Multiplexer select inputs

[patroclus]

Hello,
I'm building a circuit that uses a 74LS153 4:1 multiplexer. It has 2 select inputs, and I want to toggle one of them from 5V to GND using a switch.

I connect switch's middle pin to the multiplexer select input pin, one of the other switch's pin to GND and the other to 5V. Is this enough?? Should I add a series resistor in the 5V pin or something?? Better way of doing this??

thank you!!

 

[Sebi]

No need a toggle switch, just use a pull-up resistor (10k) for inputs, and tie with simple switch to GND.

 

[patroclus]

Yes, I sometimes use pullup resistors.
but, can they always be used as integrated circuit inputs?

I mean... (this is a theory question)
If you supply 5V across a resistor to a circuit, and this circuit has a finite resistance, can it make a voltage divider??

 

[instruite]

Normally ICs like multiplexer have specifications for input current in the specification, the value of pullup or pull down resistor is determined depending on this current specification
your resistor value should be such that the input current passing through your resistor should not cause a drop such that the voltage at the pin will be less than the minimum voltage specification at that pin of the IC
In mathematical terms
Vccmin - Iinmax x Rpullupmax > Vihmin
Where
Vccmin is the minimum value of your supply voltage (at pullup resistor)
Iinmax - maximum current going into the pin of the IC (if you refer to the **broken link removed** its specified as Ii
Rpullupmax - maximum resistance value of your pullup resistor
Vihmin - minimum high level voltage needed at select input

 

[patroclus]

Thanks, I understand