I am just testing this component MOC3021 optoisolator, its datasheet can be found here.

This is my circuit in Proteus: opto-isolator circuit

Please forgive me for my ignorance of electronics, I have my background in computer science.

I tried the whole day yesterday trying to make out what's written about it in the datasheet, the point of connections was easy to understand, rest I don't know much. However, I did learn a lot and welcome to accept more.

However as you can see the LED didn't glow, now for one thing I know it's an AC optoisolator but shouldn't current flow from one of the diodes of the DIAC?

Thanks a lot :)

Update : Problem Solved

Thanks a lot, all of you it's a great community here ( especially for noobs like me ). This is the final working circuit.

Final Image


All answers were correct but I could only choose one, sorry others :|


3 Answers 3


You've got three problems.

1: no current limit on the MOC3021 LED. With 5V there the LED inside the MOC3021 will not last long, so add 220 ohms in series with the input.

2: not enough voltage on the output. The output is is a phototriac which has a forwards voltage drop of 1 to 1.5V - to light the blue LED you want 3V or so and a resistor to limit current, 9V with a 470 ohm resistor in series would be a good starting point.

3: the output is a triac: once triggered on, it will continue to conduct until something else stops the current - using an AC supply to power the output side circuit would help here.

  • 1
    \$\begingroup\$ You're right, I didn't even realize it was a triac output. \$\endgroup\$
    – dim
    Nov 2, 2018 at 8:32
  • \$\begingroup\$ Thanks Jasen, how did you calculate the resistance required to be 470 ohm, did you know the current required or the resistance of led and thus dividing the potential drop accordingly ? Is it mentioned in the datasheet ( sorry I don't yet know what all to make of the datasheet , still trying to figure out the important bits from those datasheets ) . Thanks :) \$\endgroup\$ Nov 2, 2018 at 14:21
  • \$\begingroup\$ I was lazy, precision isn't required. 470 will get you somewhere below 20mA from 9V into a short circuit there's less than 9V available because of the coupler and LED voltage drops so you get 5 to 12 mA somewhere and most LEDs will work pretty well with that. \$\endgroup\$
    – Jasen
    Nov 3, 2018 at 3:20

You have two small problems:

No resistor in series with the opto led. So the current is not limited, and will burn the opto if you try in real life

A voltage source too low to light a blue LED. Such LED typically require about 3 to 3.5V. You have 1.5V on this side which is far from sufficient. With a 100 ohm resistor, something like 4.5V would give something like 10-15mA.

  • 7
    \$\begingroup\$ And a third problem: the LED is backwards. \$\endgroup\$
    – Hearth
    Nov 2, 2018 at 12:20
  • \$\begingroup\$ Yup dim, I did burn a few optoisolators and only later did I get my hands on this software . Thanks :) \$\endgroup\$ Nov 2, 2018 at 14:34

In addition to what dim commented, Your voltage source BAT2 has the wrong polarity. The LED D2 is now in reverse bias by the voltage source BAT2. You could change polarity of either BAT2 of LED D2 in order to let current flow throug the LED when MOC3021 is enabled by the primary side.


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