74HC04 Hex Inverter

[ICX]

I am new to electronics, but I started buying kits for practice and learning. One of my kits (Elenco MM8000) isn't working properly and I think I've isolated the problem with the 74HC04 hex inverter.

I looked at the data sheet (https://www.electro-tech-online.com...HC04E-Texas-Instruments-datasheet-8442375.pdf) and noticed that this inverter should always have a high voltage on the output when no voltage is applied to the input and vice versa. However, the chip has been flaky. Sometimes it will have the correct output, but not consistently. Also, once voltage is applied to the input and then is subsequently removed, the output takes a while to restore to its original high voltage setting.

I have purchased other identical chips and found this to be the case even when it has come straight out of the package from the manufacturer. I applied 5 volts to pin 14 and grounded pin 7. Am I missing something? Shouldn't the truth table always be accurate for the corresponding pins listed on the datasheet?

 

[Reloadron]

Generally when using a chip like this you want to tie all unused inputs to either VCC or GND and not leave them floating.

Ron

 

[Roff]

I am new to electronics, but I started buying kits for practice and learning. One of my kits (Elenco MM8000) isn't working properly and I think I've isolated the problem with the 74HC04 hex inverter.

I looked at the data sheet (https://www.electro-tech-online.com...04E-Texas-Instruments-datasheet-8442375-1.pdf) and noticed that this inverter should always have a high voltage on the output when no voltage is applied to the input and vice versa. However, the chip has been flaky. Sometimes it will have the correct output, but not consistently. Also, once voltage is applied to the input and then is subsequently removed, the output takes a while to restore to its original high voltage setting.

I have purchased other identical chips and found this to be the case even when it has come straight out of the package from the manufacturer. I applied 5 volts to pin 14 and grounded pin 7. Am I missing something? Shouldn't the truth table always be accurate for the corresponding pins listed on the datasheet?

What the datasheet does not tell you is that input voltages are indeterminate when they are floating. That is because the CMOS input resistance is essentially infinite, but the input has a small amount of capacitance (10pF max, in this case), which can store charge. Charge stored on a capacitor causes a voltage to appear across that capacitor, which can be a logic 0, a logic 1, or an intermediate voltage which can cause the output to be "noisy".
For this reason, you must apply a valid logic level to an input if you want a predictable output. Unused inputs should be connected to one of the supply rails, either directly or through a resistor.

 

[crutschow]

I looked at the data sheet (https://www.electro-tech-online.com...04E-Texas-Instruments-datasheet-8442375-2.pdf) and noticed that this inverter should always have a high voltage on the output when no voltage is applied to the input and vice versa.

The data sheet does not say no voltage applied, it says logic zero or 0V, which is not the same as no voltage. As Roff noted, an open input is not necessarily a logic zero with CMOS.

 

[tom_pay]

Hi

As Roff said we must never leave inputs floating. That is why you use pull up or pull down resistors. These resistors stop the inputs from floating by connecting them to 5v or 0v until a signal arrives.

If a signal consisting of a logic level low is given to the IC (to give a high output) you would use the pull up circuit, the one on the left. The pin voltage will be at 5v all the time, unless a 0v signal drives the pin low. For simplicities sake I have just drawn in a switch, in you circuit it may be a transistor or sensor or whatever.

If your signal is a logic high then you must use the diagram on the right, it is a pull down circuit. It is just like the pull up circuit, but does the opposite. It holds the pin at 0v waiting for the signal to drive the pin voltage high.

In each of the circuits I have stated to use a 10k resistor. This resistor can be virtually any value, as long as it's not too low. I try to keep them above 2k, but if you have lots of 1k2's, they will do just as good. If it is running on batteries, try to use larger values 10k+ this way they standby current is kept to a minimum and the batteries last longer.

These circuits are in most digital circuits, they avoid noise, inconsistent results and a lot of other things.

Tom