Powering a LED watch - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: January 16, 2019, 01:56:41 am
content: 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:RJD2450https://www.mouser.com/ProductDetail/Illinois-Capacitor-CDE/RJD2450?qs=sGAEpiMZZQ#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?

author: DaJMasta
author_rank: Super Contributor
timestamp: January 16, 2019, 02:53:25 am
content: 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.

author: alank2
author_rank: Super Contributor
timestamp: January 16, 2019, 02:57:51 am
content: 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.

author: mariush
author_rank: Super Contributor
timestamp: January 16, 2019, 03:13:10 am
content: 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 ...seehttps://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)

author: Audioguru
author_rank: Super Contributor
timestamp: January 16, 2019, 04:30:43 am
content: 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?

author: ArthurDent
author_rank: Super Contributor
timestamp: January 16, 2019, 06:18:27 pm
content: 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.

author: mariush
author_rank: Super Contributor
timestamp: January 16, 2019, 06:25:16 pm
content: Here's some more inspiration:

author: alank2
author_rank: Super Contributor
timestamp: January 16, 2019, 08:03:57 pm
content: Some of these are power hungry, but some are quite bright with little current. This model is very nice at only 5mA.

author: alank2
author_rank: Super Contributor
timestamp: January 17, 2019, 12:30:44 am
content: 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?

author: mariush
author_rank: Super Contributor
timestamp: January 17, 2019, 05:31:14 am
content: Look in datasheet for that battery:https://media.digikey.com/pdf/Data%20Sheets/Illinois%20Cap/RJD_Series_Rev2016.pdfCharging Voltage 4.2VDC + 0.03VCharging current0.5CACharging Time < 3.0 hoursCharging 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 <= 100mAThat 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/5054547It'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/7404517MCP73831-32 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/1979802Ireg (mA) = 1000 / Rprog (kOhm), so with a 10kOhm resistor Ireg = 1000/ 10 = 100mAYou 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

author: alank2
author_rank: Super Contributor
timestamp: January 17, 2019, 01:00:53 pm
content: Thanks Mariush - I was looking at many of those rectangular batteries last night and I agree that they might be better suited.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: January 19, 2019, 08:27:51 am
content: Quote from: alank2 on January 16, 2019, 08:03:57 pmI like that. Who sells it ?

author: alank2
author_rank: Super Contributor
timestamp: January 19, 2019, 05:42:10 pm
content: 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-2905hcms-2915hcms-2925hcms-2965hcms-2976Don't let the 3.3V ones dissuade you, they still can handle 5V and are even brighter than the 5V versions.hcms-3906hcms-3966hcms-3916hcms-3976hcms-3907hcms-3967hcms-3917hcms-3977There 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 20, 2019, 11:37:05 am
content: Quote from: Audioguru on January 16, 2019, 04:30:43 amWas 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!

author: Kilrah
author_rank: Unknown
timestamp: January 20, 2019, 01:41:27 pm
content: Quote from: Audioguru on January 16, 2019, 04:30:43 amYou 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.

author: alank2
author_rank: Super Contributor
timestamp: January 20, 2019, 01:53:02 pm
content: That is one of the cool things about these displays - they use so little power than you can leave them on all the time...