Shouldn't they be in anti-parallel?

Partially! The transistors in parallel, the LEDs in antiparallel.

 

Can you post the sim output?

Here are the results on Falstad online simulator. I am using a borrowed PC so no access to LTspice or Proteus.

 

Why are there two 20K resistors in the circuits?

 

Here are the results on Falstad online simulator. I am using a borrowed PC so no access to LTspice or Proteus.

In LTspice:

ADDED:

Danko,
Can you run the sim again without R3 and set R1 at 131K please. V1 = 230vac

See:

 

In LTspice:

Danko,
Can you run the sim again without R3 and set R1 at 131K please. V1 = 230vac

 

Following circuit worked:

However, when the sensor gets off, there is a delay in switching off of the PC817.
Need to discharge C1 quickly. Alternatively, increase the reference voltage at pin #3 of LM358.

 

Need to discharge C1 quickly.

Could try a smaller value for C1 but I think a better solution is to use the PC814 (U3) on the pc board as Ian0 mentioned.
The PC814 is designed for AC voltage as it has two internal LEDs.

 

Could try a smaller value for C1 but I think a better solution is to use the PC814 (U3) on the pc board as Ian0 mentioned.
The PC814 is designed for AC voltage as it has two internal LEDs.
View attachment 299509

Means, use PC814 with only 3 resistors? C7 100nF to be removed?

 

Means, use PC814 with only 3 resistors? C7 100nF to be removed?

Yes, and leave C7 installed.

 

I suggested a capacitor instead of R1 to keep the heat dissipation down.

 

I suggested a capacitor instead of R1 to keep the heat dissipation down.

Agree but want to see if this circuit will work before trying the cap.

 

Agree but want to see if this circuit will work before trying the cap.

Fair enough.

 

You have already got a convenient AC input optoisolator (PC814) and that allows you to eliminate the rectifier.. I had to simulate it with two PC817 in parallel,
The 150k in series with the source in the second circuit simulates the leakage resistance.
You can use the existing 1k 100nF network. The external capacitor must be a class X (but it doesn't dissipate any power unlike @sghioto 's 20k resistor which needs to be 3W) and the external 1k needs to be 1Watt.
Top trace is with proximity not detected, bottom trace is with it activated.
View attachment 299431

Sorry guys for responding late.
This simple configuration worked perfectly.

Thank you everyone for his inputs.
Kind regards.
Abu-Hafss

 

Hello Guys

Since, August 10, 2023 the following interface circuit has been working fine. But now, all of sudden (without any change in the operating conditions) the circuit is not performing. The output of LM358 is continuously high, as if the sensor is continuously ON.
Here is the interface circuit:

And here is photo of my complete set-up.
This whole set-up is usually enclosed in a metallic console box. The interface circuit is on the small veroboard. I had been using 2 separate SMPS for 24V and 6V but I replaced them with linear ones assuming that the SMPS might be causing some interference.

The Arduino Nano on the main PCB is programmed to run an AC servo upon receiving signal from the sensor. For last 5 months, the circuit offered me peace of mind but now severe frustration. I just failed to diagnose the problem. The scenario is that the sensor is continuously ON which runs the servo continuously.

Very interestingly, I have another exactly same set-up which is running without any problem. Both these units are placed just 6 feet apart. I interchanged one-by-one, the main PCB, the Nano, the interface circuit; all are worked fine in the set-up B. And the same parts from B installed in this set-up (A) would not solve the problem.

Any suggestions???

 

the following interface circuit has been working fine.

Shouldn't there be a 20kΩ resistor in series with the sensor?

 

Shouldn't there be a 20kΩ resistor in series with the sensor?

I'm sorry, I missed the 330nF cap in series with the sensor as suggested in post #57. I will post the correct schematic shortly.

 

I interchanged one-by-one, the main PCB, the Nano, the interface circuit; all are worked fine in the set-up B. And the same parts from B installed in this set-up (A) would not solve the problem.
Any suggestions???

Interchange sensors.

 

Here is the correct schematic. The interface circuitry is enclosed in red box.

 

Interchange sensors.

Yes, already tried new sensors but no change.

 

In this test circuit, the mains voltage are 225VAC. The voltage at A-B is 1.31VAC when sensor is ON and 0.87VAC when sensor is OFF. When the sensor is OFF, the LED glows very faintly and the voltage across the output of the opto coupler is 3.65V.

Another test circuit, the voltage across 22K is 216VAC when sensor is ON and 22VAC when OFF.