inverter chip not inverting

[ravener96]

i got myself a toolkit of logic chips for a project i'm working on, and thought i'd test a few out to be sure they work like they should.
unfortunately a 74ch05n inverter chip doesent seem to be cooperating.

as per the datasheet i have an LED with a resistor connected to positive on the output, but the led is on with the input high and off with the input low. it's effectively a repeater at this point.
am i just being dumb? the outputs are open drain, and i might just be misunderstanding what it implies, i just assume logical high is the pin being open to ground, and low being closed.

in the attached picture i have the same circuit (i couldent figure out how to get the LED icon the right way, and the chip name is wrong) in this example the LED is on.

[ravener96]

i might have worded it strangely, it's not A chip not working. i have tested a few and they ALL work the same. sorry for confusion

[ravener96]

read the text
http://www.ti.com/lit/ds/symlink/sn74hc05.pdf

[rstofer]

It seems correct to me, if I understand what you are saying.

With the LED connected to Vcc, a Gnd or 0V is required to turn it on.  The output of an inverter will be 0V when the input is high.

So, logic '1' on the input should turn the LED on.  Logic '0' should turn it off.

[ravener96]

i guess it makes sense, i could just be misunderstanding how it would interact with other logic gates.

[ravener96]

thanks a lot guys, i just tried connecting it up to an and gate with a few pullup resistors, and it works swimmingly. most of the time the problem is between the keycaps and the chair

[rstofer]

The thing about LEDs in general is that we need to pull them down because, in the old days, TTL could pull down a lot more than it could pull up.

Given that we pull down to turn on, it is kind of backwards to how we view logic.  In fact, there is an entire way of thinking about negative true logic.

With the inverter things are easy to understand:  Logic '1' to turn the LED on, logic '0' to turn the LED off.  Wired the same way to an Arduino output (pull down configuration), the logic is upside down.  Logic '0' to turn the LED on, logic '1' to turn the LED off.

It all works out...