555 weirdness - Page 1
Forum section: electronics / beginners

author: Techalyzer
author_rank: Newbie
timestamp: August 26, 2022, 10:14:04 pm
content: Hi there! I've been watching the YouTube channel for years, but now I have one of those very rare and weird problems.I am doing a few mini-modules with 555s for my car. One of them - a remote engine start system using a 2 remote controlled relays module. For the engine to start, I need a circuit that acts as a toggle for the main power line, and another to turn on momentarily to power up the starter motor. The second one is done directly through the relay module that I bought, as it powers on the relays for about a second, which is perfect to start the engine. The problem is with the mains power toggle, because I need to first power up the car permanently, and only then power up the starter motor. So I built a classic 555 toggle switch circuit.And here comes the weirdness - on the bench it works perfectly. On the car, I can't make it work properly. It first turned off randomly. After a full day of tinkering, I figured out some resistor values and I found out that I needed to put a 0.1 uF capacitor in parallel to one of the resistors, pin 2 to power, about 6.8K. The problem - the command wires (that come from the relay module) acted as touch switches. So whenever I touched them, or whatever is going on in the car circuitry, it triggered the 555 as if the command relay clicked.I tested it thoroughly on the bench, all fine, then I mounted it back on the car. Now, the instant I power up the starter motor, the 555 toggler turns its relay (output) off.The first thing I did was adding up a ripple filter and a 7812. However, I have zero experience with these, but I put a 100uF capacitor in parallel with Vcc, then a choke, after the choke a 1000 uF capacitor and then a 7812. I don't have an oscilloscope, and it's impossible to get one now, but I improvised with a laptop, a mic wire and CoolEdit. Not the most precise thing in the world, but I used it before for a DIY seismometer and it was enough to see that indeed there's a lot of wobble on the main 12V line, mainly from the ignition, I suspect.At this point, I am all out of ideas and I'm just a hobbyist, so there are still a few things that I have no idea about. I was thinking about using a shielded enclosure, maybe it can help? It seems that there's something spiking at engine start, and the 555 sees it as an input command.If I really have to, I'll add some schematics, but it's all really simple and exactly what I said it is.P.S.: I am aware that an Arduino would have been way cooler for this, as I am doing many other modules, maybe even a screen, sensors and all that (it's an older car, no screens), but I don't currently have the money and I don't know how to program for Arduino and it was not really the time to start doing so.P.S.2.: I am having this exact same issue on an ozone generator, where I use the 555 toggler for an on/off switch. I suspect that it has something to do with the high voltages, it just randomly turns on or off once in a while

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2022, 08:41:09 am
content: A schematic would help us give you a better answer.The 555 is often touted as a beginner friendly IC, but it's not really. Layout and proper decoupling are important.Is pin 5 connected to 0V via a capacitor?An LM7812 won't help because it needs a few volts higher input voltage, which you don't reliably have in a car. You could try the LM7805.

author: EPAIII
author_rank: Super Contributor
timestamp: August 27, 2022, 10:39:53 am
content: A car is a horrible environment for electronics. And a 555 can be triggered by a very fast spike. It could take a lot of work to de-bug your circuit. You could start with putting it in a solid metal box and solidly ground that box to the frame of the vehicle.But it may be easier to start over and use a mechanical relay with a simple R-C filter to delay it's action.

author: rooppoorali
author_rank: Regular Contributor
timestamp: August 30, 2022, 07:38:28 am
content: I think you'll get more fruitful responses if you share the circuit diagram.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 31, 2022, 04:17:55 pm
content: I fell out with 555s a very long time ago.No friend of mine.

author: B J
author_rank: Contributor
timestamp: September 02, 2022, 10:26:52 pm
content: My first gut shot would be with EPAIII. The environment is beyond horrible. To start with, how about the power feed. Is it direct off the car battery? Make sure the circuit has its own on board, isolated buss, decoupled supply, A very large cap. along with smaller high frequency caps right on the boards. . All this to a very robust ground at the site of the circuitry. I am guessing that you do not have any high speed signals required, so are all lines in and out bypassed with caps? On the power circuit, come into the power buss through a resister, and maybe an inductor would also help. ---- I like 555s and have been using them for years. Sometime they are a challenge, but stay with it, and they are great. Sitting here, I can look at some river monitoring systems that have 555s doing their thing. The system has been running steady now for about three years, and hasn't missed a beat.

author: tooki
author_rank: Super Contributor
timestamp: September 03, 2022, 04:59:51 pm
content: Quote from: Zero999 on August 27, 2022, 08:41:09 amQuote from: Terry Bites on August 31, 2022, 04:17:55 pmThank you both. I’ve never been a huge fan of the 555, and have never understood people’s fascination with it, and above all, the way its fans push it as the go-to solution for every timing problem, even when much better alternatives exist.

author: Zeyneb
author_rank: Regular Contributor
timestamp: September 04, 2022, 06:43:53 pm
content: Quote from: tooki on September 03, 2022, 04:59:51 pmA former friend of mine also was this enthusiastic about the 555 timer. His comfort zone was in analog electronics, not digital and certainly not in microcontroller programming. He did go out of his way to prove me "you don't need a microcontroller for that."For me microcontrollers are quite handy because of its versatility. It's a lot easier to change to code somewhat as to change a circuit.

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2022, 08:13:13 pm
content: Quote from: Zeyneb on September 04, 2022, 06:43:53 pmIt appears the original poster has lost interest. Perhaps he didn't like my post?To be fair, there are a few applications for which the 555 is the best IC for the job, but there aren't many. One of the good things about it is, it can work up to 16V (18V for the SA555) and has a high current output which can drive up to 200mA, which makes it good for driving MOSFETs and BJTs.

author: Techalyzer
author_rank: Newbie
timestamp: September 17, 2022, 12:16:40 pm
content: Quote from: Zero999 on September 04, 2022, 08:13:13 pmStill here, it's just that this year has been hell for me. I fell ill after this (nothing serious), now I bought an apartment and all the mayhem with repairs and renovations started, a whole bunch of never ending stories.I gave up on the 555 and built a classic transistor toggle switch circuit which works just fine. The 555 did the same thing even completely shielded with the shield grounded. The instant it "feels" a discharge it turns off. For what I have to do, there was really no point in going any further with investigating what actually happens.Again, I know this would be perfect for an arduino or something like that, but as I said, I don't have much time, I'm in a hurry and I work with stuff that I know for now.