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I've spend days debugging this circuit and have exhausted all possible solutions. I'm trying to use an Arduino PWM pin to light a 120V AC lightbulb. In order to do this I'm using a MOC3010 opto-isolator and a BT136 Triac. The goal is to be able to fade up and down a 120v light.

Below is my circuit (Vcc is 5v arduino pin):

a busy cat

I'm certain I soldered everything correctly. I'm certain the components are working (tested with multimeter). The issue is getting the photocell within the opto-isolator to trigger the gate of the Triac. Currently when I plug this circuit in - the light doesn't turn on.

One weird thing is that when I touch the line between pin 4 and the gate with a multimeter- sometimes the light flickers. I'm thinking that the voltage difference isn't correct and that my probe is inducing a difference that allows the gate to be triggered for a very very brief moment.

Has anyone been able to make this circuit work before? Is the 180 ohm resistor too small/big for my appllication?

This simple circuit has now become the hardest circuit I've ever worked on... Please help!

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  • \$\begingroup\$ What does your zero-crossing detector look like? \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 19 '13 at 5:07
  • \$\begingroup\$ there's no zero-crossing detector. I know it's more stable to have one, but from my understanding the circuit should still work without one. \$\endgroup\$ – user34365 Dec 19 '13 at 5:41
  • \$\begingroup\$ What is the source of your circuit diagram? You can't fade up and down the light without a zero cross reference. Specifically which opto are you using? \$\endgroup\$ – Andy aka Dec 19 '13 at 8:14
  • \$\begingroup\$ My circuit diagram is from the chip MOC3010 (figure 9 - pdf.datasheetcatalog.com/datasheet/QT/MOC3012.pdf). hmm...i thought i could fade it up and down without a zero cross reference... \$\endgroup\$ – user34365 Dec 20 '13 at 4:46
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Connect a 470 ohm resistor as shown, and the problem of flickering when pin 4 is touched will be solved:

solution of flicker when pin 4 is touched

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  • \$\begingroup\$ I've changed the formatting a little, but you could improve the answer if you explain how it solves it. \$\endgroup\$ – PeterJ Nov 15 '14 at 11:35
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I'm writing this down here because I was unable to comment on the question directly(due to reputation).

The fade that you're trying to achieve is not possible with this circuit or with whatever approach you are trying to follow here.

Reason: MOC30XX is an opto-isolator. It consists basically of a Gallium-Arsenide-Diode Infrared Source and Optically-Coupled Silicon Triac Driver. See here

The led will trigger the Triac driver inside, at whatever threshold it is supposed to trigger. Let's say supplying 2.5V to LED will bright it up sufficiently to trigger the Triac, then below 2.5V the output from MOC20XX will be default. As you rise the voltage from 0-2.5V, you'll see nothing happening to your 120v light. As the voltage crosses 2.5V your 120v light should light up(here I suppose the issue is with the 180 ohm resistor, change it to 10K and try).

I hope you got the point that you cannot dim using this circuit. I suggest looking out 'more' on the internet might help you.

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  • \$\begingroup\$ Read about PWM. \$\endgroup\$ – kelin Feb 18 '16 at 19:42

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