I found this https://www.edn.com/isolated-circuit-monitors-ac-line/ circuit in my quest for a low cost isolated AC line voltage monitor and constructed it. On testing however, I am not getting the correct voltage as expected from the formula provided. Ve is supposed to be 48V but my calculation by rearranging the second formula to get Ve, is giving around 133V.
Is the formula wrong or have I made a mistake somewhere?
Additional information: I changed the values of R1 and R2 to 100k each as I found the 10k 0.5W resistors were getting quite hot and the 200k obtained should provide enough current to turn on the optocoupler. I used EL817 as I had a few and the diode is 1N4007. AC RMS voltage is 230V for which I got a low pulse width (Ttotal in the formula) of 0.0073s (it should be about 0.0091s if Ve is 48V). I used an STM8S microcontroller's input capture for measurement. Unfortunately I do not have a DSO to verify the pulse width. I did verify that the pulse width was changing using a variac and an old analog scope.
Also are there design errors in the circuit which should be corrected?