Difference between revisions of "User:Bertrik Sikken"

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(Audio over networks)
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== Project ideas ==
== Project ideas ==
This is a list of ideas I'm thinking about, but have not fully developed into an actual project yet.
This is a list of ideas I'm thinking about, but have not fully developed into an actual project yet.
=== Raspberry pi airplane tracking ===
Apparently now you can also participate in MLAT tracking of planes that don't transmit GPS coordinates themselves.
=== Honeywell PM sensors ===
=== Honeywell PM sensors ===
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I plan to experiment with this kind of network connectivity for perhaps a bat activity monitor or air quality monitoring.
I plan to experiment with this kind of network connectivity for perhaps a bat activity monitor or air quality monitoring.
Hardware is taking a while to arrive...
Hardware is taking a while to arrive...
=== Audio over networks ===
I'd like to experiment a bit more with sending digital audio from an embedded system and play it back on a laptop (for example).
Perhaps create a basic intercom-like system, or even apply this for recording ultrasonic bat audio.
==== audio over ESP now ====
The idea is to create a simple battery powered intercom transmitter:
* sample audio from a digital microphone, like the INMP441
* send it to another ESP using the esp-now protocol, see https://hackaday.io/project/166868-my-attempt-at-an-esp-now-mesh/log/166398-072419-maximum-transmission-throughput-of-esp-now
* play it back on the other ESP, using https://github.com/earlephilhower/ESP8266Audio
* instant-play: pressing the button powers up the ESP, starts sending audio ASAP
==== Protocols ====
* seemed complicated wrt setup when I looked at at
VBAN protocol:
* [https://www.vb-audio.com/Voicemeeter/VBANProtocol_Specifications.pdf specification]
* audio over UDP with a simple fixed size header in each packet
=== APRS gateway ===
=== APRS gateway ===

Revision as of 14:27, 1 March 2020

User info Bertrik Sikken
Name Bertrik Sikken
Nick bertrik
Tagline heb ik niet

You can reach me at bertrik@sikken.nl or bertrik@gmail.com

Studied Electrical Engineering at Twente University.

Main interests:

  • reverse-engineering things (USB stuff, mp3 players), working on http://rockbox.org
  • studying bats and making electronics for recording/listening to bat sounds
  • radio stuff, in particular software-defined radio

Projects I work(ed) on (refresh):

 Project Status
Pico TrackersAbandoned
Secure iButtonCompleted
AntiLostIn progress
AudioMothIn progress
CubeCellIn progress
EspAudioSensorIn progress
Esp32camIn progress
FMCWRadarIn progress
LoraWanDustSensorIn progress
LoraLuftdatenForwarderIn progress
Sim7020In progress
StereoBatRecorderIn progress
StofAnanasIn progress
UltrasonicPlayerIn progress
... further results

Project ideas

This is a list of ideas I'm thinking about, but have not fully developed into an actual project yet.

Raspberry pi airplane tracking

Apparently now you can also participate in MLAT tracking of planes that don't transmit GPS coordinates themselves.

Honeywell PM sensors

NB-IoT module

Ordered a SIM7020 and a starter kit, see: https://www.hackster.io/voske65/arduino-nb-iot-with-sim7020-and-t-mobile-027f8f

I plan to experiment with this kind of network connectivity for perhaps a bat activity monitor or air quality monitoring. Hardware is taking a while to arrive...

APRS gateway



Small inexpensive modules that play mp3 from an internal flash. Could be nice for a custom door bell for example.

More info at:

Implement code for samenmeten API


ESP mesh networking

The makers of the ESP8266 provide a software stack to run a 'mesh network' between ESP8266 nodes. As I understand this allows several ESP devices to join together and create a kind of dynamic network.

What is not quite clear is how this is all modelled. For example, do devices have addresses. And if so, what kind? Does this thing run on top of TCP/IP, or does it run on raw wifi frames? Can you designate one particular node as the 'root node' and let it bridge communication with other networks? Can you do transient low-power communication with the mesh network, e.g. wake-up, send a message, go back to sleep?

The plan is to experiment a bit with this, perhaps use it in the hacker space for the sensor/actuator infrastructure.



Cheap FPGA boards and nice applications:

Neural networks on low-end hardware

Investigate if you can run a powerful neural network on relatively low-end/cheap/low-power hardware. For example a Raspberry pi. A RPI runs Linux, run python, just like some common neural frameworks. Do we need hardware acceleration from the GPU and does the RPI GPU support that?

Read list:

Bought a MaixPy:

STM32 stlink V2

Interesting to see you can flash inexpensive stlink v2 clones into even more useful debuggers:

investigate quadcopter remote control

It turns out that the typical little cheap Chinese quadcopters use a remote-control protocol that can be easily recreated using the famous NRF24L01+ chip (< $1 and easily connected to an arduino). This gives nice opportunity to either:

  1. transmit our own control signal, to control a quadcopter from something different than the manual remote control, e.g. automatic control
  2. receive the control signal, so the manual remote control that comes with a quadcopter can be used to steer other things (like a model car).

I haven't found a good overview of quadcopter remote control protocol specifications yet, there seem to be plenty examples of "here-is-the-code" however.

mini word clock in dutch

Basically an monochrome 8x8 word clock, in Dutch, showing local time in the Netherlands.

This git repo has the current code.

See here for a demo running on an arduino nano.

The plan is to run this from an ESP8266 instead of an arduino nano, so it can get the time from the internet using NTP. Andreas Spiess demonstrated on youtube how existing libraries on the ESP8266 can be used to do the local time (including summer-time) calculations.

Cypress PSOC5

Play with the Cypress PSOC5 platform, which combines a ARM Cortex-m3 processor with configurable analog blocks. I'm thinking of combining it with a 24 GHz doppler radar sensor, to process the signal and present it as a USB audio device (stereo signal contains I and Q parts). See RadarOnAStick.

Simple Doppler motion sensors

You can find basic doppler microwave motion sensors based on a single transistor, with some weird traces on the PCB very cheaply, for example

Typically the microwave part of these consists of a single transistor with a rectangular area on one leg and a meandering trace (with lots of vias to the other side) on the other leg. The output of this circuit seems to go into a chip very much like the ones used in PIR sensors.

See also https://github.com/jdesbonnet/RCWL-0516 for a reverse engineering effort of these doppler radar modules.

Bare-bones Arduino bat detector

This is an idea for a very basic heterodyne bat detector, doing signal processing on an Arduino, requiring minimal external components.

The basic principle of a heterodyne detector is that it just mixes (multiplies) the audio signal with a square wave, low-pass filters the result and puts it on a speaker.

Multiplying with a square wave can also be considered to be just alternatively inverting and not-inverting the signal. So if you sample an ultrasonic signal at twice the rate you want to multiply, you can just subtract odd samples from even samples and low-pass filter that.

How this can be done in an AVR Arduino:

  • sample the audio signal at twice the detection frequency, say 84 kHz. An AVR should just be able to do that.
  • apply a 1-pole IIR high-pass filter to remove DC bias, this takes one shift instruction and one addition.
  • multiply by the detection frequency, this means just inverting the odd samples.
  • low-pass filter the signal, this can be done using a moving average filter, say 16 samples long (first null at 5.25 kHz). Theoretically, averaging 16 samples should result in two bits extra accuracy. This operation takes some storage, an addition and a subtraction.
  • output the filtered signal using PWM, possibly at the same rate that we are sampling the input audio.

The microphone can be a 40 kHz piezo transducer, to keep it cheap (but also limited to 40 kHz). The pre-amplifier can be a single transistor with some resistors around it, providing about 40x gain. The arduino does the signal processing (mixing, low-pass filter) to shift the bat audio to human range. The speaker amplifier can just be a simple two transistor push-pull circuit, since the output from the Arduino is digital/PWM.

AVR Arduino sample rate

As far as I understand, the ADC clock can be set to 1 MHz. Conversion takes 13 cycles, so this can be a problem to reach a sample rate above 80 kHz.

Bat call cleaner

This idea is about a simple push-button tool to clean up bat call recordings, to make them more suitable for playback in a bat lure.

By cleaning up, I mean removing the background noise and turning it into a pure sine wave like signal characterized only by an instantaneous frequency and amplitude. Any harmonics will be lost.

This way, you can pick a nice bat call recording and turn it into a file suitable for playing back at relatively loud volume from a bat call player / lure, without the broadband noise.

The signal is put through the following signal processing stages:

  • Apply a high-pass filter to get rid of non-ultrasonics like speech, say at 10 kHz
  • Convert the signal into a complex analytic signal, by creating the imaginary part using a Hilbert transform.
    • The Hilbert transform can be approximated by a FIR filter of appropriate length to get the desired bandwidth.
  • Split the complex signal into an instantaneous frequency and amplitude.
    • The frequency is determined by differentiating the instantaneous phase (inverse tangens i/q).
    • The amplitude is determined by calculating the norm of the complex signal (sqrt of i^2 + q^2)
  • Average/low-pass the frequency component, say at 0.1 ms intervals. Do the same to the amplitude.
  • Re-synthesize the signal s using the simple model s = A.sin(2.pi.t/f) where A is amplitude, t is time and f is frequency, interpolating A and f.


  • Apply a high-pass filter to get rid of non-ultrasonics like speech, say at 10 kHz
  • chop the signal up in segments of (say) 100 ms and determine the total energy content in each segment
  • choose the quietest segment and use this as a "background noise template"
    • perform a fourier analysis on the template
  • reduce the noise on the signal using the template:
    • for each segment, apply for each frequency an attenuation based on the signal level compared to the template level
    • resynthesize from fourier back into the time domain using an overlap-add method

=> perhaps there is already software which performs these steps?

ESP8266 modbus adapter for solar converters

This idea is about programming an ESP8266 to talk to solar converter over RS485 on one side as a MODBUS/serial master, and making it appear as a MODBUS/TCP slave on the other side. This way you can talk to your solar converter using an existing MODBUS/TCP slave application.

The particular solar converter is the "EP Solar MPPT Tracer A 3210A". This github project may give some clues to which modbus registers contain what.

The RS485 side is done using a (soft-)UART on the ESP8266 with a TTL-to-RS485 converter.

The TCP/IP side is done using the built-in wifi of the ESP8266.

ESP8266 as modbus slave (over network):

ESP8266 as modbus master:

GPS repeater

This idea is about experimenting with a cheap GPS repeater built out of an "active" GPS antenna.

The problem this solves is that often indoors you have no GPS reception, but you like to have some signal to experiment with (e.g. a LoRa tracker).


  • get a cheap active GPS antenna from AliExpress (some as cheap as E2,- !), most just mention one frequency (1575.42 MHz)
  • get a bias-T circuit to feed it the supply voltage (e.g. from a KOPPLA) and pass the RF signal onto an indoor antenna
  • the indoor antenna may be as simple as a 1/4 wave coax dipole: center conductor sticking up (about 47 mm), coax shielding is divided into 3 of 4 ground radials sticking sideways
  • build it and test it with a smart phone, tracker hardware, etc.

See also:

Indoor radar speed sign

This idea about placing a simple IQ-output radar sensor indoors in the hacker space, do some basic signal processing on the IQ doppler signal and determine movement speed and direction, then display this on a LED display. This is of no immediate practical use other than fun, but helps me to gain a bit more experience with microwave radar sensors and eventually build a more effective setup for detecting/counting bats flying in and out of a roost.

Implement this on a PSOC5 platform or on the STM32 using Arduino.