Hi, I'm trying to use an NTE7426 quad NAND gate IC with my Arduino Uno. I have tried prototyping with just an LED and pushbuttons on one of the gates. The LED won't light. I have triple-checked the pinout and my wiring. I have tried replacing the IC and LED. I have checked the polarity on my LED. But it still won't work. Help will be appreciated. Thanks!
Its open-collector... How are you connecting the LED/resistor?
does the LED work? (resistor + voltage)
does the output of the nand gate deliver enough current to light the LED?
Guess it's easier to see your circuit 
I'm connecting the positive lead of the LED to a resistor and the NAND gate IC. I tried reversing the polarity of the LED, but it still won't work. And when I attach +9v to Vcc on the IC and the positive side of the LED to the NAND gate's output, and the negative side to ground, it lights up. But as soon as I attach the NAND gate's ground, it goes out.
element74:
I'm connecting the positive lead of the LED to a resistor and the NAND gate IC. I tried reversing the polarity of the LED, but it still won't work. And when I attach +9v to Vcc on the IC and the positive side of the LED to the NAND gate's output, and the negative side to ground, it lights up. But as soon as I attach the NAND gate's ground, it goes out.
uh oh.
First rule, unless it's from the 4000 series (Due to their high voltage tolerances usually up to 12-15v) of IC's, always look at the data sheet for max voltage specifications.
5.25v is the MAX supply voltage for the NTE7426.
http://www.doctronics.co.uk/pdf_files/hcf4011.pdf
check out the 4011 for this project or lower the voltage and pray to the IC gods for a working chip when you lower the voltage to 5v... now if that checks out and works, try it again use a 300 ohm resistor to your LED (unless you are i'm not scrolling all the way down).
The circuit is still going to be handy to check your NAND logic inputs.
Because its open collector you must connect the LED anode to Vcc and cathode (via
resistor) to the 7426 output since it is only capable of sinking current, not sourcing.
And it you've put more than 5V on any of the 7426 inputs you've likely burnt them out,
TTL is extremely sensitive to overvoltage on the inputs (more so than the Vcc supply
itself which I think is 7V abs max). But 9V is certain death/damage.
Normally with TTL any inputs you want to tie high you do via a 1k resistor to prevent
over-voltage damage.
And really, you paid way too much for that part. Like almost a $1.00 more than you needed to. Friends don't let friends by NTE parts.