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7404 Hex Inverter - Dumb question

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New Member
I've just started tinkering about with electronics and have a question regarding the 7404 Hex Inverter. I've put together a very simple circuit to test the output from a 7404.

The 7404, as everyone here will know, has a Ground pin. I have pin 2 connected to the anode of the LED. Pin 1 is connected to one of the joystick lines (which can be changed to output). However, if I connect the Ground pin to the Ground rail, the chip seems to bypass all the power to the rail and the LED fails to light.

When I disconnect the gnd pin, everything seems to work ok. I'm just wondering if there'll be any damage to the chip by doing this as the 7404 seems to need to have the Vcc connected (unlike other logic devices) - obviously to provide power to the outputs when the inputs are logic 0.

FYI: The board is being powered by the joystick port on an Atari 800XL computer.


New Member
Which version of 7404?
You should never have applied power to it without a ground at #7.
You could have toasted it.
Remove the joystick connections, LED etc. and connect the 7404's ground [#7] & power [#14] only.
Unconnected inputs usually float high. [TTL & LS]
Does grounding the input pin make the output go high? It should.
Now connect your LED with the current limit resistor.
Does grounding the input pin make the LED light?
If you get this far try your joystick signal again.
Good luck.


New Member
Tried everything but can't get the LED to work. Even tried changing the IC to a 74AHC138 to see what happens (the 7404 is a 74AHC04) and I get exactly the same problem. As soon as the ground pin is connected, the LED refuses to come on.

Must be something I'm doing, but what?

Also, I know I need a current limiting resistor at one end of the LED, but don't know why. Can anyone fill me in on that?


New Member
Sorry to hear you are still having troubles. :(

The 74AHC04 is a CMOS version of the 7404.
CMOS is better at sinking current than sourcing.
This means it is better at providing a ground than a source of power.
You hooked it up alone and watched it change the output as you changed the input, right :?:
What is the LED resistor's value :?:
You probably only have 8 mA to work with.

When you connect the LED anode to the 74AHC04 you use it as a source of power.
Try reversing the LED circuit:
Connect the LED cathode end to your 74AHC04 and change the current limiting resistor to the positive power rail.
A positive input to the 74AHC04 gate should now provide a output ground to sink the current, lighting your LED. :idea:

The 74138 is a more complex decoder with several pins that need attention before it will do much for you.


Active Member
After connecting everything i.e. GND, have you tried changing the input logic state? If not try doing it. May be when you power up, the input is high and so LED refuses to go since output will be low.


New Member
Finally got back to it today and got it working. Just put together a very simple logic tester with the LED and resistor, instead of putting the LED in the circuit ... and the outputs on the 7404 now seem to work okay.

Figure that one out! :?
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