Sharing of data inputs

Discussion in 'General Electronics Chat' started by Flug540, Mar 14, 2014.

  1. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    I'm using two identical data logging devices that each have two sensor inputs for 4 total inputs to log temperature changes using thermistors. I currently have over 10 sensors that I need to log, but I don't really need to log them all at once. I can define several groups of up to four sensors each to log at a time. The problem is how to do this and not to kill the devices in the process.

    One thing to note is that the inputs inside the devices are not implemented exactly the same. IN1 requires the use of an extra 1K resistor wired to a 5V source, while IN2 has that done internally inside the device.

    I'm attaching two schematics, the first one is of the device inputs and the second one is what I was thinking of putting together originally to be able to connect multiple sensors to the four inputs. The problem I'm seeing is that if the switches are turned to the same positions (say all switches are at position #1), then all four inputs are interconnected together and I'm not sure whether the devices will be OK with it or not. In terms of data logging itself it's not an issue, since this would be an invalid configuration, I'm only worried for the safety of the devices.

    What do you guys think? Maybe add a diode to each of the inputs? Not sure if this will affect the measurements though.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    Begin by giving us the part numbers of all your devices.
     
  3. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    The microcontroller is MC908SR12CFAE, IN1 is connected to pin 1 and IN2 is connected to pin 46.
     
  4. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    And for clarity... all the X-devices on the schematics are just 2-pin connectors.
     
  5. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    Any thoughts, guys?
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
    804
    What is inside logger1 and logger2? Why are they wired differently?
     
  7. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    I decided to redo the sample schematic (attached) to avoid any confusion. The two data loggers are identical and are commercial devices that I purchased. The schematic of the device inputs I know from opening the device myself and reverse engineering the board. Not sure why the company decided to do the two inputs differently, but that's the way it is. As far as I can see though, with the addition of the external 1K resistor, they become symmetrical.

    I also added more thermistors to the schematic to make more sense of what I'm trying to achieve, which basically is the flexibility to configure the array of N thermistors into groups of four to be sampled at the same time.

    The main question is if this is safe for the devices in the case where two or more switches are set to the same position (like in the attached schematic, all set to #1), effectively shortening all data inputs. As I mentioned, as far as data logging itself, this would be considered an invalid configuration and the results are undefined, which is fine. I'm only worried for the devices, don't want to burn anything.
     
  8. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    Anyone? :)

    In the meantime I contacted the device manufacturer and they say it should not harm the devices, so I'm going to give it a shot.
     
Loading...