This project happened mostly by accident - I wanted to design a small ESP board with mounting holes / LiPo charger, and I decided to stick a temperature sensor on it. I initially chose the SHTC3, because it wasn't too expensive (£1), and it had good specs. However, later on, I wasn't able to solder it consistently. Instead, I added a DHT22 temp/humidity sensor on a seperate breakout board and connected it with wires.
It features a buck converter rather than a simple linear regulator for greater efficiency, and also because the lowest dropout voltage I found in a linear regulators with 1A rating has been about 0.4v. This means it needs at least 3.3 + 0.4 = 3.7 volts to operate, but a single LiPo will be under this voltage about half of the time! I chose the TLV62568 buck converter, mainly because it was the cheapest one at quantity 1 on Digi-Key at the time.
I made a few mistakes as it was targeting JLCPCB's 4 layer offer ($2 for a 4 layer board under 50mmx50mm) and they advertised it as ending on 25 Dec - namely, I forgot to add decoupling capacitors for the ESP32! I added one between the pins on the module, and it seems to be fine. Also, the JLC offer still stands, so I didn't have to rush it at all...
I also didn't manage to get the crystal oscillator working - my best guess so far is that I have the wrong load capacitance, meaning the crystal doesn't oscillate.
I designed a case with OpenSCAD - it seems quite useful, definitely worth it for smaller/simpler designs.
The software can be found on my GitHub: https://github.com/Stary2001/esp32-thing-fw
At the moment, it's hardcoded to one temp/humidity sensor and uploads to both Influx and Home Assistant, with a hardcoded password for both. Eventually I'll make it more generic.
ESP-IDF is the official SDK for writing software for ESP32 chips - it has lots of functionality that I need out of the box, like HTTP and MQTT libraries. Much of this is based from the wifi/http/mqtt examples.
MQTT was used for integrating with Home Assistant - with the right configuration, the sensor can automatically advertise itself. https://www.home-assistant.io/docs/mqtt/discovery/ Just send a JSON document with the configuration as outlined in the relevant sensor type. For a temp/humidity sensor, the relevant type is Sensor: https://www.home-assistant.io/integrations/sensor.mqtt/.
I chose to build it in C with cJSON (included in ESP-IDF):
cJSON_AddStringToObject(device_json, "identifiers", mac_str); cJSON_AddStringToObject(json, "name", s->name); cJSON_AddStringToObject(json, "unique_id", unique_id); cJSON_AddStringToObject(json, "state_topic", s->state_topic); cJSON_AddStringToObject(json, "unit_of_measurement", s->unit); cJSON_AddItemToObject(json, "device", device_json); cJSON_AddStringToObject(json, "device_class", s->class);
I also chose to use 3 different MQTT topics (and device entries) - one for each value (battery/temp/humidity), rather than a single topic with a JSON blob and the JSON template functionality. I'm not sure if that would be better, but possibly more efficient as it only has to send one message.
The ESP32 datasheet: RTC timer + RTC memory 10 µA
The buck converter datasheet: 35-µA Operating Quiescent Current
The DHT22 datasheet: 40-50uA in standby
My resistor divider:
> 3.3V/ (20kohm + 90kohm) 30 microampere (current)
adding up to ...
10 + 35 + 45 + 30 = 120uA
Which means it can sleep for
1000mA * h / 120uA = 49 week, 4 day, 5 hour, 20 minute, 0 second on a 1000mAh battery. Clearly, this doesn't model the time WiFi is on, but it gives a rough idea.
Assuming WiFi is on for 2 seconds out of every 5 minutes, and that it uses 200mA while on (the ESP32 datasheet quotes 180mA for 802.11n transmit),
> (1000mA*h / ((120uA * (5min - 2s)) + 200mA * 2s)) * 5min = 4 week, 16 hour, 27 minute, 8.492748 second
When measuring the current to the battery with my multimeter I'm getting anywhere between 80uA and 150uA...