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Circuit not working

Discussion in 'General Electronics Discussion' started by kingofjong, May 19, 2018.

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  1. kingofjong


    Aug 14, 2014

    I have created a 2 bit adding machine that outputs a 2 bit number. I am trying to connect the machine to a shift register and store my 2 bit numbers. the shift registers should output the 2 bit numbers one at a time. I am using the LED's to simulate a 1 or a 0. But for some reason my circuit is not working. I am not sure what I am doing wrong. The circuit schematic is bellow.


  2. kellys_eye


    Jun 25, 2010
    Your logic inputs (switches) should not 'float' - use a resistor (10k will do) between each input and ground to set a logic 0 level. Same for the /MR pin on the SR.

    You can't tie the two OR gate inputs together without setting a logic level either......

    You will also need another diode at the output of the NOR gate (don't tie two outputs together unless they are open-collector) that doesn't have one - you are creating a wired OR function here.

    You are sinking current to the LEDs too - I doubt the output of the SR has the capability to sink current for the 8 LED's in one go - ONE of them maybe, but not all 8 at once.
    Cannonball likes this.
  3. kingofjong


    Aug 14, 2014
    Thanks for the help. I finally got it to work.
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