Wiring a 7408 TTL Gate

I had the toughest time getting this right. With help from the electronics lab forum (thanks, audioguru), I got it figured out. Now, I’m not the brightest bulb in the pack when I’m learning something new (a fact to which those of you who know me can attest), but once I get my head wrapped around something, it’s mine! Now, this dude at the electronics lab forum (the aforementioned audioguru) tried like hell to get me to freaking get why I was seeing a voltage on the input pins of the 7408 I was messing with. Like I said, when I’m learning something new, I can appear kinda dense. Finally, the tumblers clicked into place and it dawned on me that what audioguru was saying is that I needed to connect each input pin through a 270 Ohm resistor to ground to pull down the “float high” constant current present at the input pins. I actually used a 470 Ohm resistor for each input pin, but that still worked.

Now, I know there is a fairly constant current at the input pins because I tested the voltage while switching out resistors between the input pin and ground. The higher the resistor value, the larger the voltage. Ohm’s Law tells us V = IR, so at constant I, changing R makes V change. Higher R for the same current I makes the voltage V higher.

Okay, cool. So, here’s a picture of the proper wiring of a 7408 TTL AND Gate. It is shown using a DIP switch to act as the “inputs” (with switches 1 & 2 needing to be in the “on” position in order for the AND to work).

Thank you! Goodnight!

Wiring of a 7408 TTL And Gate

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