Archive for the ‘Programming’ Category.

LED candle light with timer


I did a lot of research on battery powered LED candle lights circuits and code and was unable to find a time controlled LED candle light.

As I have some ATTiny13 left, I decided to use this little chip. As I want the LED to do a candle light for about 4hours and then stay of for another 20 hours, I need on timer and counter. Another timer/PWM is needed to immitate a candle flickering. But the ATTiny13 has only one timer. On the ATTiny13 you can either use the timer or PWM.

Fortunately the ATTiny has also a Watchdog timer that can be used to call an interrupt function. The watchdog runs with a separate 128kHz clock, independent from the CPU clock. The largest timeout is 8 seconds. So I need to count this 450 times to have one hour.

// watchdog interrupt
ISR (WDT_vect)

  //flash heart beat LED
  digitalWrite(LED2, HIGH);
  digitalWrite(LED2, LOW);
        bLedIsOn=0; //switch to OFF mode
        bLedIsOn=1; //switch to ON mode
}  // end of WDT_vect

The code makes the ATTiny13 sleep for another 8 seconds or light the LED. The ON phase is 4 hours and the sleep phase will be extended to 20 hours.

void loop ()
  //sleep 20 hours and work 4 hours
    digitalWrite (LED, LOW); //ensure LED is OFF
   goToSleep ();
}  // end of loop

The sequence starts with the ON phase, when power is applied. In the ON phase I measure about 3mA and in the OFF phase the circuit needs 300µA.

The circuit is documented in the Arduino code file. Running the ATTiny13 at lower clock than 9.6MHz did not change the power usage. But I switched to the 1.2MHz internal clock and disabled BOD (auto power down for low power) to get a longer runtime with two or three AA batteries.

UPDATE 2019/03/10: The wdt calculation is wrong and I changed the 450 cycles for one hour to 388. See github README for full update. The main issue is that the ATTiny13 datasheet says 128kHz but means 131072Hz and not 128000Hz and the wdt oscilator never cycles that fast but more or less at around 113000Hz.

CREDITS to all that share their knowledge, especially:

Openvpn server an Deutsche Glasfaser IPv6 only Router und

OpenVPN auf BananaPi M2 Zero W mit Raspbian

Deutsche Glasfaser Genexis Router in Verbindung mit zur Umsetzung ipv4 nach ipv6

Ich versuche hier meine Installation basierend auf den nachfolgenden Quellen zusammenzufassen.

Quellen: und

Nachfolgend sind persönliche Angaben in < und > gesetzt. IP Addressen sind in der Form durch x ersetzt. Universal Portmapper einrichten

Über die öffentliche IP6 Adresse des Servers (VPN Gateway) ermitteln. Portmapper

Bei habe ich eine Port Weiterleitung auf die öffentliche IPv6 Addresse des BananaPi (dem OpenVPN Server) und den Port 443 unter “Universelle Portmapper” angelegt.

Ich habe hier den Port 443 gewählt da dieser auch in vielen Fremdnetzen (zb Mobilfunk Netze) funktioniert. Der Standard VPN Port 1194 ist möglicherweise in dem einen oder anderen Netz gesperrt.

Ich habe einen Alias gewählt den ich mir leichter merken kann als den DNS-Namen. Ausserdem laufen auf demselben BananaPi M2 Zero W noch drei weitere Dienste. Diese sind über den gleichen Alias unter drei anderen Ports erreichbar.

In der Client.conf muss später der Alias Name und der gemapte Port angegeben werden, da die Verbindung nicht über IPv6 und Port 443 hergestellt wird. Siehe client.con Zeile:

remote <aliasname> <auf 443 umgeleiterter port>

Openvpn Server einrichten

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FHEM: Home Automation remote display using small OLED and MQTT

I wanted to have a small display with some essential Home Automation data. So I buy an 1.3″ OLED display with I2C interface and started to look for some existing code for an ESP8266. The display should be feed using WiFi and be portable, so I can place it where needed.

I started with ESPeasy, as there is already support for multiple pages with text lines. Unfortunately the code is written to show sensor data of directly connected sensor only. No option to publish free text or data from a Home Automation system. The ESPeasy code will save the text to flash memory on every change when using the Web GUI. The flash memory would wear out after some month with my use case.

Finally I decided to start my own code. First I had to find out that the OLED display is not an SD1306 but a SH1106 one. Although some people state that these are more or less the same, the SD1306 libraries did not work for me. Then I found which works well for me.

Starting with SSD1306UiDemo.ino I added WiFi and MQTT libaries and code.

My code will show 6 lines of text on two frames (3 lines each). The frames will change every 5 seconds. The content of the text lines is published by a FHEM server. As there was no easy way to integrate the display using a MQTT_DEVICE definition in FHEM, I wrote my own perl mqttmsg sub function and used the FHEM notify definition to publish changes to the display.

a small video

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Android: Print PDF to Thermal Portable Wireless Printer

Recently I got the request to write an application that prints a PDF file on a portable Thermal printer.

Normally that does not make sense, as the protable printers come with 2, 3, 4 or 5 inch paper width only. But the PDF files are created for 3 inch paper and os this Receipt printing makes sense.

The idea was to render the PDF to a bitmap and then print the bitmap to the printer. These printers do not support PDF or Postscript, they come with special Printer Language support as called ZPL, ESP/P, CPCL or others. So we need to create a Bitmap of the PDF first and then print the Bitmap.

I started using the Android Google PDFRenderer but this fails with two issues: the bitmaps created are always transparent and print with black background where white has to be; the text was not rendered as the Fonts did not render.

Among others I found PDFbox and after fixing a small issue with scaling of bitmaps the solution works very well.

The CPCL_Sample code and application can load a PDF file. This is then converted to a scaled bitmap which can then be printed to a CPCL compatible printer. The CPCL performance is very different and printing on a Zebra takes about a minute for a 3 by 4 inch sized bitmap.

The code has to use a lot of intents and background tasks as most processing takes some time. The bitmap is created by an IntentService. The Sewoo SDK prints the bitmap first to a queue buffer and there is no control or feedback about the status of the print job. So it hopefully prints after some time.

By replacing the print SDK you should be able to print the bitmap of the rendered PDF to any Bitmap-Printing capable printer.

Be warned: there are Printers that are compatible with a Printer Language like CPCL but may not support Bitmap printing.

Source code and apk at github.