- Arduino / ESP8266 ? Arduino is more conservative but doesn't seem to be the popular choice. I mean, speed might be an issue with transmission so bang it out on a Wemos with WIFI disabled.
- Clamshell
- Nokia 5110 screen / e-ink
- Buzz tune on receipt
- Opening wakes up device screen
- Small as poss, but poss 18520 batteries, so..
- Lora communication over 300m
Mobile keypad text is a fun way to go. Revive that old skill.
Would be nice, but bandwidth is limited, as well as rendering capabilities so.. it'd be a whole thing. Couldn't be real-time.. but still an option if I make a larger device.
| Send Device | Listen Device | User |
|---|---|---|
| Listening always | Listening always | |
| Emit (n?) | Send | |
| Receive | Buzz | |
| Store to EEPROM | ||
| Emit message received? meh | ||
| Stop emitting | ||
| Screen on, display message | Open device | |
The power concerns are real. So let's make sure to do all dis:
WiFi.mode(WIFI_OFF);
If possible, I mean perhaps it could sleep half the time? Let's have a shufty at some power requirements. Complete conjecture, but anyways:
-
Microcontroller General: 20mA ESP w/out WIFI or 5mA for pro mini Sleep: Negligible
-
LoRa (3.3v) Receiver: ≦ 10mA (LnaBoost off, band 1) Transmit: ≦ 120mA (+ 20dBm) Sleep: Negligible Total, if listening always and transmitting like 1% of the time: 11mA
-
Screen Unlit: 1mA Backlit: <= 80mA, so maybe don't bother!
-
18650 2400mah 3.7v 5v converters are supposedly 90% efficient so 2160mAh
So, assuming they're always listening and transmit at glorious 28.8kbps modem style speed now and then:
Battery capacity (mA/h) / Current draw (mA) = hours of battery life .. right?
2160mAh / 32mA = 67 hours
900mAh / 32mA = 28 hours
240mAh / 32mA = 7.5 hours
Woah. I mean, these are conservative numbers too. Not three bad. So yeah, sleep both chips if possible when closed and poss use an Arduino (5mA).
Literally anything is fine. Peak current, with lit screen and full transmit is like 220mA, so 0.5C @ 500mA is enough. (18650 are 1C)
Just a reminder to sleep this along with the microcontroller.
5V step ups bitch out under .5v, which won't matter much, but the efficiency varies with input voltage (and current): http://www.hobbytronics.co.uk/u1v10f5-5v-regulator
| Battery | V | mAh | Effic | ->~mAh | Life(h) mAh/32mA | Notes |
|---|---|---|---|---|---|---|
| AAA (r/c) | 1.5v | 1000 | @70% | 700 | 22 | |
| AAA (r/c) x2 | 1.5v | 2000 | @70% | 1400 | 44 | |
| AA (r/c) | 1.5v | 2400 | @70% | 1680 | 53 | |
| Lipo 50x30mm | 3.7v | 2000 | @90% | 1800 | 56 | Better form factor than AA |
| 18650 | 3.7v | 2400 | @90% | 2160 | 66 | All lipos easy to charge |
| Lipo 90x60mm | 3.7v | 4000 | @90% | 3600 | 113 | Phone bat size, affordable |
Lipo batteries supposedly self-disarge at only 5%/month (despite my experience), not far off alkaline. So any lipo really, high mAh as possible for the best form factor. But deeefinitely invest in a voltmeter setup (see below). Perhaps flash the screen hard and/or chirp for a turn off.
Feed battery into analogue input, with a voltage divider to reduce it.
Wemos A0 has a voltage divider built in. 220k / 100k (2/3rdsness). People say it has a range of 3.3V, but I should test that.
+++++++++
|
[10k]
|
|----(100pF -> 1nF)---> analogue input
| reduce wobble? Didn't work..
|
[10k]
|
---------
V divideness = (R1 / R1 + R2)
= 1 / 2
So if my lipo spits out a mad 4.6V, A0 would receive 2.3V. Update: 1024 max value is 6.1V, in reality, through my divider.
| Lipo V | Desc | Dividered | A0 (theory) | Human |
|---|---|---|---|---|
| 4.6 | Pumped | 2.3 | 713 | 100% |
| 4.2 | Full | 2.1 | 652 | 100% |
| 4 | Good | 2 | 621 | 75% |
| 3.86 | Mid | 1.93 | 599 | 50% |
| 3.75 | Low | 1.875 | 582 | 20% ! |
| 3.7 | Low | 1.85 | 574 | 10% ! |
| 3.5 | Dead | 1.75 | 543 | 0% off |
A0 (theory) = Dividered V / 3.3 * 1024
Empirical realities are close but a little different:
| Lipo V | Desc | A0 (theory) | Human |
|---|---|---|---|
| 4.6 | Pumped | 762 | 100% |
| 4.2 | Full | 706 | 100% |
| 4 | Good | 660 | 75% |
| 3.86 | Mid | 634 | 50% |
| 3.75 | Low | 615 | 20% ! |
| 3.7 | Low | 608 | 10% ! |
| 3.5 | Dead | 577 | 0% off |
It really should power itself off, LoRa et al, with low batt for safety. Might not be worth it though, I mean, do my charge protection circuits handle that? Probs eh?