Connected Power Meter using PZEM-004T, Wemos D1 Mini and a 1602 I²C display

Parts needed

Wemos D1 Mini
PZEM-004T
1kOhm Resistor (optional - see alternate wiring)
Enclosure
5V buck converter power supply (search for "700ma 3.5w 5v" on usual stores...)
I²C 1602 LCD Display (I had issues with green one, I²C address 0x3F, while no problems with blue ones, address 0x27)
Mains Power cable
Mammuth Clamps

Preparation

You need to compile your own Tasmota firmware as none of the pre-compiled binaries have support for display and PZEM module.

Set up your preferred IDE as described in wiki

Enable IDE to Use Custom Settings

Create user_config_override.h in the sonoff folder and paste the contents of this sample configuration file.

PlatformIO

Edit platformio.ini. Uncomment the statement by removing the ";" in front of the line:
-DUSE_CONFIG_OVERRIDE
Enter platformio run -e <variant-name>
Examples:
- platformio run -e sonoff-sensors
- platformio run -e sonoff-DE

Arduino IDE

Edit my_user_config.h. Uncomment the statement by removing the "//" in front of the line: #define USE_CONFIG_OVERRIDE
Click compile

Flash the binary on the Wemos D1 Mini and confirm it is functional before connecting the PZEM-004T to its serial interface.

Tasmota Parameter Configuration

Device Template
PZEM-004T version prior to V3:

{"NAME":"HW-655 PZEM","GPIO":[0,63,0,62,6,5,0,0,0,0,0,0,0],"FLAG":0,"BASE":18}

PZEM-004T version V3:

{"NAME":"HW-655 PZEM","GPIO":[0,98,0,62,6,5,0,0,0,0,0,0,0],"FLAG":0,"BASE":18}

use I2CScan to detect your device address
use DeviceAddress XXX (where XXX is the decimal converted address found) to set the I²C address
set TelePeriod 10 to have the display refresh every 10 seconds (you can't go under this value)
set DisplayModel 1, and DisplayMode 0
finally, add a Rule to display values (I choose these):

Rule1 ON Tele-ENERGY#Power DO DisplayText [z] [x1y0]%value%W ENDON ON Tele-ENERGY#Today DO DisplayText [x8y0]%value%Wh ENDON ON Tele-ENERGY#Voltage DO DisplayText [x1y1]%value%V ENDON ON Tele-ENERGY#Current DO DisplayText [x8y1]%value%A ENDON

remember to enable the rule, with Rule1 1

Images and Wiring diagram

DANGER - MAINS VOLTAGE. Be sure to crimp connectors and use heat-shrinking tube wherever possible/needed, and tightly secure any screw.

How it looks, from web GUI:

How it looks, from enclosure:

You can set the contrast using the little trimmer/pot on back of the display. I cut a bit of the corners from the display to have it flush with border, and used two hexagonal plastic standoffs with nuts and bolts to secure it to transparent top.

Mains IN, mains OUT, all sealed: