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PiFace Digital I/O maximum voltage on inputs pins

plmsherman

I'm using a Pi with Digital I/O board as a secondary controller for a mechanical device. Control of the device motor using the relays is simple but I'm not sure what to do about the Pi input pins that need to detect switch closings. The other (not to be named) controller normally provides either 9V or 14V which the switches short to ground.

 

I know I can disable the Pi's pullups on the input ports. This would allow the Pi to sense the same voltage that's being measured by the other box. Unfortunately, I haven't been able to find any documentation that states what the maximum input voltage that can be applied to the input pins. If it's limited to 5V, I'll just have to build a little voltage limiting circuit for each of the input pins I'm using.

 

Phil

  • 0

    Hello Phil,

     

    Although i have not found any specs for the input pins of the PiFace Digital cards, i would be very much surprised if they are designed to be used with more than 3.3V. The Raspberry Pi for which they were designed is not designed for more than 3.3V to be applied to the inputs. The pullup resistors only serve to secure a high input signal if the device which supplies the signal to the input of the PiFace card is only capable of supplying an active low signal but in the high state is not supplying any voltage reference. If i am not mistaken, you would most likely damage your PiFace Digital card (and perhaps the Raspberry Pi as well) if you apply more than 3.3V to the inputs.

     

    The outputs of these cards are buffered and in the default configuration is configured so that the 5V output contact can be supply 5V to the connected output device, and the output contacts on the PiFace cards are open collector circuits which, when activated, will sink the current at the output pin (connect it to ground). When not activated it will effectively only break the connection to ground.

     

    I am quite sure that it is safe to leave the input and output signal contacts that are not in use on the PiFace cards unconnected if you have no use for them in your project.

     

    urkraft

  • 0

    Can't remember if those pins are buffered assume a logic voltage of 3.3V which will still work for 5V logic.

    Even better use an optoisolator and gain some flexibility and a bit of safety from any over voltage your inputs may have unwittingly

  • 0 in reply to Problemchild

    Also the PIFACE outputs are coming from the MCP???? chip not the Pi directly so you need to program the pull ups there if they have internal pull ups. The Pi it's self is only there to program the device in this scenario

  • 0

    I decided to play safe and built a simple voltage limiting circuit to protect the inputs. Starting with the voltage to be sensed, the circuit is:

    Vin -> 10k resistor -> 3.3V zener diode -> ground

     

    The joint between the resistor and the zener diode is fed to the pifacedigitalio board input pin. The Pi pullup is disabled.

     

    Phil

  • 0

    Even better would be to split the 10K.  A 'T' with the horizontal bars being 5KRs and the vertical being your ZD.

  • 0 in reply to D_Hersey

    A T and the zener will work well but I needed the circuit to be capable of handling inputs starting at 5V. The 10K + zener gives a (maximum) voltage drop across the resistor of a bit less than 7V when the voltage to be sensed is 12V. 7/10K = 0.7ma, which yields a total power consumption of less than 5mw. Using your recommended T circuit would more than double the power consumption. This normally wouldn't be an issue at such low power levels but this is installed in a solar powered facility and every milliwatt adds up. The current flow through the zener isn't enough to raise it's temperature much above ambient.

     

    Phil