Wireless sensors for Arduino and the Internet Of Things

Low cost and (soon) open source!

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Lambda Nodes

Hardware

There are currently four Nodes in development: temperature sensing, weather monitor, soil moisture sensing and the receiver node

Lambda Nodes provide a hassle-free solution: no soldering, no wires, almost zero coding: the sensor node detection happens automatically. Just drop in the battery and the data is delivered to your device. A very simple way to get sensor data wirelessly to your Arduino and Arduino compatible platforms. There are examples ready to stream the data to your favorite MQTT host, thingspeak, plot.ly and others like Cayenne. Or just save the data locally on an SD card.

Wireless

The nodes use a very low power wireless transmission module based on the NRF24L01 chip. It uses the same frequency as wifi does but uses a different protocol to make transmissions very fast and low power. The transmission of the sensor data only takes a few milliseconds.

Transmission range

I tested the wireless modules outside and transmission range of the nodes is around 50-70m in free space. A test inside an office building still gave a range of about 30m through several walls. In a modern building with thick concrete walls the range depends the building structure but the transmission power is still enough to go through one wall but not a floor or an outside wall.

Battery Powered

Except for the Alpha Node the sensors are battery powered. Measurements of the power consumption show that the nodes can operate for years on a small CR2032 coin cell battery. The battery life is of course influenced by the measurement interval but even at an interval of 15 seconds it lasts over a year and at 1 second still over a month.

Epsilon Node – the plant sensor

This was the first node designed around a measurement circuit I came up with that can be implemented on any microcontroller and uses only two digital inputs, a resistor, a diode and a reference capacitor. The circuit works similarly to the commonly used charge transfer circuit but it does not require any additional switches.

epsilon_node

What it does is to measure the capacitance of the insulated electrode. The big advantage of measuring capacitance instead of soil resistance is that the electrode does not corrode over time when left in a moist environment and it also does not influence the soil. The capacitance measured strongly depends on the dielectric: the water in the soil.

The circuit’s sensitivity is automatically adjusted by the software and the probe can measure the full range from completely dry up to fully soaked soil. Just put it in the pot of your plant and using the Alpha Node or any other internet capable board it can notify you on your smartphone when it is time to water the plant.

Optionally a NTC resistor can be added to measure the temperature, just like the Theta Node does.

Name: since it measures the relative permittivity (aka. dielectric constant) of the object in front of the electrodes, i.e. the soil, it is named Epsilon Node.

 

Theta Node – temperature sensor

This node measures the temperature using an NTC resistor. These types of resistors lower their resistance the higher the temperature gets. Using precision resistors the measurement can be quite accurate. To my calculations it is within ±0.5°C accuracy and the resolution at room temperature is an impressive 0.02°C so you can measure the slightest change in temperature.

thetanodeWith the batterybeing magnetic it just takes a small magnet to attach it to your fridge, a heating pipe or a railing.

 

Name: the greek letter used for temperature is ‘Theta’

Kappa Node – the weather station

With its high precision environmental sensor it measures temperature, relative humidity and barometric pressure. It uses the latest generation Bosch sensor: the BME280.

The Kappa Node makes it really easy to build your own wireless weather station. And with the I2C and GPIO interface even more low power sensors can be added
like a light sensoror a rain sensor. Actually the Epsilon Node could also be hacked as a rain sensor.

kappanode

Name: the name for this node is actually not physics related. As it measures multiple physical values I decided to name it after the first letter of the Greek word for weather: καιρός (kairós) which is the letter ‘kappa’

Alpha Node – wifi bridge

This is the base station that receives and processes the data. It uses a ESP8266 module to transmit the data through your wifi to the internet where it can be displayed in a graph and can send you notifications by email, text message or notifications on your phone. It also logs the data to an SD-card if enabled.

For easy setup and configuration it can act as a wifi accesspoint that contains a web page to setup your wifi password and to send configuration data like the measurement interval or calibration data to the nodes.

The built in color LED shows you the current status of the node and during normal operation it lights up brighter whenever a value is received from a node.

I even designed a casing that I mould out silicone from a 3D-printed mould. When checking with a manufacturer it turned out to be really expensive to have it made so I am not sure if these can be manufactured for the crowdfunding campaign.

Project background

Lambda nodes are wireless sensor nodes that are battery powered and run for a long time. The Idea is to be able to use sensors without having to wire them up or to hassle with the programming to actually get them up and running. The project also includes the Alpha Node which acts as a receiver for all the sensor nodes but basically any microcontroller based hardware can be used: Arduino and Raspberry Pi are just two examples. You just need to add a NRF24L01 wireless module.

epsilonnode_inaction
wireless soil moisture sensor

The project started at the end of 2014 when I was experimenting on how to do highly sensitive capacitive sensing with as little components as possible. This was after I discovered a great project for plant monitoring but I wanted to improve the design. I came up with a circuit to do capacitive measurements on any micro controller by just using two digital pins and three components.

I wanted the circuit to be low power and use commonly available and cheap components. So I did some research and built the first prototype. Having already used the NRF24L01 wireless data modules before and finding that they are indeed one of the lowest power transmitter available they were an obvious choice.

The prototypes worked quite well and after a few months testing and coding I was ready to design the next round of prototypes along with a receiver station (the Alpha Node) using the ESP8266 wifi module. It turned out that these wifi modules are much more powerful than an Arduino and it did not make much sense to just use it as a modem. Also the AT command firmware was very annoying and buggy.

The next round of the Alpha Node prototypes were based purely on a ESP8266 module and worked much better. Having all the hardware working I dug into the code, tried various protocols and improved code stability and speed. Also I tried various online services to display data or events but nothing was really satisfying. I asked a friend to help me out here and he programmed a great web interface to log and display the data (work in progress).

The project is in an  advanced state now and I decided to start publishing it bit by bit. Check out the more detailed descripton of the sensor nodes over in the Hardware section.

I really enjoy working on this project and I learn something new each time I try to improve it. If you have any suggestions for great additions to this project, please feel free to send me a note.