Raspberry Pi/Linux with the nrf24l01+ & RF24Gateway

 A guide/example of advanced RF24/nrf24l01+ usage and monitoring on Linux devices

Overview:

There are many examples of basic RF24 usage, but not so many demonstrating complex communication scenarios and networking. The RF24 communication stack is free, with all development information, examples, code and documentation available on GitHub, designed with Arduino and RaspberryPi/Linux devices in mind.

The RF24 stack generally follows the OSI model, and provides a separate library for each layer, with efforts toward proper documentation for each layer. This allows any type of data/communication to pass over the radios, including TCP, UDP or any other protocol, and users can write their own code to link devices together using any layer(s).

So, what can we really do with these little radios?

The nature and low-cost of the system allows literally anybody with interest in wireless communication to setup, deploy, and experiment with small scale, self-healing wireless networks, or start writing their own code with the core RF24 driver, in a very short period of time.

The stack currently does not include sensor specific code or functionality, it is simply a communication stack which enables all kinds of different functionality.


Requirements, Hardware Setup & Installation:

1. A Raspberry Pi or Linux device a compatible radio device connected
    RPi Install Script

2. An Arduino device with a compatible radio device connected
    Arduino Install (Select RF24, RF24Network, RF24Mesh, RF24Ethernet libraries)

See the RF24 docs for hardware setup info & installation
Note: Please report any issues at https://github.com/TMRh20/RF24/issues

Demonstration and Information:

The video below demonstrates how to use RF24Gateway in general - Setting up nodes and communicating over RF24 using RF24Network, RF24Mesh, and standard protocols (ICMP, HTTP, SSH), along with monitoring

1. Config on RPi & Arduino devices
2. Complex mesh networks: How the different layers (RF24Network,RF24Mesh & RF24Ethernet) can be used with RF24Gateway
3. RF24Ethernet: Simple TCP/IP communication and web server running on Arduino over RF24
4. RF24Mesh: Node config & interaction with RF24Gateway
5. RF24Network: Node config & interaction with RF24Gateway
6. How to use WireShark to monitor user payloads with RF24Gateway and LUA Wireshark dissector script (using laptop w/SSH & X11 forwarding in this example)
7. How to configure Raspberry Pi as a standard node in an existing RF24Mesh
8. How to sniff RF24Network, RF24Mesh, and RF24Ethernet traffic using RF24Gateway & Wireshark(optional)
9. Tacos... oops they were already eaten, maybe next time

As shown in the video, any nodes running RF24Ethernet, RF24Mesh or RF24Network can be used with a master node running RF24Gateway, allowing users to try out or utilize the different layers quite easily.

RF24Ethernet: Provides TCP/IP communication. Very simple and reliable way to control on/off devices via a web browser or scripting. Has all functionality of lower layers.

RF24Mesh: Provides automatic (mesh) networking for RF24Network. RF24Mesh networks are self-healing and dynamic in nature.

RF24Network: Provides addressing, routing, fragmentation/reassembly of payloads via manual/statically defined wireless networks.

See the detailed overview and related pages at http://nrf24.github.io/RF24Ethernet

HB-100 / GH1420 10.25GHz Microwave/Doppler Radar Motion Sensor w/Arduino

I picked up some of these modules out of curiosity, as these modules can detect the motion and speed of objects through walls or other non-metal obstacles.


Overview:

The modules themselves are fairly simple in use, since they output a signal with the frequency/pulse width indicating the speed, and the amplitude/voltage indicating the strength of the reflected signal.

The speed is very simple to calculate once the frequency of the incoming signal is detected:

km/h = hz / 19.49
mph = hz / 31.36

These HB-100 modules require a pre-amp circuit to boost the IF signal, so I used an LM358N and the circuit at https://hackaday.io/project/371-hb100-radar-shield to boost the intermediate signal.

The output of the circuit sits at about 2.5v when quiet, and moves up & down to represent the received AC waveform.

In pure digital form, this would be very easy to detect using interrupts, but with an amplified analog signal, weaker signals may be missed.

Problem:

Tests with this radar shield using standard frequency counters/libraries seemed to show that interrupts and edge-detection have limited sensitivity to weak signals. They also provide no means for detecting the amplitude of the signal. It seemed that using the on-board ADC may provide better results in this frequency range. (0 to 4000 Hz or 0 to 205km/h)

Goal: 

Detect the frequency and amplitude of low frequency (0-4khz) signals from HB-100 modules, while potentially increasing the sensitivity of waveform detection using the HB-100 & circuit as linked above.

Result:

The resulting code uses the Arduino ADC to sample incoming signals using very simple peak-to-peak detection.

Signals are detected when the waveform transitions above or below a set "midPoint" which represents the 0v value of the AC waveform. Typically, it will be ADC_RESOLUTION / 2.

This is very similar to the edge detection used for interrupts and digital signal detection, but using the ADC to detect weaker transitions.

ADC Resolution / VCC == Volts per Increment ( 1023/5v = 0.005v )
Sensitivity of 10 == peak-to-peak detection of +/- 0.05v signals


Technical Info:

The ADC is driven in free running mode, taking samples at about 38.5khz while looking for specific deviations in the measured voltage that correlate with a peak-to-trough waveform.

The results are summed in a single variable, and returned either as a single result, or an average, depending on how often the results are read in relation to the received signal.

Information about the signal, including frequency, amplitude and number of samples allows more complex analysis.

Conclusion:

The code was developed in a very short time, and can still be considered a work in progress, although it seems good as-is. It tests well up to about 2-4khz with another Arduino producing square wave output. When used with the radar module, it is very sensitive, and allows the sensitivity of the device to be defined in software, and/or results to be filtered.

Code & Examples:

https://github.com/TMRh20/AnalogFrequency