Wireless Humidity Sensor Part 1 – Hardware Design

Humidity Sensor PCB Assembly


Project Synthesis

It had come to my attention that there was a growing damp spot on my kitchen ceiling – after ripping through some drywall and inspecting for leaks, the prognosis was bathroom humidity issues due to faulty grout in the shower.  In my most optimistic take on the situation, I reasoned a fun project could be to make a humidity sensing solution to monitor for humidity problems (after a fix was put in place).  And so I began designing!

My main design goal is to place humidity sensors in the walls while the drywall is off, and leave them sealed in there.  Therefore, I want them to be cheap enough that I don’t care to get them back again.  Bonus points if they look sufficiently creepy for any future homeowners performing renos.


With my economic goals in mind, I quickly found the cheapest radio solution I could – the nrf2401 -the exact one I purchased is available here: kuman 10pcs nRF24L01+ 2.4GHz Antenna Wireless Transceiver RF Transceiver Module Arduino Raspberry Pi Compatible K19.

nRF24L01+ transceivers

These devices perform basic ISM 2.4Ghz band packet-based communications, and have a number of useful features.  One receiver can receive from up to 6 transmitters simultaneously.  Note – this device does not use any TCP/IP stack!

Note – the ESP8266 may have been a better solution overall, as it has an on-board micro.  I have not yet evaluated it!

The next step was to decide on a microprocessor and humidity sensor.  I decided to stick to the Atmel family of microcontrollers, due to my familiarity with them, allowing me to get something working as quickly as I am able.  I opted for the ATTiny series due to their minimalist features and low power consumption.  In terms of minimizing time to get something working, I already have an Olimex AVR-ISP-MK2 capable of flashing them.  You can get yours here:
Olimex AVR-ISP-MK2 USB Compatible AVR programmer with ICSP PDI TPI Support

AVR-ISP-MK2 Programmer


Ultimately I settled on the ATTiny87 as it had the following features:

  • Programmable via AVRISP 6-pin interface
  • Features I2C Master peripheral – necessary for talking to some humidity sensors
  • Features SPI Master peripheral – necessary for talking to nRF radio
  • Features analog input – useful if I opt for an analog-based humidity sensor
  • It can run directly from a coin cell battery

I purchased mine from digikey – they are also available via amazon at: ATTINY87-SU 1 Lot/5PCS

ATTiny87 Microcontroller


For the humidity sensor, I settled on the Silicon Labs Si7006, as it was the cheapest I2C-based humidity sensor that I could find on digikey.


Circuit, PCB Design for Transmitter

With the major components selected, I plugged my design into KiCAD and got a basic PCB layed out:

Humidity Sensor Schematic

Humidity Sensor Schematic

Humidity Sensor PCB

Humidity Sensor PCB

I have open-sourced the design files, the KiCAD project is available here: https://github.com/trdenton/humsensor/tree/master/tx_pcb

You can also order the PCBs directly from PCBWay here: https://www.pcbway.com/project/shareproject/W04152ASD12_humxmit_v2.html

You can see (most of ) the parts populated here (note – the white wires are for power and ground connections during testing).

Humidity Sensor PCB Assembly

Humidity Sensor PCB Assembly


BOM is as follows:

Part Digikey part # / spec
switch CKN9112TR-ND
micro ATTINY87-SU-ND
HTU21D 336-3124-ND
R1,R2,R3,R5 generic 10k, 1206 size
R4 generic 220R, 1206 size
C1,C2,C3 generic 0.1uF, 1206 size
batt holder 952-1737-2-ND
led 1516-1076-1-ND

Software Implementation – Transmitter

The current status of the project is as such:

  • Software load for transmitter board is communicating with the nRF24L01+, sending arbitrary data
  • I2C peripheral is configured for the ATTiny87
  • Have not tested reading the Si7006 sensor yet

Source code for the transmitter is available here – still a work-in-progress: https://github.com/trdenton/humsensor/tree/master/hum_sw/humtx

The repository contains a Makefile with the following targets:

  • make all
    • Compiles the source code to target .hex file
  • make flash
    • Uploads the hex file via avrdude

Circuit for Receiver


The receiver circuit is based on an arduino mega, but any SPI-enabled arduino should suffice.


Arduino Mega Receiver

Arduino Mega Receiver

Arduino RX Breadboard

Arduino RX Breadboard


Software for Receiver

The software for the receiver is currently a work in progress – it’s goal is to receive from multiple transmitters, and log the data back to a PC via serial port.  Eventually, this data will get put into a historical database.


You can check out the current software here: https://github.com/trdenton/humsensor/tree/master/hum_sw/humrx


Next Steps:

Stay tuned for the following tedium action-packed items!

  • Test reading the Si7006 device
  • Test wireless range v. TX power for nRF24L01 – it should transmit through a sheet of drywall
  • Enable low-power mode on the ATTiny87 for optimum battery life
  • Populate another board,
  • Write a program that can log the receiver’s readings to a database

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