GTEM ESP32 ATM90E26 – Connections


The GTEM, Grid Tie Energy Monitor board, has been designed to be flexible and whilst providing Energy Monitoring functionality, it also has other options for experimenting.



  • AC Input
    • Power safely derived from a SELV / Wall AC/AC Power Supply
    • Provides AC waveform reference for accurate Real Power measurements
    • Connect ideally 12V AC (can be slightly lower), to the AC/DC In terminals. (It does not matter which way round the wires go).
    • DC will power the board but not provide ATM functionality such as AC Voltage or Mains Frequency
    • The AC will then flow into a half-wave rectifier and to a 3V3 SMPS.
  • I2 Input
    • 120uV to 600mV RMS (When Gain is 1)
    • This is generally normally unused.
  • CT Clamp
    • Typically a YHDC SCT-013 100A/50mA Current Transformer
      • These current transformers can measure current up to 100A
      • Can fit around cables that are 13mm in diameter
      • They have 3.5mm jacks which plug directly into the GTEM
      • A TVS diode is generally built in for safety, and to prevent open circuiting
      • These output 50mA when reading 100A RMS
    • Connection is a 3-Pole 3.5mm Socket
      • Centre ring not used.
  • User GPIO
    • GPIO 12 – Input or Output
      • Note: Must be LOW at ESP Boot
    • GPIO 14 – Input or Output
  • OPTO Input
    • GPIO33 – Input or Output
      • Example devices 3DU5C, TSL257 or GL5528
    • Could be used for monitoring Meter Pulses
  • USB
    • Micro USB Connector
    • GTEM board is WeMos D1 Mini flashing compatible
      • Set the BOARD to ESP32, ‘WEMOS D1 MINI ESP32’ DEV Module (or similar).
    • You can also set the BAUD rate to 921600 to speed up flashing
  • I2C/OLED
    • I2C Expansion
    • Standard OLED connections
      • Ground / 3V3 / SCL / SDA


Important Foot Note.

These Wiki pages are MOVING to the GitHub Wiki.   Simply select a repository pertaining to the board and click Wiki.


Other Details

Further information on this product may be found in our downloads and Wiki pages.  Other technical information and code examples are also shared in our GitHub Pages.  Project information may also be found on

We value our customers and should you have any other questions, please feel free to contact us.

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