multiple Pull-up/down resistors or not

Hi all,

does one need to use a pull up/down resistor for every PIN used, or is it OK to use one pull-up resistor and connect all the needed PIN's to this one, the same question for the pull-down resistors

for my project I need 8 PIN's to be connected to a pull-down resistor and another 3 PIN's to be connected to a pull-up resistor.

can i work with one resistor for each type i need? or is it better to solder 8 & 3 resistors separatly

the pull-ups are 1K, the pull-downs are 10K

grtz, Yves

You need a resistor for each pin. When you have a bunch of pins pulled down with one resistor, if one pin is taken high, all the connected pins will be taken high as all of the inputs are connected together. Same for pull-ups. There are internal pullups that can be enable using pinMode(pin, INPUT_PULLUP. If you can change your design so that all inputs are pulled up (and enable internal pullups) you will not need any external resistors.

If you want a pin to permanenty read LOW, just wire it to GND, ditto for permanently HIGH, wire to Vcc.

However if you want the input to be controlled by something pull ups or pull downs may be needed, and of course you need one per pin otherwise you've just connected all those input pins together so they are always the same.

If the thing controlling the pin drives HIGH and LOW (aka totem pole or push-pull) then no resistor is needed.

If the thing driving is open-collector/open-drain or a push-button to ground, then a pull up (or internal pull-up) is needed.

Somethings pull-downs are needed, but less commonly (MOSFET gate is one case).

OK guys, Thanks....

don't know where my mind was, "all the PIN's togehter", that's in fact a single read-out on all the pins simultaniously

Grtz, Yves