Nixie clock: 3 – RTC and expander

luca 16/03/2012 13


A RTC (Real Time Clock) module keeps track of the current time.

One of the most used RTC is Maxim’s DS1307 (datasheet) that has an handy I2C serial interface. I chose a module by SparkFun with DS1307 chip, the needed 32KHz crystal  and a lithium coin cell battery to keep RTC updated even without external power supply.

LadyAda, starting from Jeelab one, developed a very good Arduino library (RTCLib) for DS1307. The connection is very simple:

  • (Arduino) PIN 5V -> (RTC) PIN 5V
  • (Arduino) PIN GND -> (RTC) PIN GND
  • (Arduino) PIN A4 -> (RTC) PIN SDA
  • (Arduino) PIN A5 -> (RTC) PIN SCL

Arduino offers indeed an I2C interface (“Wire” as the official library is called) on PIN Analog 4 and 5. For more informations, I suggest the reading of the following tutorial, by LadyAda too.


With 74141, you need 4 digital PINs for each Nixie tube: the Arduino I chose hasn’t enough I/O PINs. There are some integrated circuits, called I/O expander, that allow you to expand the number of pins using a communication bus.

The PCF8575 IC (datasheet) offers 16 I/O PINs and an I2C interface: with the same 2 PINs you use to communicate with the RTC chip, you can also drive 16  outputs connected to 4 Nixie drivers.

This IC is available in SOIC package; I soldered it on a SOIC->DIP adapter to make tests easier:

Finally I connected the two modules on a breadboard:

In the next page, I’m going to show you some Arduino sketches…

Pages: 1 2 3 4


  1. pitchoilcan 06/04/2012 at 06:19 - Reply

    I get :error:
    PCF8575_Led_rolling_Demo.cpp: In function ‘void loop()’:
    PCF8575_Led_rolling_Demo.cpp:24:8: error: ‘class TwoWire’ has no member named ‘write’
    PCF8575_Led_rolling_Demo.cpp:25:8: error: ‘class TwoWire’ has no member named ‘write’
    I able to compile by change it for
    but still now a blink from any of the LED’s. Are you using any resistor? I”ve tryed it using just the pcf8575 anf an arduino board, no RTC module. Is that doable and if so what I’m I doing wrong?

    • luca 10/04/2012 at 08:07 - Reply


      You’re probably using an older (< 1.0) version of Arduino IDE… anyway you don’t need a RTC module for this example, could you post your connections so I can check?

  2. pitchoilcan 06/04/2012 at 07:15 - Reply
  3. Fernando Garcia 25/12/2012 at 18:27 - Reply


    I’m trying to build a circuit with a RTC and a PCF8575 .

    It happens that the RTC does not tell the correct time.
    There must be some conflict.

    The code I’m using to test is this:

  4. Fernando Garcia 25/12/2012 at 18:43 - Reply

    The code does not appear correctly.

    Here’s a link to it.

    Best regards.

    • luca 11/01/2013 at 08:32 - Reply

      Hi Fernando,

      are you using an Arduino Mega?

  5. Dmitry 19/03/2015 at 09:15 - Reply

    Hello Luca.
    Thanx for your manual!

    Could you advise IO extender on 24 pins (for 6 tubes)?

    I’ve found the Tca6424rgjr but probably there is something better.

    • luca 19/03/2015 at 11:15 - Reply

      Hi Dmitry… it depends on you: on the I2C bus you can connect more than one expander, therefore you can use 3 8-pins expanders or the one you found that works fine!

  6. DRS 30/11/2015 at 11:50 - Reply

    Hi, where i can read more about power supply ?

  7. Mauro 06/09/2016 at 10:07 - Reply

    Ciao Luca,
    non riesco a compilare NESSUN esempio che utilizza il pcf8575!
    Ho scaricato le librerie da github le istallo ma gli esempi Blinkled non si compilano . dice che manca il file PCG8575.h.
    Dove sbaglio ?

  8. Mauro 19/10/2016 at 14:57 - Reply

    Devo usare tre 8575 in I2C e collegare ai pin digitali 2 e 3 di Arduino Nano il modulo Bluetooth HC-06 .
    Le librerie 8575.h e SoftwareSerial.h sembra che non possano convivere assieme.
    Con qualsiasi IDE con cui cerco di compilare mi da sempre:

    SoftwareSerial\SoftwareSerial.cpp.o: In function `__vector_3′:
    \\Srv01\dati\Comune\Arduino\IDE_ARDUINO\arduino-1.5.8\hardware\arduino\avr\libraries\SoftwareSerial/SoftwareSerial.cpp:306: multiple definition of `__vector_3′
    PCF8575\PCF8575.cpp.o:\\Srv01\dati\Comune\Arduino\IDE_ARDUINO\arduino-1.5.8\libraries\PCF8575/PCint.h:104: first defined here
    SoftwareSerial\SoftwareSerial.cpp.o: In function `__vector_4′:
    \\Srv01\dati\Comune\Arduino\IDE_ARDUINO\arduino-1.5.8\hardware\arduino\avr\libraries\SoftwareSerial/SoftwareSerial.cpp:313: multiple definition of `__vector_4′
    PCF8575\PCF8575.cpp.o:\\Srv01\dati\Comune\Arduino\IDE_ARDUINO\arduino-1.5.8\libraries\PCF8575/PCint.h:107: first defined here
    SoftwareSerial\SoftwareSerial.cpp.o: In function `__vector_5′:
    \\Srv01\dati\Comune\Arduino\IDE_ARDUINO\arduino-1.5.8\hardware\arduino\avr\libraries\SoftwareSerial/SoftwareSerial.cpp:320: multiple definition of `__vector_5′
    PCF8575\PCF8575.cpp.o:\\Srv01\dati\Comune\Arduino\IDE_ARDUINO\arduino-1.5.8\libraries\PCF8575/PCint.h:110: first defined here
    collect2: error: ld returned 1 exit status
    Errore durante la compilazione

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