emonPlug: OpenEnergyMonitor-compatible energy monitoring US plug

I’ve been working on a way to monitor energy use of home appliances using OpenEnergyMonitor, and I now having a working unit that I would like to submit for comment/review/use by the OpenEnergyMonitor community. I have been calling it emonPlug, or emonPlug US, for lack of a better idea of what to call it.

For GitHub users, I have posted it here:

For non-GitHub users, I will post my description of the project here.

emonPlug Overview

OpenEnergyMonitor-compatible energy monitoring US plug.

emonPlug is an energy monitoring transmitter unit that can monitor AC electrical consumption of one appliance connected to its NEMA 5-15R receptacle.

emonPlug is designed to be used together with an emonBase web-connected base station from OpenEnergyMonitor. It can be thought of as an emonTx+emonVs for monitoring one appliance.

Make sure to read the warning and disclaimer below before starting any work on emonPlug assembly or use.

emonPlug Architecture

emonPlug uses a CT to perform non-invasive current monitoring, similar to emonTx from OpenEnergyMonitor. emonTx3 User Guide — OpenEnergyMonitor 0.0.1 documentation

It uses a voltage transformer for voltage monitoring and includes a built-in power supply, similar to emonVs from OpenEnergyMonitor. Voltage sensors — OpenEnergyMonitor 0.0.1 documentation

It is firmware compatible with emonTx3 (with appropriate calibration settings). emonPlug-specific firmware, based on the emonTx3 firmware, is provided below.

To use emonTx34_CM firmware change the following settings:

float vCal_USA = 111.686;  
float i1Cal = 14.68;                                       
float i1Lead = 7.6;

bool  USA=true;

The emonPlug-specific firmware omits CTs 2, 3, and 4 and provides a more detailed packet for CT1: Vrms, Irms, PowerFactor, Frequency, LogPeriod, RealPower, ApparentPower, WattHours.

emonPlug Parts

The CT is an AC1015 from Talema Group. It is rated to 15A with 1000:1 turn ratio. https://talema.com/wp-content/uploads/datasheets/AC.pdf

The voltage transformer is the ZMPT101B. This is the same transformer used in the emonVs. The current emonPlug is US-specific, so the 3x10K, 1/4W current-limiting resistors are designed to maximize the waveform of 120V AC. More resistance, more power dissipation, and a different burden resistor would be necessary for anyone modifying emonPlug for use with 240V AC outside the US.

emonPlug uses the Polycase PM2425 AC wall plug enclosure with US 15 AMP 120 VAC plug configuration. PM2425 | Wall Plug Enclosures.

The emonPlug MCU is the Microchip ATMega328 and the radio is the HopeRF RFM69CW, similar to emonTx3.

emonPlug Photos




Warning and Disclaimer

Make sure to read this disclaimer before starting any work on emonPlug assembly or use. Credit to Felix Rusu of LowPowerLab for the wise wording of this advice and disclaimer.

emonPlug is isolated from mains electricity via the RECOM PSU (3kV isolation). Additional safety devices in the emonPlug include a current limiting fuse and a MOV (metal oxide varistor) device to help protect against transients.

Electricity WARNING: whenever you touch anything that is or was connected to mains voltages you have to really know what you are doing, otherwise you may shock yourself to shivers. If you are not 100% sure, do yourself a favor and don’t touch anything. ATX power supplies, TVs, CRT/LCD monitors, among most other high voltage devices contain very large capacitors that can hold lethal charges that may or may not discharge after the device is unplugged. You should discharge these caps before touching the circuit.

You should not attempt to assemble or use an emonPlug if you are not familiar with mains wiring! Mains voltage can cause a fatal shock or start a fire when used improperly or negligently! Please consult this electrical safety handbook to understand how you can protect yourself and your equipment from electrical shock.

The emonPlug is not an end consumer product but an open-source design that requires assembly/soldering and is intended for users comfortable/qualified to assemble it. Certain parts are polarized and require proper orientation before soldering otherwise emonPlug may malfunction or not work. emonPlug also requires programming prior to it being functional. The designer of emonPlug is not liable for how the end user decides to assemble or use and customize the sample firmware code. The information and sample code is meant as an educational resource only, to help guide the end user with assembly and better understand how emonPlug can operate. The end user is fully responsible to ensure the final product will be electrically safe to operate. Although as a whole, emonPlug is not UL/CSA/CE certified. It uses a certified power supply. However emonPlug comes with NO GUARANTEES expressed or implied and the designer is not liable for any injuries or damage caused by improper assembly or use of emonPlug. It may be against code to install non UL certified equipment in your area, so you are responsible to make sure you are compliant or take any risks on your own. If you do not agree or understand the terms of this disclaimer you should not use emonPlug. Use it at your own risk.

Alternatives to emonPlug

Switchbot Mini Plug with Tasmota is a good way to monitor energy consumption via emonBase through MQTT. SwitchBot Plug Mini Plug (W1901400) Configuration for Tasmota The Tasmota firmware can be loaded without disassembling the unit. GitHub - kendallgoto/switchbota: Replaces the factory firmware on the SwitchBot Plug Mini via OTA, enabling the use of Tasmota without disassembling the unit.


emonPlug v1-1.brd (204.5 KB)
emonPlug v1-1 Board Layout.pdf (44.4 KB)
emonPlug v1-1 Bottom GND Pour.pdf (541.2 KB)
emonPlug v1-1 Schematic.pdf (31.1 KB)
emonPlug v1-1 Top GND Pour.pdf (539.8 KB)
emonPlug v1-1 Traces.pdf (547.3 KB)
emonPlug v1-1.sch (1.5 MB)
emonPlug BOM.ods (5.9 KB)


emonPlugv1-1_CM.ino (15.2 KB)
emonTx34_CM_config.ino (12.1 KB)

1 Like

These will do it.

Nice work @brandock and thanks for sharing this.

Developing hardware like this is always a significant amount of work, so hats off to you!
It looks like a really nice and tidy board!

Nice work as well getting this into a suitable enclosure!

Thank you, Trystan! It is a testament to you and the OpenEnergyMonitor team that someone like me can go to the Guide, get help here in the community forum, and learn to create a board like this. And the fact that you would take the time to look at the board and give some positive feedback … you and the OpenEnergyMonitor team constantly amaze me.

1 Like