That's what I want to accomplish with the upper 555 timer. To synchronize it with the mains AC so that I can clip the waveform better and hence dim the light. In other words. Zero cross detected --> turn on 555 ---> 555 turns on the photo coupler which controls the switching on and off of the triac

It seems ill-advised to add the 555 to the mix here. It doesn't contribute anything that the usual dimming circuits don't already do, and it's a relatively finicky piece of hardware.

But in order for a triac to conduct, do we not need a current(control current) applied at the gate terminal??

@ZiphoLunika - to start conducting, yes... the gate is irrelevant when you want to stop conducting.

but what if the gate is connect to an opt coupler? So that if the opt coupler LED is off, there is no current that enters the triac gate?

Okay, maybe I should show you the circuit. The program seems to show it turning on and off. Very strange

Check this out: ziphoblogbucket.s3.eu-north-1.amazonaws.com/triac.JPG

I'm unsure what you are trying to show me that isn't covered in my answer.

If you look at the circuit that I just showed you. Do you think the AC circuit will turn on and off according to the 555 timers' on/off state?

Either the simulation is flawed, or you may have it configured wrong... What is the AC frequency in your model, and what time period is the simulation / step running at? If the simulation doesn't give you a good number of frames for each AC cycle, then it's possible it will appear to "work", when it won't in reality.

Perhaps its the simulation flawed. I have gotten strange results. the main point is ultimately to achieve 60% dimming. thats the ultimate goal. Perhaps I should think of another solution

@ZiphoLunika you said: Do you think the AC circuit will turn on and off according to the 555 timers' on/off state? and my answer is a categorical no.

Dude. What happens if the triac's gate current goes below the "holding current?" Ask chatgpt

Put ChatGPT in the bin... en.wikipedia.org/wiki/TRIAC#Gate_threshold_current,latching_‌​current,_and_holding‌​_current "_Holding current is the minimum required current flowing between the two main terminals that keeps the device on after it has achieved commutation in every part of its internal structure"

Let me quote chatgpt with my question included: My question: "Suppose a control current is applied to a triac's gate allowing it to conduct. Now what happens if the current at the gate drops below the triac's holding current?" chatgpt - "...depending on the specific circuit conditions: Triac turns off: If the current at the gate drops below the holding current, the triac may turn off if the load current also drops below its holding current. This situation occurs because the triac requires a minimum holding current to remain in the conducting state once it's triggered."

@ZiphoLunika you came here to get advice. Good advice has been given. Your use of chatgpt is the problem. You asked about holding current and that is not a parameter associated with the control current. So, although chatgpt is a PoS for most electronics questions, you have compounded your misinterpretation by using incorrect terms. If we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification.

"Now what happens if the current at the gate drops below the triac's holding current" ... holding current is not a related to the gate. Your question is malformed, and ChatGPT can't deal with it appropriately - put it in the bin. Read my quote from Wikipedia.

@ZiphoLunika my usage of PoS means piece of slime.

I got your point. Read the wiki. The simulation misled me. It seemed to work.

But then again these simulation might work for LEDs when in reality, there would be too much current and the LED should blow up. The simulations show the LED is still on...so there's that. So that's the problem probably

So the lesson here is to educate myself on triac theory 101.

BTW andy. Do you think a gate turn-off thyristor would be a better replacement in this case(en.wikipedia.org/wiki/Gate_turn-off_thyristor). I'm just gonna scrap this idea and start over

I'm no expert on this type of device. If wanting to turn on and turn off irrespective of the zero crosses, consider using a solid-state relay or use a regular MOSFET "inside" a bridge rectifier (optically driven by an opto-voltaic coupler).