add lora sml page

input/projects/smarthome/lora_sml.md

@@ -0,0 +1,77 @@
+title: Smart Meter Bridge
+description: Reading SML power meter data transmitted via LoRa
+parent: smarthome
+position: 150
+git: https://git.xythobuz.de/thomas/esp-env
+github: https://github.com/xythobuz/esp-env
+date: 2024-07-06
+comments: true
+---
+
+<!--% backToParent() %-->
+
+A while ago my power net company installed new smart power meters, specifically the [Iskra MT681](https://wiki.volkszaehler.org/hardware/channels/meters/power/edl-ehz/iskraemeco_mt681).
+It has a standardized infrared interface to get some data from it.
+There are different [designs](https://wiki.volkszaehler.org/hardware/controllers/ir-schreib-lesekopf-ttl-ausgang) available to attach a microcontroller, both open-source and commercial.
+I've gone the lazy route and bought one [from eBay](https://www.ebay.de/itm/353940190755).
+
+<!--%
+lightgallery([
+    [ "img/lora_sml_1.jpg", "IR TTL Reader with magnetic base" ],
+])
+%-->
+
+Unfortunately the meter is mounted in the basemet of my apartment building, too far for any WiFi signals.
+So I decided to try out [LoRa](https://en.wikipedia.org/wiki/LoRa) to transmit the data, using two [Heltec LoRa32](https://heltec.org/project/wifi-lora-32-v3/) dev boards.
+
+<!--%
+lightgallery([
+    [ "img/lora_sml_2.jpg", "LoRa SML transmitter, outside" ],
+    [ "img/lora_sml_3.jpg", "LoRa SML transmitter, inside" ],
+])
+%-->
+
+The interface to the meter is an infrared UART with 9600 8N1.
+Over this port it sends messages in a format called [SML](https://de.wikipedia.org/wiki/Smart_Message_Language).
+To decode these I'm using [sml_parser](https://github.com/olliiiver/sml_parser).
+
+<!--%
+lightgallery([
+    [ "img/lora_sml_4.jpg", "LoRa SML receiver" ],
+])
+%-->
+
+To get useful data from the meter you have to unlock it with a PIN from the net provider.
+Without this PIN you only get the current energy count in `kWh`, the same number you can see on the display.
+After unlocking all data, you get the energy count in `1/10th Wh`, so much finer resolution.
+And you also get the current power in `W`.
+
+<!--%
+lightgallery([
+    [ "img/lora_sml_5.jpg", "Testing the transmitter" ],
+    [ "img/lora_sml_6.jpg", "Readout of SML messages" ],
+])
+%-->
+
+For the LoRa PHY I'm using [RadioLib](https://github.com/jgromes/RadioLib/), which already comes integrated into the [unofficial Heltec LoRa 32 library](https://github.com/ropg/heltec_esp32_lora_v3/).
+
+I'm doing lots of deep sleep on the transmitter side, to safe as much battery capacity as possible.
+And I had to use the watchdog in the ESP32 and also do an auto-reset when no messages have been received after a while.
+So somewhere the code hangs, sometimes.
+But with these hacks in place it seems to work well.
+
+The code is integrated into my [ESP-Env project](espenv.html) and [repo](https://git.xythobuz.de/thomas/esp-env).
+
+<!--%
+lightgallery([
+    [ "img/lora_sml_power.png", "Example of my power usage" ],
+    [ "img/lora_sml_battery.png", "Battery usage of the LoRa transmitter" ],
+])
+%-->
+
+Here you can see a normal weekend for me, going to bed at 4:30 in the morning, and waking up at around 13:00.
+The spike with 1.3kW is heating water for coffee.
+The other power readings come from tasmota sockets or my NAS UPS.
+
+Also the battery usage of the LoRa transmitter is very low.
+I will update when I have to recharge them for the first time.