Im trying to learn input shift register to increase number of inputs for my Arduino. And in googling I found this schematic enter image description here

I think its not for 74HC165, but I believe its almost same way for 165 also. But my concern or confusion is the resistors there. What is the purpose of those resistors there, which are connected to ground??


The inputs of CMOS logic parts (CD4000 series, 74HC, 74AC) are very high impedance, so if left unconnected may randomly appear as logic High or Low. With the switch open, the resistor holds the inputs Low, but allows a closed switch to pull the input High.

If you want the inputs to be High by default, you would connect the resistors between the input pin and the positive supply, with the switches connected between the input pin and Ground.

  • \$\begingroup\$ Ok.. so to avoid the random buggy inputs when unconnected, we are using the pull down resistors.. isn't it? In second scenario mentioned that time the resistors we can call as pull up resistors.. isnt IT?sorry I was trying to understand that also and you answered it clearly.. thanks a lot. \$\endgroup\$ – Sandeep Thomas Sep 2 '17 at 5:16
  • 1
    \$\begingroup\$ @sforsandeep yes, you understand it right. Pull up and pull down resistors are exactly that. \$\endgroup\$ – Chupacabras Sep 2 '17 at 6:10
  • \$\begingroup\$ @sforsandeep If you want to reduce power consumption (e.g. your circuit runs on battery), 10k may be increased to 100k (or larger, ... 220k, would not go to the 1 Mohm range) with CMOS circuits ... input impedance is much larger. \$\endgroup\$ – andrea Sep 2 '17 at 7:51
  • 1
    \$\begingroup\$ The downside of big resistors is it makes your circuit more sensitive to noise/interference. \$\endgroup\$ – Peter Green Sep 2 '17 at 12:53
  • \$\begingroup\$ @PeterGreen Thank you sir for the bonus info too.. That gave me the idea of pull up and pull down resistors.. For me such beginners those were really confusing terms \$\endgroup\$ – Sandeep Thomas Sep 2 '17 at 13:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.