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Need help for 74LS08 one logic input with switch

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PCBwire

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74LS08

74LS08 is simple AND.

I would like to control one logic input, with switch.

When switch is closed, both inputs are Vcc, and output is Vcc.

When switch is open, one input is open circuit, I would like for output to be Gnd, but i do not think it will be?

Mabybe to use switch which is changing form Vcc to Gnd, and not from Vcc to open circuit.
 

Re: 74LS08

By opening the switch you don’t change the stage of the input, it just goes from Vcc to floating (or undefined) potential that by the 08 gate is still read as “H” ..
With TTL gates you have to physically connected them to GND or use low-ohm pull-down resistor;
For TTL-LS gates (Iil<400uA, Vil<0.4V) the pull-down resistor can be 1kΩ ..

Rgds,
IanP
:|
 

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    PCBwire

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Re: 74LS08

try something like this?
gate.jpg


EDIT: sorry cameo_2007. you hadn't posted when i clicked reply..
 

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    PCBwire

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Thanks for help, good solution, i should think a little. :D
 

if this is all your circuit, i think you can avoid that AND gate... just that switch and resistor should do your job. ;)
 

Nope, that second input is not always logic one, it is output of multiplexer so can be anything, and AND is needed. :)
 

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