I would like to use a shift register to control source-select in a pre-amplfier. After watching this very nice video on https://www.youtube.com/watch?v=Y4djl3oYqDQ, I made the same setup on my bread board using a SN74194N shift register and an old telephone wal wart as a 5Volt power source. To test the clock I used a led with a 330Ohm resistor to ground.

The shift register did not behave like in the video. After a lot of effort I reduce the 10k resistors to 1k resistors.Pins 4, 5 and 6 are now connected to ground with a 1k resistor. Pin 3 to Vcc (and a resistor to gnd) This setup is for bootstrapping the circuit with bit one to high. To make the shifting go in a loop I connected pin 15 to pin 7 and pin 12 to pin 2. Pin 9 and 10 are connected to push buttons.

After power on and setting the register by pushing both buttons one led is lighting up and I can shift the bit left and right by keeping one or the other buttons pushed in.

There is some strange behavior though that I do not understand

  1. Why do I need 1k i.s.o. 10k resistors?
  2. The chip boots up with random bit turned on. This can be overcome by making S0 and S1 high at start up, but is a bit annoying none the less.
  3. This is really a problem: If I push one of the buttons rapidly, it does not shift the bit, but makes random bits go high. This is really unwanted, because it make more than one bit (input source) selected.

Can you help me understand what's going on and tell me how to solve the third issue?

  • \$\begingroup\$ (1) can be answered from the TTL Data Book, looking at the input currents and input voltages for the logic family you are using. (3) suggests you forgot to debounce the button. \$\endgroup\$
    – user16324
    Commented Jan 10, 2021 at 12:37

1 Answer 1


She uses a 74HC ... which is high impedance CMOS

You are using TTL which is lower impedance for "0" due to input current

Put a cap across a switch to prevent contact bounce such that RC=T is greater than your bounce time ? 2ms? 3? It seems she uses better switches with no bounce. (nice but poor assumption)

  • 1
    \$\begingroup\$ Thanks to your pompt answer I solved all issues.. The circuit is now wel behaved :) \$\endgroup\$
    – dre
    Commented Jan 10, 2021 at 18:23
  • \$\begingroup\$ Vote and close pls. \$\endgroup\$ Commented Jan 10, 2021 at 18:25
  • 1
    \$\begingroup\$ I do not have enough merit points to vote, so you'll have to do with a well meant "thank you" :) \$\endgroup\$
    – dre
    Commented Jan 13, 2021 at 14:24

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