light dimmer using optocoupler

Thread Starter


Joined Jan 3, 2010
hai all,

here i have some idea to create a AC light dimmer, if u could see from attachment bellow i want replace TRIAC and LDR with TRIAC opthocoupler that is (MOC3051M / TD3052 / TD3082). As i see TD3082 have zero volt switching and it different with MOC3051M and TD3052 which not have zero volt switching. So here i couldn't figure out what different output for each opthocoupler. Before this i have try using LDR opthocoupler (MOC3011) then give PWM signal to opthocoupler input which in pin 1 and 2, the problem.....the light not dimm but only like i switch on/off the light. I just guest that maybe MOC3011 have wide range of resistance (270k-15kohm) so it difficult to control and it not match with my range that is (25k-0 ohm), so to avoid this problem i want try to use TRIAC opthocoupler, but i did not have experience with TRIAC opthocoupler...................if u all have some experience using all the opthocoulper above i hope u all could share with me your experience......

i hope i could figure out:

1) what output TRIAC opthocoupler give. (MOC3051M/TD3052/TD3082)
- is it opthocoupler input could control the firing angle of TRIAC
2) posible or not i give PWM signal to opthocoupler input, if yes how to control it.
3) how could i control opthocoupler input..

* all precaution is in my responsibility *

thanks 4 reading..........:) ENJOY TROUBLESHOOTING!!!! :D



Joined Nov 6, 2005
You can use an instantaneous type triac optocoupler with a phase control circuit.

This is something you can experiment with at low voltage, using a small filament bulb on 12V or 24V AC from an isolating transformer. The forum rules do not allow experiments working on the AC mains voltage.

The lamp brightness is proportional to how far before the AC zero crossing you trigger the triac, as the triac only turns off again at the zero crossings.

It's a form of PWM in a way, but the timing must be exacly synchronised to the AC zero crossings.

Thread Starter


Joined Jan 3, 2010
ohooooo ic

so you mean that if i use triac opthocoupler, pwm must synchronized with AC. It seem very difficult to using pwm in controlling brightness of lamp. Is it true if i used any opthocoupler i need synchronized the pwm with AC????

if any design/circuit/idea to control the light brightness of lamp is also allowed......

thanks viewing......i happy sharing with u heeee....


Joined Jun 26, 2012
In fact while dealing with triac to control dimming we not at all use the pwm wave for example if we use a pwm wave say 20% duty cycle then you indeed on for first 2ms(10ms since we sync with ac i.e,50Hz) then because the triac will switch off again for the next zero crossing even though you make it off for the next 8ms it will be on as it is the basic property of the triac.......... so this will result in no variation in the brightness only the maximum voltage........

So in order to vary the brightness what you have to do is that !!!!!!!!!!

for example if you want 20% brightness take 10 ms(that is the positive half cycle) you make delay for 8ms (off for first 8ms) then trigger triac for 0.1ms( ie on for 0.1ms ) and again off it for the remaining time ........ so this will make the triac trigger @8ms and repeat this for the negative half cycle also so this will make your lamp to have 20% brightness...

I hope I am clear........


Joined Aug 7, 2008
'Could try a dusk to dawn light control & use a LED for control element, worked for me on 120V AC, 60 Hz. It keeps your fingers out of 240 V