what is the need of Monostable multivibrator in this circuit?

Thread Starter


Joined Feb 1, 2018
I am wondering what is the need of NE555 configured as mono stable multi vibrator in the below circuit? I am with an understanding that what ever the signal is being fed into NE555 has a output with larger time duration/ larger duty cycle. the same can be achieved with ESP-12E firmware, changing the PWM.
My idea is to remove Ne555 circuit and drive output of Pin3 PC 817(Zero crossing detector) to ESP-12E GPIO pins and then driving PWM to pin 1 of MOC3021.
Please explain me the need of NE555 here. I am with an understanding that even if we remove NE555 and change the PWM in firmware it should work in a similar way. Correct me if I am wrong.



Joined Jan 29, 2010
hi ashok,
Looking over that circuit, IMO it is over complicated.
I would remove the NE555 [ which is d/s rated at a min operating voltage of 5V, yet it shows a 3.3v supply]
Remove the Q2 MOSFET, R10 and C8.
Drive the MOC3021 opto pin #1, with a timed pulse from the ESP-12, via series resistor.
You will need the PC817 to provide a Zero Mains crossing pulse to the ESP-12.


Joined Aug 1, 2013
Not your normal monostable. Charging current for C5 is dependent on both the PWM duty cycle *and* the transconductance of Q2

What is the source of the schematic? Link?

Is this supposed to be a color organ?