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

author: alank2
author_rank: Super Contributor
timestamp: November 24, 2024, 01:51:11 am
content: I'm wanting to build a BCD binary watch using some low current LED's.My initial thought it to use a cr2032 to power it, but I know it will probably eat them!The current plan is 35mm wide x 45mm tall, so that is the size I'm looking at. It will be SMD, probably a sandwich of two pcb's with some type of hardware to mount them to each other.The microcontroller I plan to use is an atmega328p, so it can handle 3-5V no worries.What about a lithium polymer type battery that is rectangular? I don't mind any voltage fluctuation it might have 3.7V to something lower, but what is the best way to keep something like that charged? A charge management IC of some type?

author: ataradov
author_rank: Super Contributor
timestamp: November 24, 2024, 02:11:52 am
content: If you are not after high visibility in a direct sunlight, modern LEDs take a fraction of a uA to be clearly visible indoors. MCU is going to be a bigger source of drain.

author: alank2
author_rank: Super Contributor
timestamp: November 24, 2024, 02:35:58 pm
content: I'm going to have a setting for brightness. I usually use APT2012LSYCK/J3-PRV, but APTD2012LSYCK has a tighter angle and more brightness. I don't have the latter on hand yet to see how it differs. It looked pretty nice at 3mA, but that is what I'd call very full brightness. I could get by on 1/4 of that probably. If the domed LED could focus that 1/4 and make it as bright as 3mA, that would be great, but I'll have to order some and test them to find out.I think I can put the atmega328p to sleep to a pretty low uA consumption.Any comments/thoughts on the lithium polymer battery or another power source?

author: ataradov
author_rank: Super Contributor
timestamp: November 24, 2024, 03:52:46 pm
content: Rechargeable batteries increase overall complexity, but realistically unavoidable if you are going to have lots of LEDs at 3 mA.Li batteries will have 4.2 V at the maximum charge and 3.7 V is already pretty low, I personally would shut down high current consumers at 3.8 V. You would need an IC to mange charging for sure, but they are not hard to find.Small batteries like this would have 350 mA or so capacity. So, if you have 10 LEDs running at 3 mA, the battery would last ~10 hours.

author: tooki
author_rank: Super Contributor
timestamp: November 24, 2024, 04:30:35 pm
content: Indeed. The green LEDs I’m using as power LEDs for use indoors in brightly lit laboratories are so efficient I run them at 0.5mA. And that’s just to make them pleasantly bright, not blinding. They could easily be reduced significantly and still be readily seen. In another project, I used modern green LEDs as internal status/diagnostic indicators, and since it’s battery-powered I kept current to a minimum. I ran them at something like 5 μA. Definitely not what you’d call bright, but eminently visible indoors.What RTC are you using, by the way?As for LiPo batteries: a charging chip is absolutely unavoidable. A basic one like the ubiquitous TP4056 would do the job, though I prefer chips from mainstream manufacturers due to the better documentation. (For example, the STC4054 from ST.) There are more advanced ones specifically for wearables (for example MAX17335 or STBC03) that integrate the protection IC, fuel gauge (state of charge monitoring), charger, and in many, load switches and regulators, in one chip, but they’re in tiny BGA-style packages that are challenging for the hobbyist.The ’328P has one nice advantage for this application: since it’s happy from under 3V to beyond 5V, you can run it directly from a LiPo battery (or 3x1.5V alkalines) without any regulator. Of course, the LEDs are easier to drive if you have a constant supply voltage.

author: alank2
author_rank: Super Contributor
timestamp: November 25, 2024, 12:41:53 am
content: Thank you both for replying!What green LED's are you using tooki? I am very impressed that can be powered by 5 uA and be visible outdoors!I'm going to connect a 32.768 kHz crystal to the tosc1/tosc2 pins and not use any sort of external RTC, but implement it in software. My hope is to have the AVR wake up, make changes it needs to make to scan the keyboard (5 wide, 2 rows) light the LED's (7 wide, 4 rows), and go back to the lowest power sleep mode it can until it is awaken again. Those plus the RTC crystal and a couple more pins to drive a piezo finishes out the available pins on the 328.I'll check out the lipo management IC's you suggest!I don't mind the LED's being a variable brightness from the battery voltage fluctuation. I plan on adding a brightness setting where you could have a high/medium/low/off (unless a button is pressed to show the time or date for 10s).

author: tooki
author_rank: Super Contributor
timestamp: December 14, 2024, 09:00:45 pm
content: Quote from: alank2 on November 25, 2024, 12:41:53 amSorry for the slow reply, I overlooked that you had responded.I think you misread, at 5 μA it’s just barely visible indoors, you’d never be able to see it outdoors during the day. I haven’t tested how visible the 500 μA (0.5mA) one is in bright sunlight (and given the weather here, you’ll have to wait a few months for me to test it! :p ).As for the kinds, I don’t have specific models, they’ve been random ~525nm (“pure green”) LEDs, some 3mm and 5mm THT, some 0603 SMD. That wavelength LED is almost always exceptionally efficient. I’d just do what I did and order a few different ones from AliExpress.One theoretical solution is to use an ambient light sensor to adapt the display brightness automatically. I do expect you’ll need quite a significant dynamic range to be both visible in outdoor daylight but also dumb low enough to not be blinding at night indoors with the lights off. I have no idea how much power the most efficient ambient light sensors need, but of course you don’t need to check it constantly.

author: tooki
author_rank: Super Contributor
timestamp: December 14, 2024, 09:02:58 pm
content: Quote from: ataradov on November 24, 2024, 03:52:46 pmThat is a ridiculously high voltage to consider “low”. A more typical voltage to stop discharging at is 3.0-3.2V. The battery datasheet (if a real one is available) will specify what to use.