Hi all, this is very much a "help the noob" post. I'm working on a hat for the Moteino w/trace antenna that will turn on a string of flickering LED lights as part of props for a theater show. I'll be making 100 of these modules. This is my first PCB, and really my first real electronics project outside of digital control of relays. So before I screw myself over 100 times, I'd love some feedback.
The module will run off a 9V battery. I know that's not optimal for the Moteino and battery life, but it's an existing constraint of the LED lights.
There are three main components:
- a BC817 transistor for controlling the LEDs; the LEDs draw 30-50mA total. I haven't actually tested with a BC817 yet, but it seems to be similar to a 2N2222. I have it connected to a PWM pin because I'm hoping that will allow me to control brightness. Not a requirement, but if it works, cool feature to give to the directors.
- A voltage divider to monitor the 9V battery voltage and theoretical life remaining.
- a PCF2123TS RTC, to be used as a remotely programmable wake-on-interrupt. I might be overdoing this... I want to put the Moteino in the deepest sleep possible between shows (no power switch is accessible), with a goal of reaching 200 shows and 2 months on one battery. I need it to wake up and check for radio broadcasts to see if a showtime is approaching. If a show is near, I'll switch to a routine that uses the watchdog timer and the Moteino progressively wakes up more frequently as the showtime approaches. My plan is to programmatically tweak that wakeup time, starting at every half hour and if I need it going up to every 3 hours. But maybe in the big scheme of things there really isn't much power difference waking up every 30 minutes vs 120 minutes... I did select a low-power RTC, it consumes about 100nA with the clock output turned off (I don't need the clock, only the interrupt). If my calculations and tests are right, every performance activation uses up about 2mAh. At 200 performances, that's 400mAh, leaving 100mAh to keep the Moteino active for 2 months.
Also, I'm not sure if the ground plane for the RTC is actually effective...
Any thoughts, or reasons why I'm doing something really stupid or wrong? Thanks!
In electronics, the only real way to learn is to make mistakes.
So don't feel bad about making many mistakes, and a few revisions before the thing shows signs of life, we all did that. ;)
Component choice seems OK. I would stay away from 9V. I am sure there are ways to get your LEDs working on a different lower voltage. If not, allow parallel cells to plug into the board, at least if 1 battery doesn't make it through 2 months, 2-3 might.
IMO the RTC is overkill. In a way simpler scheme you can deep sleep and wake up periodically and ASK your gateway when "it's show time" rather than use an RTC to wake and "listen".
Thicken traces wherever possible.
Spin a revision, assemble, fail, find bugs, fix, repeat. Eventually stuff will work.