well can smbody pls tell me what voltage values r associated with logic levels 0 & 1 . is it always
0v for logic zero
& 5v for logic 1 ?

 

Designers have several choices. A positive voltage can represent logic zero & ground can represent logic one. This was once called "negative logic." Or the opposite may be used, as is more often the case. We used to call this "positive logic."

Voltages can be five volts, three volts, twelve volts, or any other value needed for the specific application. Most commonly, we'll see devices running at either 5Vdc or 3.3Vdc, and using positive logic.

 

Sometimes it's defined by the family of logic devices. In the good old TTL series, a logic 0 was anything below 0.8v, and a logic 1 anything above 2.8v - even though powered by 5v (+/- 0.25v)

4000-series CMOS can be used from around 3v to 16v. If Schmitt-trigger inputs are used, anything lower than 1/3 Vdd (Vdd = the positive supply voltage) is a logic zero, and anything higher than 2/3 Vdd is a logic 1.

 

Here's a chart I ran across for 5V logic for various logic families.

http://www.interfacebus.com/voltage_threshold.html

Best bet is to look at the data sheet for the specific digital logic or microcontroller chip your're using. Google is your friend there.

Ken