Issue with CMOS gate switching

Discussion in 'The Projects Forum' started by Synthetase, Mar 2, 2013.

  1. Synthetase

    Thread Starter New Member

    Mar 2, 2013
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    Hi all, I'm a bit of a noob when it comes to cmos and digital circuits in general. I'm currently building a circuit to electronically switch the inputs to a DAC/preamp project and remove the need for the old-school rotary switch. The idea is to have a series of momentary push button switches that pulse a 4043 latch flip-flop. The outputs from the 4043 turn on NPN transistors, which in turn supply current to the switching relays and the corresponing indicator LED. Each switch is connected directly to the flip-flop it needs to set, and through a passive OR gate to the rest of the 'flops to reset them all, ensuring only one input may be selected at any one time. I've built this part of the circuit (everything above the red line in the attached schematic) and tested it thoroughly and it works very well.

    The problem is that when power is applied, the 4043 is (naturally) low on all outputs and I'd really like to be able to have a "default" input selected at power-on. So I decided to add a three-input NOR gate taking its inputs from the 4043 outputs and feeding any one of the three inputs to the 4043 (selectable by jumper switches J1, J2 and J3). Theoretically the NOR output should be high at power-on due to the outputs from 4043 being low. (This is true, I've measured the voltage.) Then the NOR should set one of the flip-flops and go low due to the presence of a high on the 4043.

    Except it doesn't work. I thought it might be an issue with the 4043 only switching at the rise of the pulse, and it not getting one because at power-on the output of the NOR was already high. I've tested this by inserting a momentary switch between them and got nothing.

    I've attached a picture of the (relevant parts) of the schematic. It's a bit of a mess but you should be able to follow it. Any ideas on what I'm doing wrong here would be greatly appreciated.

    Cheers.
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Try this instead of the gate:

    View attachment 52792

    The cap will send a pulse only at power-up to one of the set pins.
    The cap could probably be a smaller value - 10uf.
     
  3. Synthetase

    Thread Starter New Member

    Mar 2, 2013
    3
    0
    The link is broken, it's returning a "Invalid Attachment specified" error.
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Sorry 'bout that :confused:

    Power-On Reset.png

    Drawings wrong Swap Resistor and capacitor positions :)
     
    Last edited: Mar 2, 2013
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    197
  6. Synthetase

    Thread Starter New Member

    Mar 2, 2013
    3
    0
    Thanks very much for that. It's a very elegant solution.
     
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