Simple 555 timer Delay ON not working on LTspice - Page 1
Forum section: electronics / beginners

author: sparksflying
author_rank: Contributor
timestamp: November 21, 2013, 06:14:07 am
content: Hello,I am trying to make a simple delay on 555 timer circuit, but can't seem to get it working on LTspice. I have gone off this schematic, which is the same as many others on the internet.http://www.electro-tech-online.com/attachments/delayed-on-png.59362/When I start the simulation the output should stay LOW for 5sec and then go HIGH, but it is going high from time zero.What have I done wrong?

author: codeboy2k
author_rank: Super Contributor
timestamp: November 21, 2013, 07:53:51 am
content: modify your.tran 10line to be.tran 10 uicwhich means Use Initial Conditions (and don't calculate the DC operating point)You can edit the line directly or set it with the menu command:Simulate->Edit Simulation Cmd->Transient->  Skip Initial Operating Point SolutionAt simulation startup, time t=0, SPICE changes capacitors to independent voltage sources and inductors to independent current sources in order to calculate the DC operating point of the circuit for a transient analysis. You usually want this step for simulation of AC signals. For pure DC signals and varying DC signals such as your 555 timer, you may not want spice to do this initial operating point calculation. Often times, you will actually want to set specific initial conditions at specific circuit nodes using the.icspice directive (to set an initial condition)However, in your case, this circuit is simple and doesn't need specific initial conditions set, but you will still want to tell SPICE to use initial conditions (of which you have none set) so that all node voltages in your circuit start off at 0V, and that is what you really want in your circuit, to simulate the actual voltage conditions at power on.

author: sparksflying
author_rank: Contributor
timestamp: November 21, 2013, 09:40:50 am
content: That worked, thank you

author: codeboy2k
author_rank: Super Contributor
timestamp: November 21, 2013, 12:48:22 pm
content: you're welcome

author: cydtulkas
author_rank: Newbie
timestamp: February 23, 2014, 09:14:38 pm
content: Hey,Seems I'm having an issue with this very same circuit in real life. I built it on a breadboard, but when I apply power the led immediately lights up. There's no way to modify the.tran 10line to.tran 10 uicin real life unfortunately.What do you think should be done to get this thing actually working?Thanks for your help.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 24, 2014, 04:15:42 am
content: This simple circuit should work as it is, but perhaps it's something with your 555...check the wiring carefully.

author: cydtulkas
author_rank: Newbie
timestamp: February 24, 2014, 07:40:30 am
content: That was my first thought, and tried to rebuild the circuit a few times. Still it is just not working for me. Maybe I am doing the same mistake all over again. Can you please take a look on the attachments? Maybe you pinpoint my mistake.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 24, 2014, 10:53:53 am
content: Your capacitor on pin 5 seems a little too large, although I don't think that is the cause of any problem. I've never seen more than about .1uF on the control pin of the 555. You have about 10uF there. it should be a small ceramic disc.Also, try putting a large decoupling capacitor on the breadboard near the IC. Lots of things go weird when you don't have decoupling near the IC, especially on a breadboard. So perhaps try a 10uF decoupling capacitor across the power supply on the breadboard. Place it across the chip VCC and GND.Other than that, are you sure that 555 is not damaged? Does it work in other modes, like astable mode, to blink the LED on and off?If I was in your shoes and baffled by why the 555 timer wasn't doing what it was supposed to do, I'd try to make it do something else, just to see if the chip works in other ways.

author: cydtulkas
author_rank: Newbie
timestamp: February 24, 2014, 12:04:18 pm
content: I think the 555 is OK. It works just fine in another circuit on the other side of the paper I used to separate the things a bit. I know the 10uF is a bit large for the control pin, but I'd think that just affects when the OUT pin gets High. I did not have a small one at hand, so thought this would do it. But I go and get a much smaller one.I try the decoupling capacitator and let you know how it went.Thanks so far!

author: cydtulkas
author_rank: Newbie
timestamp: February 26, 2014, 09:57:43 pm
content: So I went to buy a new 555 timer, a ceramic disc capacitor, rebuilt the circuit from scratch, used a 10uF decoupling capacitor between the VCC and GND legs of the 555 IC, but the behavior is the same. I am running out of ideas.May I ask you - if you have the spare parts available - to build this on a breadboard? I know this circuit can be found on a number of places all around the Internet, but I'm getting really confused.Thanks.

author: cydtulkas
author_rank: Newbie
timestamp: February 28, 2014, 10:48:59 am
content: I figured it out. Lamest mistake ever. 550 ohm <> 550k ohm. I was simply using the wrong resistor.And since the resistor was waaaay too low, the timer kicked in so quickly that I thought it was immediate. Me = potato.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 28, 2014, 08:57:11 pm
content: oh see... it had to have been somethingglad you got it working.