1 NE555 timer and 1 LED circuit question - Page 1
Forum section: electronics / beginners

author: Lucky-Luka
author_rank: Unknown
timestamp: June 01, 2019, 07:49:14 pm
content: Hi allI've done a simple flashing LED circuit using a NE555 IC.I've simulated it in LTspiceIV.And no, I haven't got an oscilloscope.It looks like that the LED takes two moments to flash to its full brightness.This is the video I've done regarding my circuit:https://www.youtube.com/watch?v=-vjMvDEyco4&feature=youtu.beI expected a crisp ON and OFF behaviour but it looks like it's not the case. Can anybody help me to understand what's going on?Cheers

author: soldar
author_rank: Super Contributor
timestamp: June 01, 2019, 08:30:52 pm
content: I always put a 0.1 uF cap between pin 5 and gnd.

author: Audioguru
author_rank: Super Contributor
timestamp: June 01, 2019, 10:05:31 pm
content: When an old fashioned 555 switches its output then it draws 400mA for a moment (shoot through) that sags your battery for a moment.Any other kind of power oscillator dont doo dat.

author: Lucky-Luka
author_rank: Unknown
timestamp: June 02, 2019, 12:11:38 am
content: That's interesting.ThanksCheersp.s. is there a way to find all my threads or posts in this forum?

author: drescherjm
author_rank: Contributor
timestamp: June 02, 2019, 12:57:37 am
content: QuoteIts in your profile.Profile->summary

author: ArthurDent
author_rank: Super Contributor
timestamp: June 02, 2019, 02:52:39 am
content: Here's a post I made a year ago about the 555 output problem with high current pulses just mentioned."------ there appear to be many variations of the ‘555’ and although they should functionally work the same, the circuitry inside is different and can cause many different types of problems. A lot of 555 chips may have what is known as a totem pole output where there are 2 transistors, stacked one on top of the other, between the + and – supply with the output from their common junction. This configuration allows the 555 to either source or sink current. There is an extremely small moment of time when switching that both transistors could be on, producing a short and having a large current flowing through the transistors. The way around this is to wire a, say 22 ohm, bypassed resistor in series with the + for the 555 to limit the current to a safe value.The second problem is somewhat related to the first and relates to the NE555 you are using. Here is an answer to a question I found on line.“You've discovered one of the best kept secrets of the 555. That is, they aren't all the same. In fact, I know which one you have. It's the NE555 from Texas Instrument (silver top, right?), because that's what RadioShack stocks even though the catalog says LM555. The difference is that the LM555 is a bipolar chip and the NE555 is a CMOS chip. And I bet that pin 2 of this circuit is waving in the air — picking up static electricity. When the first CMOS 555s showed up on my workbench, I had the same problem until somebody told me they were CMOS, then the light went on. When working with CMOS technology, every pin has to go somewhere, used or not. What you need to do is tie pin 2 high through a 10k resistor, that's pretty close to its bipolar equivalent, and the circuit will work.”"The above may not be your problem but not having a capacitor from pin 5 to ground and not putting a 22 bypassed resistor in series with the chip supply voltage certainly are good design choices.

author: soldar
author_rank: Super Contributor
timestamp: June 02, 2019, 02:18:48 pm
content: Quote from: ArthurDent on June 02, 2019, 02:52:39 amThis problem with bipolar transistors in totem pole configuration is not restricted to the 555 IC but can happen in any IC or discrete circuit with similar configuration if the circuit does not leave enough dead time between switching one transistor off and switching the other one on. The problem is called "shoot through" and a lot of discussion and ways to deal with it can be found online.The best way to deal with this problem is, obviously, to provide enough dead time. If that is not possible or practical a resistor and small capacitor can be used. Even better, instead of a resistor an inductance can be used as it tries to keep a constant current and returns the energy it stores.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 02, 2019, 02:31:31 pm
content: Maybe add a bypass capacitor to the input.If the problem is the high current draw it should solve it.As clearly explained in great scott's video

author: soldar
author_rank: Super Contributor
timestamp: June 02, 2019, 02:46:23 pm
content: "I once crossed the street without looking first and nothing bad happened." Bad advice."I text while driving and haven't had an accident yet." Bad advice.Quote from: Arjunan M R on June 02, 2019, 02:31:31 pmBad advice.Bad advice: costs nothing and it's worth the price.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 02, 2019, 02:51:53 pm
content: Quote from: soldar on June 02, 2019, 02:46:23 pmSorry, removed it .

author: Audioguru
author_rank: Super Contributor
timestamp: June 02, 2019, 02:56:40 pm
content: There are only TWO 555 spec's: The old bipolar one and the newer Cmos one.The NE555 is made by many IC makers. Its max output current is 200mA. Its minimum supply voltage is 4.5V.The Cmos one is made as LMC555, TLC555 and ICM7555 and is low power and has a low max output current, especially when its supply is 5V or less.The LMC555 is guaranteed to work with a supply as low as 1.5V!

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 02, 2019, 02:57:29 pm
content: Quote from: soldar on June 02, 2019, 02:46:23 pmI said so because I built this circuit a lot of times but I never had a problem =I said so because I texted many times while driving and haven't had an accident yet. = bad adviceSorry, removed the first two lines.I was not aware of that I was posting it in the beginners section.

author: soldar
author_rank: Super Contributor
timestamp: June 02, 2019, 03:13:23 pm
content: Quote from: Arjunan M R on June 02, 2019, 02:51:53 pmOh man, I did not intend to pick on you and I am sorry if it came across that way. I was just attempting a little humor.All I meant is that it is better to follow good practices all the time. You can get a way with a lot but bad practices will catch up with you sooner or later.Pin 5 of the 555 should not be left floating. It is better to put a capacitor that will avoid false triggers.

author: StillTrying
author_rank: Super Contributor
timestamp: June 02, 2019, 03:46:43 pm
content: With long power wires the 100nF to 10uF decoupling across a bipolar 555 power is more important than the Pin5 decoupling.Just to add to the confusion.On a supply with high impeadance such as a half discharged battery, there are occasions when decoupling to Pin5 to GND when the timing or triggering is tied to V+ can make false triggering worse.

author: soldar
author_rank: Super Contributor
timestamp: June 02, 2019, 03:49:03 pm
content: Decoupling the power pins goes without saying. There, I said it.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 02, 2019, 06:41:58 pm
content: Here is a pulse stretcher I designed years ago to lengthen the 25us 1pps pulse from a Z3801A GPSDO so I could have it flash an LED and also be used as an output. Note I used supply decoupling and supply bypassing and pin 5 has a capacitor to ground. It worked flawlessly for years, 24/7.