NE555 inverting phase and duty cycle. Can change it? - Page 1
Forum section: electronics / beginners

author: Maurizio
author_rank: Contributor
timestamp: November 23, 2013, 06:49:37 pm
content: Hello, I'm building a little circuit with NE555 as Schmitt trigger. It works, but (as I read in many articles) the phase of the output is inverted, so when the entrance pulse is 1, the output is 0.Also, it is affecting the duty cycle and inverted it.I need to clean a distorted square pulse that have a very long rise time, so the output is too short.There is some trick to re-invert the output of the 555? I do not care of the delay due to the phase shift, but mostly about the inversion of the duty cycleOr it's better to try some other chip?Tnx very much and sorry for my english.Maurizio

author: IanB
author_rank: Super Contributor
timestamp: November 23, 2013, 07:56:55 pm
content: If you would attach a sketch of your circuit it would be easier to provide specific answers.One possibility is to use the DISCH terminal as the output signal rather than OUT. In many circuit configurations this signal will be pulled high when the output is off and pulled low when the output is on. So it will act as the inverse of OUT.Oops, giant mistake, comment retracted. Sorry.Another possibility is to use a dual 555 chip, and then use the second 555 to invert the output of the first 555.

author: strangelovemd12
author_rank: Regular Contributor
timestamp: November 23, 2013, 10:36:43 pm
content: What did I miss that means a PNP doesn't work here?

author: Six_Shooter
author_rank: Regular Contributor
timestamp: November 23, 2013, 10:40:22 pm
content: Quote from: IanB on November 23, 2013, 07:56:55 pmOr more simply an Op-Amp using the inverting input, such as a 741.

author: c4757p
author_rank: Super Contributor
timestamp: November 23, 2013, 10:45:24 pm
content: Quote from: IanB on November 23, 2013, 07:56:55 pmDISCH and OUT are in phase with each other, unless I'm misreading the datasheet...

author: Maurizio
author_rank: Contributor
timestamp: November 24, 2013, 12:58:34 am
content: This is my schematic: I think it's a very classic one.Next picture is a scope view: input sinusoid , 5V pep, 10 kHz,output pin 3, square, 5V over ground, 10 kHzPhase shift is a little bit more than 180°Nex one: the output on pin 7 is very distorted form, around 0,5V under the ground level.Phase shift is also inverted, but the signal is "under" the ground level.IanB: perhaps woould be better changing something in schematics?Last picture: input square, 5v pep, 10 kHz, duty cycle 80%output pin 3, square, 5V over ground, 10 kHz, duty cycle around 20%Using a 556 (double 555) would be ok, but a bigger chip and some other components meaning more space. But I cannot try now because have'n this chip at home.strangelovemd12: how to implement a PNP, please?Thanks to allMaurizio

author: chasxmd
author_rank: Contributor
timestamp: November 24, 2013, 01:18:21 am
content: Would a Op Amp as an inverting amplifier work, or perhaps just a 74LS04 logic chip if you're using TTL levels?

author: Zero999
author_rank: Super Contributor
timestamp: November 24, 2013, 01:27:42 am
content: Or you could just use a NE556 or 74HC14, depending on your power supply and load driving requirements

author: IanB
author_rank: Super Contributor
timestamp: November 24, 2013, 01:55:02 am
content: Quote from: c4757p on November 23, 2013, 10:45:24 pmYes, it does appear that they are. Which leaves me strangely puzzled, since I was sure when I was experimenting with the 555 in the past that I had inverted the output signalNow I can't figure out what it was I was doing. Maybe my memory is just playing tricks with me.

author: IanB
author_rank: Super Contributor
timestamp: November 24, 2013, 01:59:11 am
content: Quote from: Maurizio on November 24, 2013, 12:58:34 amUnfortunately I made a mistake and my suggestion does not work. Sorry.I think your best option would be to use a single transistor following the output to invert the signal.

author: Maurizio
author_rank: Contributor
timestamp: November 24, 2013, 03:00:36 am
content: No problem IanB, and thanks for replaying. Also, I tried with a NPN transistor mounted in NOT configuration (i.e. "straight")and it worked flawlessy. The rise time of the transistor is a little bit slow, but it's another matter.Many tnx to you and all.Maurizio

author: c4757p
author_rank: Super Contributor
timestamp: November 24, 2013, 03:04:55 am
content: Quote from: Maurizio on November 24, 2013, 03:00:36 amTry decreasing the collector load resistor (obviously within reasonable limits).

author: strangelovemd12
author_rank: Regular Contributor
timestamp: November 24, 2013, 03:13:28 am
content: Quote from: Maurizio on November 24, 2013, 12:58:34 amIf the 555 output is wired to the base of a PNP wont the emitter be the inverse of the signal?Replies while typing. Yeah, NPN in NOT config > PNP.Thanks for bearing with me while I learn not to be an idiot.

author: Maurizio
author_rank: Contributor
timestamp: November 24, 2013, 12:39:25 pm
content: It works very well.Transistor is BC107, 10k on the base.First shot: 10k on the collector, current 0,32 mA, rise time low2nd shot: 1k on the collector, current tenfold: 3,2 mA, rise time much better.Scope (old Tektronix 2205) is a fundamental tool to see things , but also my new function gen Rigol DG1032zI'm amusing a lot also with such simple matters.TnxMaurizio

author: Maurizio
author_rank: Contributor
timestamp: November 24, 2013, 02:34:10 pm
content: Tried 74HC14, 2 port in series to double invert the signal. No contour components: Signal enters pin 1, exit on 2, enter on 3, exit definitely on 4. Pin 7 to ground, pin 14 to 5V. Only a 0,47 uF in parallel of the power supply, but seems to me it don't matter.It works perfectly, only a little oscillation at the end of the rise time.Could be more better?

author: Maurizio
author_rank: Contributor
timestamp: November 24, 2013, 03:52:04 pm
content: Only problem is the 74HC14 drains around 30 mA from the supply at start and then stabilizes at 16mA: the 555 ant BC107 drains only 3mA.Why such big difference?