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HCT08 and PWM to single out

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xmen_xwk

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I've a microcontroller generating 1Khz freq PWM and and I need to do some switching with HCT08 but the output is a single connection.
So I made 2 schematics. But I'm bit confused which one is correct.

normal.png
The above one is exactly what I want but I'm not sure if HCT08 has internal diode. Because current can flow in other part as well. Does it has internal diode ?

withdiode.png
Still, I put external diodes for some safety. But Im not sure if it will work properly or not. Because diode drops voltage and Im only using it for 3v3 to 5V. Or is there some special kind of diode that I can use ?


Thank you :)
 

FvM

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HCT08 has push-pull outputs and no means to make a wired OR function. Shorting the outputs against each other will fry the IC. Why don't you use an OR gate to combine the signals?

Your diode circuit would at least need a pull-down resistor, and has slow falling edge.
 

xmen_xwk

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HCT08 has push-pull outputs and no means to make a wired OR function. Shorting the outputs against each other will fry the IC. Why don't you use an OR gate to combine the signals?

Because sw1 needs to be high to get pwm1, and sw2 needs to be high to get pwm2. Both cant be high at same time.

Your diode circuit would at least need a pull-down resistor, and has slow falling edge.
has slow falling edge ? how it has slow falling edge ? I'm bit confused.
 

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My suggestion is to use an OR gate in addition to the AND gates.

Slow falling edge depends on your requirements. Compared to the output speed of HC logic, the diode OR gate is very slow. If you don't have any speed requirements, ignore the comment.
 

xmen_xwk

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My suggestion is to use an OR gate in addition to the AND gates.

Slow falling edge depends on your requirements. Compared to the output speed of HC logic, the diode OR gate is very slow. If you don't have any speed requirements, ignore the comment.
So I should use HC32 for OR gate ? and is there something else I can use that is fast enough and better than diode OR gate.
 

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Using OR gate seems good idea. But I have feeling I will have more than 2 output in future. That will introduce problem. Because OR Gate only support 2 normally. With diodes I could have n number of outputs. But again it might be slow in speed as you said.
 

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