Replacing a 74ls14  with a 74ls04 - Page 1
Forum section: electronics / beginners

author: andybarrett1
author_rank: Regular Contributor
timestamp: February 19, 2019, 11:57:45 am
content: Hi AllI 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 thisAndy

author: amyk
author_rank: Super Contributor
timestamp: February 19, 2019, 12:12:43 pm
content: There's a reason they make both a '14 and a '04... that circuit needs a Schmitt.

author: GopherT
author_rank: Contributor
timestamp: February 20, 2019, 04:31:11 am
content: I thought the story was going to end differently. I thought you would realize you could denounce in software and eliminate the extra hardware.

author: andybarrett1
author_rank: Regular Contributor
timestamp: February 20, 2019, 08:28:03 am
content: 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 equivalentBRAndy

author: capt bullshot
author_rank: Super Contributor
timestamp: February 20, 2019, 08:39:11 am
content: Quote from: andybarrett1 on February 19, 2019, 11:57:45 amDepends, need to see the schematic.A 74HC14 or 74HCT14 might work as a drop-in replacement.

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2019, 03:55:31 am
content: 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.

author: Km
author_rank: Newbie
timestamp: February 21, 2019, 10:50:09 am
content: hiyou attech a 0.1uf capcitor input and out put

author: jackthomson41
author_rank: Regular Contributor
timestamp: February 21, 2019, 01:08:36 pm
content: Quote from: andybarrett1 on February 19, 2019, 11:57:45 amYou should try using pullupresistorthat will solve the issue and also add capacitor around power supply of74ls04as it helps in reducing the noise.

author: rdl
author_rank: Super Contributor
timestamp: February 21, 2019, 06:36:19 pm
content: 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

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2019, 07:21:08 pm
content: The two gate monostable designs work with push-button or SPST switches.

author: andybarrett1
author_rank: Regular Contributor
timestamp: February 22, 2019, 02:15:29 pm
content: Hi Pic enclosedThere is a 1Ok pull up on the output and no diode across the 100R resistorThanks for lookingAndy

author: capt bullshot
author_rank: Super Contributor
timestamp: February 22, 2019, 04:07:29 pm
content: 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).

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2019, 04:55:42 pm
content: 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.

author: andybarrett1
author_rank: Regular Contributor
timestamp: February 23, 2019, 05:23:09 pm
content: Very Helpful this is what I was missing :-)