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.

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

 

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

 

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:

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).

 

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.

 

 

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