Pager Lora

Ingredients

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.

Goals

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

Input

Mobile keypad text is a fun way to go. Revive that old skill.

Drawing?

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.

Comms

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

Power

The power concerns are real. So let's make sure to do all dis:

Disable WIFI on microcontroller

WiFi.mode(WIFI_OFF);

Sleep

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).

C rating?

Literally anything is fine. Peak current, with lit screen and full transmit is like 220mA, so 0.5C @ 500mA is enough. (18650 are 1C)

Sleep LoRa chip

Just a reminder to sleep this along with the microcontroller.

Battery

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.

Battery monitoring

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

Power 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?

entozoon/pager-lora