01:53:02 pm »
That is one of the cool things about these displays - they use so little power than you can leave them on all the time...
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf