Hi

Here is the schematic on which I am working:

Without the sensor, the circuit works fine with direct 220VAC. However, when sensor is introduced the opto-coupler remains ON even when the sensor is off.

Diodes 1N4007
4 x 150k resistors in parallel so resistor value is 37k.
Coupler is PC817.
Sensor is 2-wire inductive. I also tried photo sensor with same results.
(Please ignore the GND sign in the schematic).

 

Presumably the sensor (or its mounting) provides a conduction path even when it is 'off'. Can you post a link to the sensor?

 

If it is a two-wire sensor then it must provide a conduction path when off, otherwise, where would it get its own power from?

 

This one is NC.


This is Normally Open (I am using this type).

 

Additional information:

If I connect an LED + resistor + 9V battery at the output of the coupler, the LED lights brightly when the sensor is ON and gets dimmed slightly when the sensor is OFF.

 

View attachment 299214

This is Normally Open (I am using this type).

Really? It says DC 6-32V on it and you have it connected across the mains.

 

Really? It says DC 6-32V on it and you have it connected across the mains.

Extremely sorry, I just picked up photo from the net. Here is my actual sensor --- it is actually for 220VAC.

 

Since the 2 wires of the sensor have no polarity, they can be connected either way.
The lower-right is the 1st pin of the opto coupler.
The LED is disabled.

I have tried different AC sensors, all have same results.

 

Look in the datasheet for the sensor, and find out what the quiescent current is.

 

From the brochure from Rico it says a maximum residual voltage of 10 volts.
Might try increasing the value of the 37K resistor.

 

I found this information.

I wonder if really need that bulky resistor for my circuit.

 

What is the voltage across the zener diode when the sensor is OFF?

 

From the brochure from Rico it says a maximum residual voltage of 10 volts.
Might try increasing the value of the 37K resistor.

I increased to 2 x 330K = 165K, no change.
In fact, the test load LED is more brighter when sensor is off. Difficult to visualize the change in LED when sensor is OFF/ON.

 

Did you measure the voltage across the zener when the sensor is OFF?
Show the schematic of how the LED is connected to the PC817.

 

What is the voltage across the zener diode when the sensor is OFF?

Sensor OFF => 5.05V
Sensor ON => 5.33V

These values are based on new resistor 165K.

 

Try connecting the sensor like this:

 

Already did that, no change.

 

I have 10k/2W resistor. I am considering to use it as bleeder resistor.

 

Sensor ON => 217VAC
Sensor OFF => 170VAC

That means 10K/2W won't be sufficient.

 

If you get the quiescent current from the datasheet, you will be able to work out the value you need, instead of faffing about substituting values until you randomly might get it to work.