I have this circuit:
I'm powering it from an Arduino but that's inconsequential to the operation, it's simply a convenient source of +5v. The orange line is +, black to gnd.
If it's not clear from the picture (the lines got a little messy, sorry), the basic idea is I have 2 switches connected to ground, both attached to a 10k ohm pull-up resistor from the +5v side. The resistors are connected to the input gates of a 74LS20 quad input NAND gate. I've tried to pair the inputs together as I want it to function as a dual input gate. I'm using a quad input due to lack of components.
When I throw the switches to the up position, as pictured, the circuit is shorted to ground, which should pull the input pins low, meaning the output pin is high and the LED lights. This works fine.
However, when I open the switches, the input pins are high (I checked this with a multimeter, they're at +4.98V), but the output doesn't change, the LED stays on.
I've tried the same circuit with a couple of IC's in case one was faulty, but the same behavior is exhibited.
Have I wired this wrong? It's been a very long time since I've messed around with this stuff so maybe I've forgotten something fundamentally intuitive about electronics, apologies if that is the case.