Category Archives: miscellaneous and news

Here I post everything that doesn’t correspond to a specific category

Keysight oscilloscope dies in standby – power supply repair


An interesting problem has arisen with the measurement technology in the laboratories at my workplace. I use the term “measurement technology” to describe the equipment of a laboratory workstation for basic training. There are a total of nineteen laboratory workstations, each equipped with two laboratory power supplies, two desktop multimeters, a Keysight signal generator and a Keysight (Agilent) oscilloscope of the Infiniivision DSO-X 20xx series. All devices are network-compatible and are connected to the corresponding workstation computer via LAN. This means that the measuring devices can be accessed using different software (Agilent VEE, Matlab, LabVIEW etc.). The devices were purchased around three years ago and replace the almost twenty-year-old laboratory equipment.

However, it has now happened that the DSO-X2012A oscilloscope at one workstation no longer showed any signs of life. It occasionally happens that during laboratory exercises or when working freely in the laboratories, a student presses the emergency stop switch of the workstation and thus de-energizes it. But this was not the case. All the devices connected to the workstation worked, with the exception of the DSO. Voltage could also be measured at the end of the IEC plug. So the problem could only be with the oscilloscope itself. The rear panel is quickly unscrewed, a shield plate removed and the power supply unit is exposed. The first visual inspection immediately revealed the large filter capacitor with its upwardly curved cap. But first things first.

Power supply unit of the Infiniivision

The mains voltage was measured at the AC pins of the mains input, but no DC voltage was measured at any of the outputs of the power supply unit. Regardless of whether the power switch of the device was switched on or off. This suggests that the power supply unit is defective.

Input fusing

First, the power supply unit was removed and examined, starting with the AC input side. The print fuse in the area of the mains filter was the first defective component to be noticed. It is a slow-blow 6.3A/250V fuse. As a blown fuse always has a reason to switch off, the search continued. The mains rectifiers were OK, but the 100uF / 420V electrolytic capacitor, which is used to smooth the DC voltage on the primary side, had already suffered some thermal damage and was bloated.

original electrolytic capacitor 100uF /420V /105°C

Its capacity was also no longer within the nominal range. But even that was not the direct reason for the fuse blowing. This was quickly found. A mosfet used to control the transformer was low-resistance. More precisely, it had a short circuit between all the connections.

Mosfet STP12NM50

The following picture shows the installation positions of the components. These have been replaced. The mosfet was replaced with an original type and the power supply capacitor was replaced with a 100uF / 450V / 105°C type. Although it is about five millimeters higher, it fits easily into the power supply unit.

Installation position of the capacitor and the mosfet

Two SMD resistors on the back of the power supply board were defective in the area of the gate connection of the mosfet. These were an SMD resistor of size 0805 with 5.11 Ohm and an SMD resistor of size 1206 with 2.0 kOhm. The picture below also shows the installation position.

Mounting position of defective SMD resistors

After all the components mentioned had been replaced, a first functional test was carried out. However, this was sobering, as the power supply unit was still not working. The fuse remained intact and the DC voltage on the primary side was stable. But the gate of the mosfet was not activated – unfortunately. Because now came the time-consuming part of the repair. On the power supply board, installed upright, there is another board on which several controller ICs are installed. If you follow the gate line from the Mosfet, it ends at a pin on this control board. So this must be removed.

Controller board removed

To do this, the cooling plate had to be removed first. Then it became a bit tedious, because the controller board is not connected to the main board via a pin header or plug connection, but the contact pins are laid out and milled out. This means that the desoldering work has to be carried out very carefully so as not to damage the conductor tracks at the ends of the milled pins.

Mainboard without controller board


Once the removal was successfully completed, the controller board could be inspected. Lo and behold, the line routed from the gate of the mosfet ends at pin 6 of a small IC. This is a UC3842B VD1R2G. The housing of this IC was blown up. In addition to the controller IC, a SOT23 PNP transistor (PMBT 2907A) was also dead and had a low resistance on all pins.

Installation position of the defective components

After replacing the defective components, the power supply unit was reassembled and a function test was carried out. The oscilloscope started up again and the power supply unit did its job.

defekte Bauteile
Result after successful repair

It would now be interesting to find out why the power supply gave up the ghost after just three years. Especially as the oscilloscopes do not run continuously, but are only switched on during the relevant courses. We noticed the following: The oscilloscope is permanently connected to the power supply. However, the oscilloscope’s power switch does not switch off the AC supply, but only the controller control in the secondary area of the power supply unit. This means that the power supply unit operates in standby mode when it is switched off. And we have noticed that all oscilloscopes that are switched off have a power loss in standby that heats up the mosfets and especially the 100uF electrolytic capacitor. This would explain the bloated, dried-out electrolytic capacitor and the subsequent death of the power supply units. To verify this, the temperature of the components was measured on several devices that had not been switched on for days.

Thermal sensor on the electrolytic capacitor surface

The following could be determined here. Both the surface of the capacitor and the cooling plate of the mosfets measured temperatures of 56°C to almost 60°C when switched off. Should this be the case?

Temperature measurement on the electrolytic capacitor


Here are the required components:

  • resistor 5R11 0,1W 0,1% Farnell Nr.: 1872688
  • resistor 2k0 0.66W Farnell Nr.: 721-9844
  • PNP Transistor SOT23, SMD Stempel 2F Type PMBT2907A, 215 Farnell Nr.: 1626500
  • PWM Controller IC, UC3842B VD1R2G / 500kHz Farnell Nr.:2845218
  • capacitor 100uF / 105°C / 450V
  • fuse T6.3A 250V

Jun2019: Order numbers updated




Integrating the heat pump (NEURA) into the Smarthome


A smarthome is no longer a rarity today and is very widespread. There are countless systems on the market that make your own home “smart”. The digital voice assistants from Google, Amazon and co. in conjunction with smart light bulbs are among the systems that are easy and quick to install. But there are also complex smart home systems, in which actuators for every lamp and socket are installed in the house distributors. The windows and doors are equipped with signaling contacts and secure the home or report if once forgotten to close the windows after shock ventilation. It goes without saying that these systems also contribute to energy optimization when programmed sensibly. I also operate Smarthome components from various manufacturers.

For years, this has included the HomeMatic system, which communicates with its actuators and sensors both wired and via the Bidcos protocol. The HUE system from Phillips talks to its smart lamps and sockets via ZigBee. The gateways of these systems are connected to a LAN network and each system brings its own web server, through which it can then be controlled and set. An inverter of photovoltaic systems can provide its data via different interfaces (RS485, CAN, RS232). To bring all of them to a central display level, I decided to use the NodeRed system. The necessary NodeRed server runs on a Raspberry PI. (On the CCU3 with the Raspbian image is still enough space to run the NodeRed server – it is even available as a separate plugin for the CCU and is called “RedMatic”). With this configuration you can “slay” almost everything in the field of home automation. With ESP32 and Raspberry you can easily transfer status information via MQTT (Message Queueing Telemetry Transport). I use this for example with the small feed-in inverters of a balcony PV system, as well as with the PV inverters of an offgrid system. Here the data is received via different bus systems in the Raspberry or ESP32 and converted into the MQTT protocol. The MQTT broker collects the data from the individual devices and via NodeRed they can then be written to a database, visualized in the browser or on the smartphone and also easily processed in the HomeMatic system, as required.

Example of a smart home network

Thus, it is possible to network almost all systems with each other smartly and, importantly for me, to visualize them on ONE platform. One single system was missing until now. That is my old Neura heating heat pump. The company Neura has not existed for several years and the web server “webidalog” developed by “b.i.t.” has never been updated. So the heat pump has a web server on a small with Linux computer onboard and builds the web application with an ancient Java version. For the operation a Java Runtime must be installed on the PC, which runs only with some tricks on a current Windows computer (keyword: virtualization). For the operation via a smartphone an html – version with limited functionality is available. My plan now was to find an interface, with which I can read out the data of the heat pump at least once, in order to have flow- return temperatures of the floor heating, boiler temperature, etc. also available in my NodeRed system. But since there is almost no documentation for the system and reverse engineering is a bit critical if the system should continue to run, I came up with the following idea:

With a “headles browser” it should be possible to parse the html version of the Neura WebDialog website and find the relevant data and turn it into MQTT topics via variables. And here I have to give a special thanks to my colleague Mario Wehr, who built the software structure to parse the website. The software is written in PHP and finally runs on a Raspberry PI. All you need is a php8-cli runtime and a few modules. The way the software works is that every time the heat pump website is called, a login is executed, then the data is parsed and sent to MQTT broker. The continuous calling of the php script I then simply solved with a cronjob that is executed every minute.


>sudo crontab -e

and the job then looks like this:

* * * * * sudo php /home/neura2mqtt/neura2mqtt.php -c

(if you put the files in the /home/ directory…). I have published the project on github at:

Neura data on the NodeRed dashboard





MIDI DB50XG – an interface for the daughter board


Rummaging through a box of my old crafts I found the box below. It dates from the time when I was still working with Amgia, but also with PCs – I guess around 1996. I labeled the box “DB50XG MIDI – Wavetable Processor”.

Das Fundstück aus der Kiste

Inside is a circuit board from Yamaha, which is called the DB50XG. This board was designed as a daughter board for PC sound cards with “Waveblaster” expansion port. She expanded the sound cards with a polyphonic MIDI wavetable sampler. In this way, the General Midi Standard and the Yamaha XG Standard could be re-established. Today nobody thinks about it anymore. At that time, if you wanted to generate sounds with a PC from midi data, then either external hardware was required, or a sound card with an onboard midi synthesizer or wavetable chipset. The PC then took over the control, the sending and receiving of the midi data via a sequencer software. Today, the midi sounds are generated directly on the PC and the samples and sound models are integrated into the software. At that time, the performance of the PC hardware was not sufficient. If someone is wondering what I’m palavering about here – what is Midi and why do you need it? – then let me put it briefly here: Midi is the abbreviation for “Musical Instrument Digital Interface” – i.e. a digital interface – a data protocol for musical instruments. Roughly explained, it serves to network and control electronic musical instruments with each other. For example, a large number of sound-generating devices can be controlled via a single keyboard. I will not explain here how the Midi standard works, what the data packets look like and how it looks electrically. As always, there is plenty of information on the web.

Inside the box

Back to the self-made box. At that time I packed the DB50XG in the plastic box and from the “Waveblaster” port, a 26-pin socket strip, led the necessary cables to the outside to start up the Midi board. And that was pretty simple. The board requires a power supply of +/-12V and +5V. There is a Midi-IN and a Midi-OUT (through) pin, a reset pin and two analog audio out pins – one per channel. The table below shows the connector pin assignment:

pin number assignment
1 Digital ground
2 not connected
3 Digital ground
4 not connected
5 Digital ground
6 Supply +5V
7 Digital ground
8 not connected
9 Digital ground
10 Supply +5V
11 Digital ground
12 not connected
13 not connected
14 Supply +5V
15 Analoge ground
16 not connected
17 Analogue ground
18 Supply + 12V
19 Analogue ground
20 Audio out richt
21 Analogue ground
22 Supply -12V
23 Analogue ground
24 Audio out left
25 Analogue ground
26 reset

The whole structure was rather spartan back then. The power supply had to be established via one or more external power supplies. There was no galvanic signal isolation using optocouplers. So I had to rely on the proper setup of the Midi IO controller that I connected to the Amiga. Of course it couldn’t stay like this. And I can’t bring myself not to use the beautiful DB50XG board anymore or to throw it away in the electronic waste. The plan that emerged from this was to develop a new interface board – or to tinker, which should be as universally usable as possible.


It’s been a few years since this idea and I’ve always worked on it a little bit. I thought the interface board should fulfill the following points:

  • a simple power supply should supply the Yamaha board with energy. Ideally, there should be a USB port and, optionally, a connection for a universal power supply. All required voltages should be generated on the interface board from the 5VDC.
  • As in the past, the DB50XG should also be able to be plugged in as a “piggyback” circuit board
  • The midi-in signal should be able to be fed in via the 5-pin DIN socket and also via a pin header – of course nicely decoupled (this means that a microcontroller such as Arduino and co. can also be connected without any effort)
  • The sound, i.e. the audio signal, should be available for acceptance via a chinch socket and also as a 3.5mm jack socket and via a pin header per channel.
  • Word repetitions SHOULD be avoided, but I don’t care 🙂

This ultimately resulted in the following circuit diagram. The 5VDC supply of the USB source is routed directly to the 5V supply of the midi board. The +12V/-12V that are also required are generated by a DC/DC converter (TMR0522). This is supplied on the input side by the 5V mains. The optional “external” voltage input goes to a LM2596ADJ. This is a step-down voltage regulator that can work with input voltages up to 40V. The regulated output side is available in many areas. I have integrated the ADJ (Adjustable) type into the circuit here, as I have a few of them in the assortment box. The voltage source can be selected with a jumper on the board.

Based on this circuit diagram, I created a layout and initially produced it in my own etching bath. The result was the following circuit board, which served as a test setup. Technically, the board worked perfectly, but I didn’t like the arrangement of the components. I placed the step-down converter and coil on the back. The distance between the connection sockets was also too close together for me. And how you do it as a PCB layouter – you always do a second design. So also it is this time.

The test setup with a fitted Midiboard can be seen in the image below. The midi signal as a test source comes from the PC and is generated by a USB midi adapter from the Far East.

So sat down in front of the computer again and redrawn the layout. The following version came out. I then ordered this version from a circuit board manufacturer.

The finally manufactured printed circuit board then looks like this. Below she can be seen with the DB50XG board attached.


When the car mirror doesn’t work anymore


I hear and read more and more often about electrically folding exterior mirrors that no longer work properly on vehicles from the German manufacturer with the four rings. The problem occurs with many models that have been in service for a few years and are operated in our local climate. In Internet forums you will find some users who know this problem. Also in my circle of acquaintances there are a few rings drivers who have a stuck electric exterior mirror. As a solution, the manufacturer always recommends replacing the entire unit. If you don’t want to spend your savings pointlessly on newly produced residual waste, you can take on this problem yourself. There is even a fairly small cause that causes this problem. And best of all – it can be repaired without any material costs. The longevity of the repair has also been proven…

The error manifests itself through the following behavior:

  • the mirror makes squeaking, creaking noises when folding in and out
  • the mirror stays in the wrong position and can only be engaged by moving it manually
  • the folding behavior depends on the weather


There are many posts about this with possible causes – from defective motors and defective door control units. The best thing to do would be to replace the mirror unit right away and get a new door control unit – yes, of course…

The solution to the problem is simpler: a small steel bolt that is supposed to be pushed out by a small spring gets stuck in its guide. The mechanical part of the mirror is of course also exposed to the environmental conditions and so the area comes into contact with rain, splash water – in winter salt water. Over time, the lubricants lose their properties or are even washed out and the whole “work” becomes stiff. So what helps? Completely disassemble, clean, re-lubricate and reassemble.

For this almost one and a half hour operation, I started by removing the mirror from the door and examining it in the cozy workshop. The easiest way to do this is to remove the inner lining of the door (depending on the vehicle, a few screws and many clips…) The mirror is then connected to the door control unit with a cable and secured with Torx screws.

The easiest way to click out the mirror glass is to use a plate lifter (suction cup). Then carefully – if present – pull off the two flat plugs from the mirror heating (it is essential to hold the contacts on the heating foil against). Next, both plastic halves of the mirror housing can be removed. A little observation helps here, which screws to remove and how the halves are held together.

Now the core of the mirror is there. The two die-cast parts are connected to each other via a hollow axle. The connection cable to the mirror adjustment drive and to the heater runs through the axle. A large steel spring sits above the axle and is attached with a spacer and a clamping ring (I don’t know if that’s the correct term). The spring exerts a fair amount of pressure between the two parts – and this is now the only slightly trickier part – the spring has to come out. To do this, the clamping ring must be levered out while the spring is held under tension. It comes out easily – but putting it back in becomes a challenge if you don’t have the right tools.

The already relaxed spring can be seen in the picture. Now the two parts can be taken apart.

Here the parts are to be recognized in disassembled form. In order to reach the Corpus Delicti, the small gearbox with the motor must be unscrewed. Underneath you can see the bolt, which in this case was stuck firmly in its hole so that the spring was no longer able to push it out.

Cover of the small gear
Bolt can be seen to the left of the mounting hole
Bolt with spring
the guide of the bolt must also be cleaned

The procedure is quite simple – clean everything, remove the corrosion and re-grease with lubricants. After that reassemble everything rejoice. 🙂 Most of the time of the whole job is cleaning.

By the way: the mirror described here comes from an A5…

UV sensor logger self-made


When summer comes, new ideas come. In the summer months, as is well known, the duration of sunshine is longer and the intensity of the sun’s rays is higher. Many use this property of the sun to boost their body’s vitamin D production, while others lie under the source of radiation to darken their skin color due to the high UV component. This, in turn, supposedly increases their attractiveness and stimulates hormone production and the willingness to mate… Unfortunately, the non-visible UV range in the spectrum of sunlight is known to have negative effects on the human body. Sunlight can also be used technically. On average, the power of the sun per unit area is assumed to be 1000W per m². Large-area P-N junctions in semiconductor materials are now able to generate electrical energy with an efficiency of up to aprox. 22%.

But the energy can also be used in other ways, or the UV component. Many retro collectors are certainly aware of the problem with yellowed old plastic cases. In order to get this under control, or to get it back to its original state from 30 or 40 years ago, you use H2O2, i.e. hydrogen peroxide and UV light, to get a bleaching process going. And so I came up with the idea for the following project.

At an online electronics store I found a UV sensor board from the manufacturer Waveshare in the sale. On it is a LITEON OPTOELECTRONICS LTR390 chip including a level shifter circuit. An I²C bus is available as an interface. A look at the data sheet revealed to me that the sensor records two wavelength ranges and outputs them separately. The ALS (Ambient Light Sensor from 500-600nm) and the UV (Ultra Violet range from 300-350nm). You can quickly make a simple logging board with this – I thought to myself. So I figured the board should be able to do the following:

  • Powered by a 18650 cell or USB
  • USB should also be able to charge the battery
  • a micro SD slot for recording the sensor data
  • an RS-232 port for direct logging on the PC
  • a cool OLED display
  • two buttons to operate the logger (interval, start/stop etc.)

The control should of course once again take over a chip from Atmega – the 328er. There are just enough of these in my assortment of boxes. To give you a quicker overview of the structure, I drew the following block diagram:

In the next step I created a circuit diagram from the block diagram in order to be able to create a layout out of  it. Parallel to the creation of the circuit diagram, I also connected the single components  together as a test using “air cabling” and tested whether everything worked as I imagined. And above all, everything should have space in the flash memory of the microcontroller.

The “airy wired” structure consisting of finished components can be seen in the picture above. An Arduino was sufficient for the first tests with the sensor and the OLED display. This enabled me to test the desired functions. So nothing stood in the way of creating the circuit diagram. An 18650 lithium cell will serve as the primary power source. Alternatively, there will also be a USB port that can charge the cell or operate the sensor. Because I’m lazy and component delivery bottlenecks are also a big problem at the moment, I use a ready-made Wemos D1 mini board to charge the battery. Like the OLED display board and the sensor board, this will find its place as a finished component on the circuit board design. As already mentioned, an Atmega328 in a TQFP housing is used as the controller. This will communicate with the OLED display (SBC-OLED01 with SSD1306 controller) and the LTR390 UV sensor board via the I²C interface. OLED and sensor are 5V compatible. However, the SD card is operated with 3.3V. For this, the circuit requires a voltage converter from 5V to 3.3V for the supply and a level shifter for the SPI data bus, via which the SD card exchanges data with the Atmega. Since the Atmega then also wants to be programmed with its firmware, I have provided a 2×4 pin header for connecting a programmer. The programmer needs six of these pins (GND,5V, MOSI, MISO, SCK and RESET) and the two remaining pins are intended for the serial interface. The two interrupt inputs of the Atmega are each wired with a button, which then makes the software operable. The battery voltage is measured and logged via a divider at one of the ADC inputs. The result of these thoughts is the following schematic:

A layout is then the next step. With a size of 12 x 4.5 cm, the circuit board is reasonably “handy”. The printed conductors are routed on both sides and the modules (charging circuit, display and UV sensor) are designed to be pluggable via pin headers.

The two images above show the preview of the “Top” and “Bottom” side of the layout. A circuit board could be created from the production data created in this way.

After some soldering work the hardware was ready. In order to breathe life into this “soldering” , software was required to do its work on the microcontroller.

When tinkering with the software, I used the free “Arduino IDE” development environment. The LTR390 documentation describes exactly which registers are used to operate which sensor functions. But there is also a ready-made library for those who are very comfortable – just like for almost all sensors and actuators that are to be connected to microcontrollers. In the Arduino IDE you can find the “Adafruit LTR390 Library” via the board manager, which you can use to communicate easily with the sensor. In my case, the OLED display is controlled by the SSD1306Ascii library. The “Wire” and “SPI” library take over the bus communication and the “SD” talks to the SD card. The includes then look like this:

#include <LTR390.h>
#include <SD.h>
#include <SPI.h>
#include <Wire.h>
#include “SSD1306Ascii.h”
#include “SSD1306AsciiWire.h”

I’m happy to post the entire code here if needed. However, it is not rocket science, but simple and certainly not optimized lines of code writing 🙂 In the current code (firmware) version 1.3d there is a small selection menu that makes it possible to set the log interval of the SD card recording and of course the start or stop recording. It is logged in a text file. The data recorded are UV index, ambient brightness and battery voltage.

I’ve included an excerpt from the datalog below:


 Ambient[lx], UV-indx, Batt[V], Loggingintervall[s]  

This data can now be processed very easily and displayed graphically. As an Office user, you can use Excel, for example, and import the data there and display them as graphs. But it is even easier and also very fast with tools like Matlab. With a script like the one below you can visualize the log file.

 %% Setup the Import Options and import the data  
 opts = delimitedTextImportOptions("NumVariables", 4);  
 opts.DataLines = [3, inf];  
 opts.Delimiter = ",";  
 opts.VariableNames = ["Ambientlx", "UVindx", "BattV", "Loggingintervalls"];  
 opts.VariableTypes = ["double", "double", "double", "double"];  
 opts.ExtraColumnsRule = "ignore";  
 opts.EmptyLineRule = "read";  
 opts = setvaropts(opts, ["Ambientlx", "UVindx", "BattV"], "TrimNonNumeric", true);  
 opts = setvaropts(opts, ["Ambientlx", "UVindx", "BattV", "Loggingintervalls"], "DecimalSeparator", ",");  
 opts = setvaropts(opts, ["Ambientlx", "UVindx", "BattV"], "ThousandsSeparator", ".");  
 datalog = readtable("F:\ingmarsretro\datalog.txt", opts);  
 clear opts  
 x=size(datalog); % groesse der tabelle  
 measurement=x(1); % anzahl messungen   
 messzeit = linspace(0,(measurement*datalog{1,4}),measurement); %zeitvektor von 0 bis zeitintervall aus datalog spalte4 * messungen  
 title('UV - Index');  
 title('UV - Index');  
 xlabel('Zeit [s]');ylabel('UV - Index');  
 xlabel('Zeit [s]');ylabel('Beleuchtungsstärke [lux]');  

If the script is executed, you get a plot that visualizes the measurement data.

The technical information on the sensor can be found in the manufacturer’s data sheet. Here are a few key points:

The LTR390 consists of two photodiodes, one for the visible spectrum of light and one that is sensitive in the UV range. The photodiode current is digitized in internal ADCs. An internal logic controls the ADCs and the connection to the outside world is established via an I²C interface. The resolution of ALS and also UVS can be configured in 13, 16, 17, 18, 19 and 20 bits. The sensor chip is housed in a 2x2mm 6pin package. The detector opening has an edge length of 280×280 µm.

Source: LTR-390UV data sheet
Source: LTR-390UV data sheet



The weather globe – or the Goethe glass


Again and again I look for simple, interesting things. This time I was fascinated by a measuring device or rather “display device”, whose operating principle is extremely simple and yet very effective. In addition, from my point of view, it is also an eye-catcher – it is the so-called Goethe Barometer. The best-known form is probably the bulbous glass hanging on the wall with a beak, similar to a watering can, in which the water level indicates the air pressure. I found a slightly differently constructed version of this glass on the net…

A little about the history of this structure:

To a gentleman named Evangelista Toricelli (1608-1647), an Italian physicist and mathematician, we owe the knowledge and proof that the air pressure is subject to fluctuations. He built the first barometer named after him in 1643. In 1644 he developed the mercury thermometer.

A small compensation area in the indicator tube protects against overflow

Der deutsche Dichter Johann Wolfgang Göthe, beschäftigte sich auch mit den Naturwissenschaften. Er machte selbst viele naturwissenschaftliche Experimente und entwickelte später ein einfaches, aber wirkungsvolles Barometer auf den Grundlagen des Toricelli.

Die Funktionsweise:

The barometer shows air changes quickly and precisely. When the air pressure rises, the water column in the indicator pipe falls and when the air pressure falls, it rises. This is made possible by the air trapped in the glass. The volume of the air always remains the same at a constant temperature. If the external air pressure rises or falls, the trapped air is compressed or expanded via the water column. Since the water cannot be compressed, it is the ideal medium to make the pressure differences visible. The height of the water column thus indicates the air pressure. If the air pressure is high in good weather, the external pressure is higher than the pressure of the trapped air and the water column decreases as the trapped air is compressed. At low air pressure, it can expand and the level of the water column increases.

the height of the water level in the pipe indicates the air pressure

This short time-lapse video shows the change in the water level when the air pressure changes:

HomeMatic smoke detector HM-SEC-SD quick repair


This time again a quick article on the subject of “Aging and Homematic Smart Home”. It’s about the following device: The Homematic smoke detector HM-SEC-SD, i.e. the older version of the smoke detector from eq3.

First of all: This article only shows how I put this device back into operation. Since it is a safety-relevant device, an acceptance test by a certified testing company would have to take place after the repair in order to be allowed to continue using it. So the contribution only provides what has become broken in the device.

So what is it about? The radio smoke detector HM-SEC-SD showed the following symptom during the monthly test (yes, you should press the test button once a month):

A short press on the button and there was no acoustic signal – instead the red signal LED flashes several times at approx. 0.5s intervals. Replacing the batteries does not change anything, the behavior remains the same. The radio module of the detector behaves normally. It can be reset and taught again. In this case, a look at the operating instructions (under point 9.2 on page 24)

– If only the LED starts to flash after pressing the button, the smoke detector is defective and must be replaced
So time to open the detector and take a look. My first suspicion fell on the detector chamber and that there is contamination here or that an animal has settled in the chamber …
Smoke detector opened

But after removing the lid of the detector chamber, no animal intruders were to be found. However, a strange pattern could be seen on the inside of the lid:

Streaks on the inside of the detector cover

These streaks, I thought at first, were created during the injection molding of the plastic component and must be like that. But on closer inspection and a “wipe” with your finger, they could be removed. In short, these streaks are dust particles. And when they are on the lid, then also in the entire measuring chamber. So blow it out with compressed air, put the cover back on and test it. -> same mistake as before. So again, put the lid down and take a closer look with a magnifying glass. The coarser dust, if you can speak of “rough”, was gone, but the surface of the photodiodes still had very fine and difficult to see streaks. So I cleaned the chamber and the diodes with a little alcohol on a cotton swab.

thorough cleaning required

Another function test showed success – better partial success. After pressing the test button, the piezo squeaked – but only very, very quietly – and by that I mean barely audible and the LED flashed nine times at an interval of one second. So actually the way it should be. Just way too quiet. So something had to be broken. So I examined the circuit starting with the piezo and quickly found what I was looking for. The piezo is controlled by a 40106, a 6-fold Schmitt trigger. In order to get enough electricity, three “Schmitts” are connected in parallel. The output was low-resistance, which is actually not allowed to be. So unsoldered the 40106 and measured it again. Between pin 1, 2 and 7 (input and output of the first Schmitt trigger and the VSS pin) there was a full short circuit. That means the IC is defective.

6times Schmitt trigger IC 40106

After the IC was exchanged, the smoke detector could finally “scream” again as usual.

Homematic actuator quick repair (dimming actuator RS485)


I have another small contribution to make on the subject of “Aging and Homematic Smart Home”. Many thanks to Fritz for the preparation and analysis.

As in the last post “Homematic actuator quick repair”, this time it is again about a device from the Smart Home series. It is the dimmer actuator with the designation “HMW-LC-Dim1L-DR”. This is a phase control dimmer actuator for incandescent lamps and low-voltage halogen lamps with conventional transformers. Many modern LED lamps can also be controlled with this dimmer. The actuator belongs to the “wired” series, which means that it is not connected to the CCU via the BidCos radio protocol, but via the RS485 bus. The actuator receives the power supply for data communication from a 24V power supply unit. This also supplies the µC in the actuator. The network side is supplied with control data from the low-voltage side via an optocoupler. This ensures galvanic isolation. On the network side there is a dimmer controller module, which in turn controls the triac. This controller must be supplied with a voltage of approx. 15V. To generate this, the manufacturer has built in a capacitive voltage divider. And this is where the aging problems begin …

The error pattern manifests itself as follows: The connected light source cannot be dimmed or switched on. However, the dimmer is communicating correctly with the bus. The red function LED lights up correctly. The commands for “Off” and “On” via buttons are also displayed in the CCU.

Circuit diagram of the mains side on the “main board”
defective 330nF X2 capacitor

The cause: According to the data sheet, the IC U2008, a dimmer control module, is supplied with a voltage of DC 15V. In this case, the supply voltage was significantly lower (at approx. 5.8V). This supply voltage is made the 330nF / 275V X2 capacitor C4. The capacitor is optically in perfect condition, but a simple capacitance measurement quickly shows that nothing fits here. The capacitor C4 only had a capacity of approx. 30-40nF. So it’s like so often -> It was the capacitor: D

Dimmer module side view

After the replacement, the voltage on the U2008 was ok again and the dimmer is doing its job again. As a preventive measure, the two other X2 capacitors on the board (C1 47nF / 275V and C2 100nF / 275) were replaced.

Installation locations of C1 and C2







Tidying up and archiving


Behind the title mentioned in the article headline is my idea of ​​tidying up the “ingmarsretro” blog and tidying it up a bit. By “cleaning up” I mean checking the individual posts again for spelling errors, maybe reformatting one or the other post and adding to it. That is why there will be no other contribution in January 2021.

There are also plenty of new posts that are digitally saved on the server, but there is no paper version of them yet. So I want to put all the posts that have been created since the last backup back on paper in the form of a book. And unfortunately that is not done in a jiffy, but requires a lot of work. The job and the “family time” with my little son only allow me to work on the contributions mostly at night. And between the posts I also work on the projects (little projects) that I then write about. I also have a lot of them in my head that may one day be realized. And then there are some that I’m working on that are still to be completed, or at least should be continued.

For the last few months I have been working on a Nixie clock that should be set up fairly discreetly. The watch has meanwhile also achieved a reasonable status, so that circuit boards from self-etched prototypes to reasonable, industrially manufactured condition have been created here.

the Nixie clock prototype

The “tube radio receiver” that I started with a few years ago is also waiting to be tinkered with.

Of course, the topic of retro computers does not let me go either. Here are a few devices left to restore and wait to be brought back to life. (This is where I remember right away: I’m looking for a RUN / STOP button for a Commodore Plus 4 – I would be happy if any reader could help …)

looking for a RUN / STOP key

The project with the MOS8501 CPU as a lattice – FPGA – mini board is also in the “carry on queue”. There is still a lot to do here (the level shifters are not doing as they should, the VHDL code still needs to be adapted, the prototypes Circuit boards have to be combined on one board and then also miniaturized …) So there is still enough to do.

Then there are also old devices that I would like to present here in the blog and a number of repairs that keep falling into my hands … Also one or the other HomeMatic tinkering is still pending.

I also toyed with the idea of ​​publishing content from the blog in the form of videos on YouTube. On the one hand, however, I cannot assess whether someone is interested and whether I would like to do that to myself, to present my face in front of the camera. It would probably make more sense to do it with a more didactically gifted person as the protagonist. And of course for Lau, as fun. On the other hand, there are quite a few Youtubers here who are very experienced here (e.g. Dave Jones with his EEVblog, NoelsRetroLab, Adrian’s Digital Basement, GreatScott, ZeroBrain, JanBeta, etc.) and have been doing this for a long time. Also not to be neglected is the immense effort involved in producing such films. When I look at my assembly videos here: It takes almost one day to record the raw material when soldering the respective kit and, in total, almost three days for the cut and post-processing. Let’s see if something can come of it one day …




The second book on the blog


After the great success of my first book on the retro blog, I have now made my way through and written a second book – NO FUN – there was no success at all. I wrote the book back then in order to have a printed work of my (in) deeds on this website for myself. Because, firstly, it is much more practical to be able to quickly check something – without always having an Internet available and secondly: what if the server (s) can no longer be reached, or is deleted, or even burns down? Or worse, if someone wipes the internet… 😀

At the time I thought to put it on with a “print on demand” option, but who knows if anyone will be interested and if I can even do it that easily. And who would spend so much money on it. Because the individual prints are also quite expensive. So the printed copies presented here are virtually unique. And I’m a little proud of it, because there’s a lot of work to be done.

So now there is the retro book no. 2 with the continuation of the blog entries from the end of the first version to the entry “Schuko socket with USB (repair)” from December 19, 2020 and that is again 96 “stories” which this time cover 498 pages. I chose “epubli” again for this print and ordered it there. This has the advantage that if I want to have one of the existing books printed, I just have to click on “order” again, since all the production data is already there.

I also learned a lot about the formatting of texts, fonts, step sizes and directories. The images are now printed at a higher resolution and all in all it looks better.

So I’ll stay tuned and in a few years, when enough content has been written again, I’ll be working on an issue again.