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Topic: Using a simple AND gate ic (Read 1 time) previous topic - next topic

Nick Gammon

Please don't start two threads within a few hours of each other about AND gates. Just let the answers flow in from the helpful people here. :)

Threads merged.


can someone please help me to connect a 74LS08 quad and gate to a dip switch? I cant seem to understand how the pull up or pull down resistors work. i want the inputs to go to low when the switch is off and high when the switch is on.

That's not easily doable with TTL as I've mentioned - ideally use push-to-break rather than push-to-make switches, or else feed the switches through inverters.

The problem is that TTL inputs are not really rated for connecting to +5V except through a resistor - the breakdown voltage of the input transistor is only 5.5V (bit close for comfort).   The good news is that I've done a bit more research and the LS series of TTL is more robust in this respect, so you should be able to get away with it - the input transistors can handle upto 7V

You will need a pull-down resistor as low as about 680 ohms though, to pull the inputs properly low, 1k might be a bit too large.

[ and the standard caveat - 0.1uF ceramic decoupling capacitor for every logic chip ]

[ I won't respond to messages, use the forum please ]

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