voltmeter circuit using BL0937

[samirahmed]

hi, I am designing circuit to measure ac voltage only of any source I am using BL0937 and add random load (capacitor 470nf) to Just complete circuit.
the output of pin CF1 is 1.8V but after I add transistor to turn on optocoupler or without transistor when I direct connect CF1 pin to optocoupler the voltage of CF1 pin drop to 0.2v so nothing work can you help me.
the output must be pulsed to measure the voltage using input capture. 

[Kleinstein]

The capacitor at the supply is rather small with 20 nF. There is a slight chance the supply could collaps under the load from the optocoupler. The current to the optocoupler is quite high - usually one could get away with well less current (more like 5-10 mA) and thus more like 220 or 330 ohm instead of 88 ohm.
Is the supply voltage correct ?

The polarity of the pulses could also be an issue. A relatively slow optocoupler that is highly overdriven may take more than 40 µs to turn off and may thus hide such a pulse.

[Terry Bites]

How much Zener current is that? 1.7A*3.3 is 5W, that'll melt your poor diode.
I doubt the application circuit for the BL0937 would meet any regulatory standard.
Don't skimp on mains supply circuits, your life and your home deserve a few bucks more effort.
Make a better protected supply that's less likely to catch fire. Check out this page from TDK, great advice.
www.tdk-electronics.tdk.com/en/373562/tech-library/articles/applications-cases/applications-cases/everything-for-capacitive-power-supplies-from-a-single-source/1380514

If you need to change pulse polarity do it on the LV side.