# A problem regarding connecting the grounds within two seperate circuits

Figure 1

I am trying to design the above circuit for my project to create zero crossing detector as shown in the above figure. This configuration was proposed originally by the member named "Transistor" in this community.

But the problem is I cannot connect the grounds at the Point A and Point B as shown in the above diagram due to the nature of my full circuitry.

My full circuit is shown below

Figure 2

simulate this circuit – Schematic created using CircuitLab

1) The red squared portion is a different section of my full circuit and its functionality does not need to be understood and it has nothing to with my zero crossing detector circuit. But it plays a significant role in preventing me from implementing the circuit shown in the previous image

2) I have a +12V and -12V power supply with the common point at the middle acting as ground. (POINT B)

3) My AC source is 3V 50Hz providing AC signals to the red squared portion of the circuit. But I am also using that particular AC source to supply power to my rectifier circuit as shown in the above image.

4) I have connected my AC source to the common point of the two +-12V supply (I have to do it to provide reference)

5) Now comes the problem, please compare the figures 1 and 2.

• In figure 1 , my point A has been connected to the point B (common point of the two +-12V sources).

• But in figure 2, point A cannot be connected to the point B since
point C has already been connected to point B to provide a return path for
the AC current.

How can I change my circuitry in figure 2 so that the results in figure 1 can be achieved?

• One option is to use a long-tailed pair to generate differential signals then feed them to the rectifier just like in the 1st image. Another and the most simplest (at least to me) is to put an opamp-based rectifier instead of a buffer like in the 2nd image. – Rohat Kılıç Jul 4 '17 at 6:44