Heltec HRU-3601

• Product page: HRU-3601 – Heltec Automation
• Schematic: hru3601.pdf
• MCU: Espressif ESP32-C3
• LoRa: Semtech SX1262

• HRU-3601
• Wall mount holder
• USB-A power adapter (Xiaomi MDY-08-EG, US NEMA 1-15/Type A 2-prong)
• White USB-A to USB-C cable

HRU-3601 is a low cost indoor LoRa environmental sensor that comes in 2 versions:

• HRU-3601A - Temperature and Humidity (GXCAS GXHTV3 - Sensirion SHTC3 compatible)
• HRU-3601B - Multi-gas (VOC and CO₂eq) sensor (Sensirion SGP30)

The only difference between the SKUs are the castellated-edge I2C sensor modules; with some SMT soldering skills, the sensor module can be swapped out for other modules from Heltec's QuickLink Sensor Family (select Need PH2.0 x 4 Socket? No when ordering).

MCU and LoRa are provided via a castellated-edge Heltec HT-C62 module.

• Sensor
• Notification LED? Meshtastic's notification module doesn't currently support NeoPixel output

• Firmware PR: mesthastic/firmware#4129
• Protobufs PR: meshtastic/protobufs#521

• Working
  • SK6812 Neopixel on GPIO2 (the color can be set with the Ambient Lighting module)
  • GXCAS GXHTV3 (SHTC3 compatible)
• Not working
  • SX1262 initialization after deep sleep (seems like an issue out of scope of the PR)
• Won't fix
  • Battery reading - Board has no voltage divider on VBAT (board has a 1.25mm pitch “JST” style connector and a TP4054 charge IC)
  • Main thread LED - Board has no LED on simple GPIO

The HRU-3601 is slightly smaller than palm-sized, and consumes about 100mA when awake and idle, and about 300mA during transmit.

There is lithium battery charging support via battery charge IC UMW TP4054, and a 403040 LiPo barely fits. However, it's probably a bad idea to have an internal battery, as there is no way to measure battery voltage, and the battery also ends up obstructing the sensor.

The HT-C62 module has a lot of flux residue. Because it is back to back with the sensor module, the sensor module must be desoldered by hand before the HT-C62 can be desoldered with an SMT hotplate. Care must be taken not to dislodge the SMD pull-up resistors for the buttons, and the SMD bypass capacitor right next to the sensor module's 3V3 pin.

The antenna is a simple coil spring antenna. The heatshrink may have unintended dielectric effects on the antenna, but it's too hard to cut away without damaging the antenna. You may wish to desolder this antenna entirely and connect an FPC antenna or pigtail to the U.FL connector on the HT-C62 to get improved range.