Circuit review for Sparkle Tree project. - Page 1
Forum section: electronics / projects

author: Youkai
author_rank: Regular Contributor
timestamp: June 06, 2020, 05:28:27 pm
content: I'm working ona projectand have drafted the attached schematic for it.Two question about it. First do you see any issues with the schematic?Second how do I go about calculating the power needs for the circuit? At the moment I'm planning on using a wall plug likethis onewhich I used on a previous project. But that one was used on a project that had three servo motors in it in addition to most of the stuff that this one has. So I'm sure it's probably overkill.After a quick bit of research I saw that each neo-pixel in the strip has an 18mA constant current driver. So I guess I can multiply 18 x 8 x 2 to get 288mA needed for my LED's. I'm having trouble finding how much power you need to drive a Sparkfun ProMicro and the DFPlayer Mini. What keywords should I be looking for to find those? If you have a link to a digestible primer for a newbe on how to do that it would be appreciated.What's the art to picking the correct power supply? Do I want to try and target the exact power requirements with a tiny buffer; or is it ok/desirable to overshoot by a fair amount so long as it's not cost prohibitive? e.g. the power supply linked above is only 9 bucks. Can I just use that even though it supplies way more power than necessary?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2020, 06:22:43 pm
content: I'd like to see an electrolytic on the 9V rail. This will add filtering (who knows how noisy the buck module is?) and help reduce hot-plugging failures.The ProMicro says it can run at 5V so I don't see the need for the diode to "RAW". The DFPlayer seems to recommend lower voltage, so might be supplied from a diode drop (1N4001?) from 5V, or if the regulator can be turned down, everything can run just fine at 3.3-4.2V I guess. (And maybe 5V is fine, check through the datasheets/schematics.)Both are little more than a chip on a breakout board, so won't draw much more current than whatever their base load is, plus whatever's connected. I would be shocked if they draw more than 20mA together.You'll usually find current draw in the datasheet. Looks like the DFPlayer doesn't specify at all, so good luck with that. (It's probably just a relabeled microcontroller anyway?)The ATMEGA32U4 datasheet is readily available, and you'll find power consumption, by operating mode, in the Electrical Characteristics section (in the Datasheet, not the Manual). Unless you tell it otherwise, you'll be operating in normal mode by default. It'll be something like 10mA.Some devices/manufacturers are very leery about giving any power consumption data, because it's application dependent; FPGAs for example might draw very little on idle tasks, or quite a lot if they're cranking some huge program with a fast clock rate. In this case, sometimes they'll provide worksheets to estimate current consumption based on resource use, application and such. A worst case estimate is also reasonably useful, as you can budget for that.Also, if the device doesn't have a heatsink, it's probably not too insane. Like, that poor ATMEGA isn't going to dissipate a watt on its own, which would be 200mA. A nice BGA package, with a metal top, might be capable of quite a lot (the fancier CPUs and FPGAs will easily sink several amperes at 1.0V or so for the core logic), and that metal top is there for a reason, it'll probably need a heatsink. Cellphone CPUs for example, if you've ever opened one up. That's a hell of a hand-wave, but it gives you a ballpark at least.Tim

author: Youkai
author_rank: Regular Contributor
timestamp: June 06, 2020, 06:49:02 pm
content: Quote from: T3sl4co1l on June 06, 2020, 06:22:43 pmOk the previous project (that I started this schematic from) had a capacitor on the 9v rail too. I can add that back in.Quote from: T3sl4co1l on June 06, 2020, 06:22:43 pmIt can accept 9v to Raw as well. I need the pin 1 connection so I can make sure that the circuit has power (as opposed to just the pro micro through USB) before I try to drive the NeoPixels. I guess I don't know why in the previous project I connected those to 9v instead of from the output of the Buck converter.Quote from: T3sl4co1l on June 06, 2020, 06:22:43 pmIt can run at 5v but the RX pin specifically needs to be at 3.3v; hence the voltage divider.

author: Youkai
author_rank: Regular Contributor
timestamp: June 07, 2020, 03:49:54 am
content: Updated the schematic slightly. How does this look? See any issues?EDIT: I noticed my pulldown for the switch is labeled 470 but it will be a 10K.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2020, 05:12:02 am
content: Seems alright.An FYI, you can always add extra resistors as jumpers, in case something ends up wired wrong. Easier to have some pads to spare, rather than having to cut traces. Don't know if there's really anything to worry about here, you explained the RAW/VCC thing that's fine, maybe on TX in case you got RX/TX confused or in case it needs a resistor or pullup or something too (but that should be fine too).Tim

author: Youkai
author_rank: Regular Contributor
timestamp: June 07, 2020, 05:19:15 am
content: I'll breadboard it while I'm programming the Arduino so not much chance of it being wired wrong by the time a put it on a stripboard.Thoughts on the wall plug to use? Is it an issue if I get an "overpowered" plug? If not I'll probably just order the same one I've used before.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2020, 05:49:06 am
content: No problem.I mainly worry about the generic, poorly rated switching supply potentially being a massive noise source...who knows.Can still get old school iron core plug packs, if that turns out to be a problem.Tim