PCF8575 driver?

Discussion in 'General Electronics Chat' started by jabss, May 29, 2015.

  1. jabss

    Thread Starter New Member

    Jan 12, 2015
    11
    0
    Hello,


    I’m trying to build a circuit with the PCF8575 (I2C 16xGPIO port expander). The outputs should be able to activate/deactivate either 5V or 12V to turn on/off several components (IC’s, relays, buzzers, etc.). I considered to use a 2N2222 transistor for that switching. However, it seems the ports aren’t able to supply enough current to drive the transistor. It seems that the outputs of this PCF8575 are open drain. In the “zero” state they are capable is sinking up an average of ~20 mA and in “one” state only ~165uA.


    According to the datasheet:

    So, as I’ve understood by reading around, I need some P-channel device. 2N2222 is a N-channel and it somehow it doesn’t work well with the “open-drainess” of PCF8575. Also, assuming a scenario where I want to have all GPIO’s at the same level (0), I have to have 100mA/16= 6,25mA for each port, which seems very little to me. (and even much less if I want to have it as (1) ).


    Having this in mind, could anyone recommend a device (mosfet, transistor, whatever) that I could use to connect to the PCF8575 and could allow to:


    A) Act as a switch for 12V load (ex: some relays, 12v buzzers)

    B) Act as a switch for 5V load (ex: some IC’s, 5v buzzers)



    Thanks in advance,

    Jabss
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    2n7000 'Fetlington'
    ULN2803
    ULN2003
    Max.
     
    Last edited: May 29, 2015
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    2n2906, 2n4403,
     
  4. jabss

    Thread Starter New Member

    Jan 12, 2015
    11
    0
    Hello,

    Thanks for replying.
    Regarding 2n2906 and 2n4403 I'm not familiar with, but I've tried before 2n7000, and the ULN's directly to the PCF8575 and it doesn't work.... Maybe I need a special circuit? Maybe pull-up resistors?
    Isn't 2N7000 a N-Channel mosfet? In that case, shouldn't a P-channel be needed in this situation? And could the same mosfet model be used to switch 12V and 5V? I don't know much about mosfets, but isn't there a limitation about the Vgs?
    Just asking.....

    Thanks,
    Jabss
     
    Last edited: May 29, 2015
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    If you add a pull up resistor to each output, then any NPN-based device )2222, 4401, ULN2003, etc.) will work as an inverting saturated switch. In this case the loads are tied to the power source and GND is applied by the switch devices. Without the pullup you can drive a PNP device (3906, 4403, etc.) as an inverting saturated switch. In this case the loads are tied to GND and power is applied by the switch devices.

    ak
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    If you want non-inverting look at the UDN2981 series.
    Max.
     
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