User:Bertrik Sikken

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

You can reach me at or

Studied Electrical Engineering at Twente University.

Main interests:

  • reverse-engineering things (USB stuff, mp3 players), worked on (sansa clip players)
  • studying bats and making electronics for recording/listening to bat sounds
  • radio stuff, in particular software-defined radio
  • energy-related stuff, visualisation
  • citizen science, particulate matter measurement, noise (future)

Projects I work(ed) on (refresh):

 Project Status
Pico TrackersAbandoned
Secure iButtonCompleted
AntiLostIn progress
AudioMothIn progress
CubeCellIn progress
EspAudioSensorIn progress
Esp32camIn progress
FMCWRadarIn progress
LoRa-E5-miniIn progress
LoraWanDustSensorIn progress
MainsFrequency2.0In progress
Msi2500SDRIn progress
Sim7020In progress
StereoBatRecorderIn 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.

Cameratoezicht in Gouda

Gemeente Gouda publiceert een lijst van camera's in Gouda, zie

Aanmelden (semi-automatisch) bij bijvoorbeeld Surveillance-under-surveillance ?


Interact with the solar edge api for PV installations.



Idee: gekleurd lampje dat aangeeft of er op dit moment een stookalert actief is



Display of current electricity usage

I have a slimme lezer connected to the P1 port of my smart electricity meter. The default firmware exposes meter readings using an event stream (SSE) at http://slimmelezer.local/events

So for example, you can read this with curl:

 curl http://slimmelezer.local/events

With result:

 event: state
 data: {"id":"sensor-power_consumed_phase_1","value":0.046,"state":"0.046 kW"}

Specification of SSE:

What I would like to do is to read these events and control a simple display to display the current usage number.

Participating in ultrasonic sound project

Use an audiomoth for this.

ESP32 C3

Flexible LED ticker

Ordered a flexible 32x8 RGB LED display:

Matching diffuser to be 3D printed:

Washing machine API


Investigate TinyML, see Apparently can run on a RP2040 (raspi pi pico).

Bat box busy signal

My brother built little pyramid 'blinkies': solar cell + Lithium-supercapacitor + harvesting circuit + LED encased in clear expoxy.

Can we add a low-power PIR sensor to this, and make a blinky that blinks if movement was detected by the PIR in the past hour?

Super tiny RFID

See an almost sand grain size RFID chip, 1.25mmx1.25mmx0.55mm

The accompanying reader "M6E Nano" is still quite expensive at $235.95 :

Uses the MuRata LXMSJZNCMF-198: "This product can be used as an ultra small tag and this can be fit on any metal objects, non-metal objects, as well as embedding into any objects by glue or adhesive and so on."

See also: ISO18000-63 and EPC global Gen2(v2)


TheThingsNetwork-Sondehub bridge

Uploads balloon telemetry to sondehub.


Actually smart boiler

The boiler for hot water is about half of my electricity bill. The idea is to use/build a smart switch that switches it on at time when electricity prices are lowest.

Currently have a "Slimme boiler module" from Eneco, which is *not* actually smart, since it allows me no control whatsoever when it switches on (besides cutting the power in the meterkast). For example, it will switch on mid-day when my price is high and eneco's price is low, perhaps good for Eneco, not for me. Apparently, they even received a subsidy for this.

A boiler is a pretty big load, about 3 kW, but it is not inductive.


Receiving gas meters

Apparently gas meters send gas consumption data to the slimme meter using a wireless link in the 868 MHz band. Probably just FSK at 38.4, as mentioned here:

Interesting leads:

Multi-network wifi manager

Figure out or create software so that ESP8266/ESP32 wifi manager can use multiple networks to connect (not just one), so it allowing storing credentials for more than one network. For example, the network at home, the hacker space, at work. Instead of reconfiguring the wifi manager for each network, you just *add* your credentials (and the existing credentials are not replaced).


Interesting candidates:

Automated bat counting

Can we automatically count the number of bats from the image of a webcam mounted in a bat colony? Perhaps using some kind of AI or machine learning algorithm?

The image in question:

Use TheThingsIndoorGateway as Helium/ThingSix hotspot

I have a spare TTIG that could perhaps be used as a Helium gateway, investigate what is possible.

One possiblity is to capture the gateway traffic from the TTN gateway events API, convert uplink data to a format that the Helium network understands and forward it. I don't really care about the Helium crypto tokens, this is just for fun and science.

What I definitely can do:

  • Capture data from the TTN gateway events API, convert it back to Semtech UP format and forward it somewhere to Helium (just not sure where!)
  • See also my project which contains an initial implementation of TTN-to-UDP conversion


  • If you want to be a gateway on the Helium network, you have to *pay Helium*! Obviously I don't want that, I just want to share data to improve *their* network.

Open work:

  • Investigate if Helium has an open uplink API for their hotspots, if so study it etc, without paying the gateway fee
  • Investigate if Thingsix has an open uplink API for their hotspots, if so study it etc, without paying any gateway fee
  • Investigate if Thingsix is just another Helium, with weird crypto money

Interesting stuff:

Adding BLE GATT interface to sensors

The GATT specification allows measurement properties to be defined and transferred continuously over Bluetooth low-energy.

With GATT you can define a collection of properties (e.g. measurement items like temperature/humidity/particulate matter/noise, etc) organised in a simple structure of a BLE service. The 'notification' method allows you to basically push the data continuously to a connected host, e.g. a smart phone.

Services collects characteristic, a characteristic has values with units. Each of these (service, characteristic, unit) have their own unique "UUID". This is described in the so-called 16-bit UUID numbers document

Interesting stuff in GATT:

  • See GATT_Specification_Supplement_v5 paragraph 3.151, it has a noise characteristic with 1 dB resolution.
  • 0x27C3 is the GATT *unit* for sound pressure
  • 0x181A is the GATT environmental sensing *service*, document name "ESP_V1.0.0.pdf"
  • 0x2A6E is the GATT Characteristic and Object Type for temperature
  • 0x2A6F is the GATT Characteristic and Object Type for humidity
  • 0x2BD5 is the GATT Characteristic and Object Type for Particulate Matter - PM1 Concentration
  • 0x2BD6 is the GATT Characteristic and Object Type for Particulate Matter - PM2.5 Concentration
  • 0x2BD7 is the GATT Characteristic and Object Type for Particulate Matter - PM10 Concentration
  • Unfortunately I cannot find a characteristic for carbon-dioxide (CO2) in the BLE GATT unit document

See also

Reverse engineering XS-8217 bluetooth air quality meter

This is a thing that measures CO2, humidity, temperature, TVOC and formaldehyde.

See also

This teardown looks a lot like my sensor: Mine contains a "TPM-300A" sensor for measuring VOC.

It has a bluetooth interface, device name is XS-8217. It has a BLE GATT profile, with the following services

  • service 0xC760
    • characteristic 0xC762 (WRITE)
    • characteristic 0xC761 (NOTIFY)
      • example data: 0x23 0x06 0x10 0x04 0xF1 0x00 0x23 0x65
      • example data: 0x23 0x08 0x10 0x04 0x01 0x9A 0x00 0x0A 0x00 0x03 0x0E
      • data shown on screen was approximately: CO2=418ppm, HCHO=0.003mg/m3, TVOC=0.013mg/m3, temp=24degC, humi=35%

So data in the characteristic 0xC761 seems to have a 4 byte constant header:

  • 0x23
  • length byte

Then we have for the first message: 0x10 0x04 0xF1 0x00 0x23 0x65

  • 0x10 0x04 fixed header
  • 0xF1 is temperature in 0.1 degree Celcius most likely (24.1)
  • 0x00 is ...
  • 0x23 is humidity most likely (35)
  • 0x65 is ... checksum perhaps

And for the second message: 0x10 0x04 0x01 0x9A 0x00 0x0A 0x00 0x03 0x0E

  • 0x10 0x04 fixed header
  • 0x01 0x9A is the CO2 concentration (410)
  • 0x00 0x0A is TVOC most likely (10)
  • 0x00 0x03 is HCHO most likely (3)
  • 0x0E is ... checksum perhaps

ribbon tweeter for bat audio

Someone gave me this idea: Use a ribbon tweeter like this for playing back bat audio:

The frequency spectrum shows no sign of dropping off at 20 kHz.

3d glasses

I got some 2nd hand 3d glasses, they look exactly like these ones:

The common name appears to be "G15-DLP".

A tear-down here:

Interesting documents:

Someone claims he got something to work with some hacks:


Op verschillende plekken in Gouda staat er water in de kruipruimte van huizen van bewoners. Kunnen we dat meten en inzichtelijk maken, voor bewoners, op een kaart bijvoorbeeld?


  • in de kruipruimte plaats je een module die waterhoogte kan meten
  • de module bestaat uit een microcontroller en een afstandsmeter, die de waterhoogte bepaalt
  • de gegevens worden via WiFi doorgestuurd naar een centraal punt, waar de data wordt verwerkt en gevisualiseerd
  • op een webpagina kan je een overzicht zien van alle meters die online zijn
  • de meting wordt gedaan door bijv. een laser-afstandsmeter of een ultrasoon-afstandsmeter
  • voeding? lastig, hoe krijg je 5v naar een potentieel natte plek?
  • kosten? verwachting < E 40,-

In Gouda wordt op veel verschillende plekken de grondwaterstand gemeten, zie , maar:

  • geen visualisatie op de kaart, je ziet alleen de meetlokaties d.m.v. een icoontje!
  • geen meetpunten in Gouda noord!

Online bat detector

Idea: use an ultrasonic microphone, connect it to a WebSDR, so people can tune into bat sounds remotely.

Raspberry pi airplane tracking

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

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:


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:

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 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.

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.