Author Topic: Replacing a 74ls14 with a 74ls04  (Read 4244 times)

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Offline andybarrett1Topic starter

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Replacing a 74ls14 with a 74ls04
« on: February 19, 2019, 11:57:45 am »
Hi All

I have a small circuit just for debouncing a pb switch into a PIC Micro. It currently has a 74ls14 (Schmitt Inverter)  in situ, I am fault finding and found one of its output was u/s. Thinking a 74ls04 is a basic inverter with same pinout I have replaced.

But then none of the outputs are changing states (should be high when switched) .... All are stuck low even through the pull ups on PIC pins.

Replacing again with a 74ls14 all is good ..... What might be reason for not liking the 74ls04.

Attempting to understand this

Andy
 

Offline amyk

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Re: Replacing a 74ls14 with a 74ls04
« Reply #1 on: February 19, 2019, 12:12:43 pm »
There's a reason they make both a '14 and a '04... that circuit needs a Schmitt.
 

Offline GopherT

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Re: Replacing a 74ls14 with a 74ls04
« Reply #2 on: February 20, 2019, 04:31:11 am »
I thought the story was going to end differently. I thought you would realize you could denounce in software and eliminate the extra hardware.
 

Offline andybarrett1Topic starter

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Re: Replacing a 74ls14 with a 74ls04
« Reply #3 on: February 20, 2019, 08:28:03 am »
Problem is I am a hardware guy in a software world...

I have since found putting a chip exactly the same back in works..... Any other chip fails even an equivalent ???

BR
Andy 
 

Offline capt bullshot

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Re: Replacing a 74ls14 with a 74ls04
« Reply #4 on: February 20, 2019, 08:39:11 am »
Replacing again with a 74ls14 all is good ..... What might be reason for not liking the 74ls04.

Depends, need to see the schematic.
A 74HC14 or 74HCT14 might work as a drop-in replacement.
Safety devices hinder evolution
 

Offline David Hess

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Re: Replacing a 74ls14 with a 74ls04
« Reply #5 on: February 21, 2019, 03:55:31 am »
The TTL denounce circuits I am familiar with wouldn't care about the difference between a normal and Schmitt input.  That it matters suggests that the circuit you are using is not very robust.

 

Offline Km

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Re: Replacing a 74ls14 with a 74ls04
« Reply #6 on: February 21, 2019, 10:50:09 am »
hi
you attech a 0.1uf capcitor input and out put
 

Offline jackthomson41

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Re: Replacing a 74ls14 with a 74ls04
« Reply #7 on: February 21, 2019, 01:08:36 pm »
Hi All

I have a small circuit just for debouncing a pb switch into a PIC Microcontroller. It currently has a 74ls14 (Schmitt Inverter)  in situ, I am fault finding and found one of its output was u/s. Thinking a 74ls04 is a basic inverter with same pinout I have replaced.

But then none of the outputs are changing states (should be high when switched) .... All are stuck low even through the pull ups on PIC Microcontroller pins.

Replacing again with a 74hc14 all is good ..... What might be reason for not liking the 74ls04.

Attempting to understand this

Andy

You should try using pullup resistor that will solve the issue and also add capacitor around power supply of 74ls04 as it helps in reducing the noise.
« Last Edit: April 17, 2020, 02:39:59 am by jackthomson41 »
 

Offline rdl

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Re: Replacing a 74ls14 with a 74ls04
« Reply #8 on: February 21, 2019, 06:36:19 pm »
There is no circuit posted, so everything is speculation.

I have a single inverter debouncer for a momentary push button, I don't think it would work without the Schmitt trigger. I've seen many debouncer circuits using two regular gates. They seem to be for switches not push buttons.

http://www.ganssle.com/debouncing.htm
 

Offline David Hess

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Re: Replacing a 74ls14 with a 74ls04
« Reply #9 on: February 21, 2019, 07:21:08 pm »
The two gate monostable designs work with push-button or SPST switches.
 

Offline andybarrett1Topic starter

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Re: Replacing a 74ls14 with a 74ls04
« Reply #10 on: February 22, 2019, 02:15:29 pm »
Hi Pic enclosed

There is a 1Ok pull up on the output and no diode across the 100R resistor

Thanks for looking

Andy
« Last Edit: February 22, 2019, 02:17:38 pm by andybarrett1 »
 

Offline capt bullshot

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Re: Replacing a 74ls14 with a 74ls04
« Reply #11 on: February 22, 2019, 04:07:29 pm »
That circuit should work with any kind of 'HC14 or 'HC04 (CMOS) chip.

A "normal" LS or Standard TTL will pull up its input (on its own), even though there's a 10k resistor to ground. So the output will be stuck low. An LS14 (or Standard '14) might be somewhat different regarding its input structure, so then the 10k is enough to pull the input down.

A Standard or LS TTL input does source current from +5V, so the "open" state is "high". You'd need a rather low resistor (< 1k) to pull such an input low. The Schmitt-Trigger inputs of the '14 apparently are different, one would have to look up the datasheet.

Modern chips ('HC basically all CMOS logic) behave different, they don't sink or source current from their inputs, so an open CMOS input is "undefined" and you must use a pull-up or pull-down (10k ... 47k ballpark).
Safety devices hinder evolution
 
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Offline David Hess

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Re: Replacing a 74ls14 with a 74ls04
« Reply #12 on: February 22, 2019, 04:55:42 pm »
If you are going to use that configuration with TTL, then the switch should pull the input to ground instead of Vcc because of the TTL gate's asymmetrical input bias current.
 
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Offline andybarrett1Topic starter

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Re: Replacing a 74ls14 with a 74ls04
« Reply #13 on: February 23, 2019, 05:23:09 pm »
Very Helpful this  is what I was missing :-)
 


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