Absolutely not. Check the data sheet!

IIH = 20uA for 74LS08 (it is 40uA for the 7408).
IIL = 0.4mA for 74LS08 (it is 1.6mA for the 7408).​

See, I remember those old things.

Sorry, you are correct. I was reading the table for 7408. Brain freeze. If I didn't have so much eggnog I would have remembered the difference.

 

The currents do not matter since the OP does not know which IC he has. Maybe somebody at ebay sanded off its markings?

 

I am using 74HC08N.

Wait so ALL even unused input pins should be connected to ground ?

I don't have so many wires and not connecting gnd to unused ones seems to work fine.

 

I don't have so many wires and not connecting gnd to unused ones seems to work fine.

Leaving unused inputs to CMOS logic gates unconnected is not recommended and can result in increased power dissipation and noise on the power supply from the devices switching randomly.

If you have good supply bypassing, you can get away with leaving them unconnected for experimenting (I do it all the time); but you need to be able to troubleshoot any problems caused by doing that. If you commit the design to something more permanent, you can avoid potential problems by tying them off appropriately.

 

Graphs from Texas Instruments show 74HCxxx ICs with a 5V supply and inputs floating at an average 2.5V
cause an output current of typically 48mA through each gate. It exceeds the maximum allowed output current of only 25mA. The output transistors are designed to switch but yours will be linear and will probably burn out. The datasheet says the maximum allowed linear duration when switching is only about 2.5us.
You have 3 gates with floating inputs then the typical total current is 48mA x 3= 144mA which exceeds the 50mA maximum allowed per IC.
The datasheet says, "All unused inputs of the device must be held at VCC or GND to ensure proper device operation."

 

Read this:

http://www.ti.com/lit/an/scba004d/scba004d.pdf

 

Read this:

http://www.ti.com/lit/an/scba004d/scba004d.pdf

Yes, it says the recommended maximum Input Transition Time for 74HCxx is only 450ns.

 

If you have the O'scope then you can measure the output pin to see the waveform when the input pins are on the floating status, you can also measure the output pin when the input pins are pull high and pull low, measure it will let you to see the real situation when you don't care about the input pins then what will be happening of the output pin.

 

I never tried it but I think floating Cmos input pins will pickup mains electricity hum and every AM radio station in town.

 

I never tried it but I think floating Cmos input pins will pickup... ...every AM radio station in town.

I should build one of these, I get bored listening to only one station at a time.

 

To give the OP some peace of mind, I breadboarded a 74HC08 with all inputs floating and cycled power a dozen times. Current was 5-20mA; higher than if inputs were tied off properly, but nothing that would likely cause damage.

I also tried the experiment with a CD4069 and CD4011. Current was around 2mA; also not high enough to cause concern.

As I mentioned, I often leave inputs floating when I'm prototyping a circuit. I'm usually using a power supply that current limits at 150mA or 400mA, so I'm pretty sure I'd notice if something was drawing 100mA more than it should.

The power supply used for testing had a built-in meter that could measure voltage or current; so the meter wasn't affecting measurements.

Floating inputs did pick up line frequency, but amplitude wasn't high enough to cause an inverter to toggle.

 

The problem of SN74LS136N (XOR) was split to another new thread --
The problem of SN74LS136N (XOR).