how to measure the voltage across dc side

hi in my circuit i have to measure the instantaneous voltage across string of LEDs and that voltage has to be given as input to the PIC micro controller.As i am directly giving the 230V A.C suppply across LEDs it is becoming a problem for me.Please give a solution for me

I'm picturing that you have a string of enough LED's that can survive at 230V. Or rather the peak V which is 230 x 1.414 = 325 V DC.

An led operates over a range of several tenths of a volt.

Say it's rated 3.6 V at 20 mA.
It may conduct 1 uA at 3.1 V.
1 mA at 3.5 V.
100 mA at 3.7 V.

Maybe you have a string of 90 led's. It will operate over a range of a several volts.
If one led conducts, they all conduct.
So the string may conduct 1 uA at 279 VDC.
1 mA at 315 V DC.
20 mA at 324 V DC.

These would be instantaneous values at several points near the peak of your AC sinewave.

Are you applying 230 VAC directly? Then the instantaneous V is according to the sine curve.

Or did you put a dropping resistor/capacitor inline with the led's? Then the calculation takes a little more effort.