How to use logic

Thread Starter


Joined Feb 14, 2009
Correct, by using a pulling resistor you insure that there will be a valid digital signal level to the chip at all times, nothing floating around the forbidden area.

Some logic families (like TTL) have an internal pull up forcing the input pin high, however it is usually a very high ohm value and the 'floating' input is very subjective to noise input.

It's required in some CMOS logic families that you wire unused input pins to ground or Vcc either directly or with resistors or the device will burn up because the gate will go into high speed oscillation and draw too much current, and burn your finger also ;)

How do you figure out the value of the resistor that is needed to pull the voltage to high yet still let the device switch it to low? My application has 5V input and the .8 max off state and 2-5 volt on state like described. I'm using a 10k resitor as currently suggested but get .524 volts out so the logic is stuck on off. When I tried a lower value resistor to attempt to up the voltage I ended up burnning the hall effect switch that I was trying to use.

Mark [mkenney]


Joined Apr 20, 2004
Pleas do no hijack existing threads. It is confusing to know who is replying to what.

This is the thread that got hijacked -

A sensible answer to your question isn't possible. We don't know the logic family you are trying to use, the reason for that pull up resistor, and how things are being done.

Can you post up a schematic?