4051 multiplexer problem

Discussion in 'The Projects Forum' started by showes, Oct 27, 2011.

  1. showes

    Thread Starter New Member

    Oct 27, 2011
    I'm working on a project that involves using a 4051 chip. I have attached a small schematic of what I'm talking about.

    Since pin 3 of the 4051 is connected to ground and A, B, and C are grounded I thought that would enable X0 to be connected to ground thus giving the input of the 540 buffer a logic level 0.

    However I'm not getting logic 0 at the D0 input of the 540 buffer. I'm getting the voltage drop of the pull up resistor which is 1.4V.

    Does anyone have a solution on how I can make D0 logic level 0 when A, B, and C are grounded? I also must have X0 either floating or a logic high when A, B, and C are something else.

  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    The datasheet I found of the 4051 tells me that at 5V it has a ON resistance of 120 to 400 Ohm. So you get the voltage drop of this resistance on your input of the 540.
    Increase the 1k to a higher value (10k or more) and it should be ok. Or is there a specific reason why you want to use a pull-up with such a low resistance?
  3. showes

    Thread Starter New Member

    Oct 27, 2011
    Thanks for the quick response. I don't need to use a 1K resistor however I already have 104 1K resistors wired in (fairly large circuit). But changing them might be my only option.