Opto Isolators in Series

In the attached image, Diagram A shows a working circuit of a Spark Fun Opto Isolator break out board using 1 power source for both input and output. Diagram B shows a working circuit of a Opto Isolator board using 2 power sources, one for input and one for output. Diagram C shows a NON-working circuit of a Opto Isolator board using one power source for both input and output for the first stage and a Opto Isolator using the output of the first stage and separate power source for the second stage output. Why does this circuit not work?

Well, I can’t attach the file so this may be difficult to interpret. Basically 1 Opto which has common power for input and output. This feeds the second Opto which has its own power on the output side. Separately each circuit works, in series they do not. I would rather not get distracted on why I would want to do such a thing before understanding why the output of an Opto Isolator can not be used as the input for another Opto Isolator…

If this can’t be understood without the attachment, maybe an Admin can help me out with privileges to attach the jpg

Thanks

Since you were a first time poster, you may have been prevented from attaching files. Try again now that I’ve approved your first post.

-Bill

Thanks Bill!

Here is the Diagram of the 3 circuits and a recap of the issue…

Diagram A shows a working circuit of a Spark Fun Opto Isolator break out board using 1 power source for both input and output. Diagram B shows a working circuit of a Opto Isolator board using 2 power sources, one for input and one for output. Diagram C shows a NON-working circuit of a Opto Isolator board using one power source for both input and output for the first stage and a Opto Isolator using the output of the first stage and separate power source for the second stage output. Why does this circuit not work? I understand the Opto Isolator board has 2 circuits and is simplified in the diagram and there may be no conceivable reason to use the circuit in Diagram C, but that still does not explain why it doesn’t work.

I’ve checked voltages and connections many times. Input a signal to circuit in diagram A and you get an output signal. Input a signal to circuit in diagram B and you get an output signal. Add 2 jumpers to connect the output of A to input of B, input a signal to A and you get no output signal from B. Any suggestions would be greatly appreciated…

If I follow the schematic of the board ([<LINK_TEXT text=“https://www.sparkfun.com/datasheets/Bre … or-v12.pdf”>https://www.sparkfun.com/datasheets/BreakoutBoards/Optoisolator-v12.pdf</LINK_TEXT>), I think you’re connecting e.g. JP1 / pin4 to JP2 / pin3.

If so, the 10K pullup-resistor on the collector of Q2 limits the current that can flow through the LED in the second optocoupler to roughly 0.5mA. So the LED in the optocoupler will never give enough light to get the transistor in the optocoupler into a state where it conducts and hence you will never get a change on the output of the second optocoupler; it will always be high.](https://www.sparkfun.com/datasheets/BreakoutBoards/Optoisolator-v12.pdf)

Thank you very much sterretje

I added a 2n3904 transister between the opto isolator boards and it now passes the signal…