Optocoupler with common ground?

[haxan7]

I recently saw a circuit using an optocoupler with a common ground.
Is there any reason you would want to use an optocoupler this way?
To me the circuit the circuit looked pointless, same functionality could be achieved using a single transistor.
Am I missing something here?

 

[crutschow]

It does seem like there would be no use for a optocoupler with a common ground. Sometimes they are used when someone wants "isolation" not realizing that such a connection does not achieve the desired result. Can you post the circuit?

 

[haxan7]

I don't have the circuit, I saw the design from a made PCB.
The cathode of the led in the optocoupler was shorted with the ground of the micro-controller, and the emitter of the transistor was tied to an I/O pin.

 

[D.A.(Tony)Stewart]

pointless, loss of common mode rejection and HV isolation.

 

[schmitt trigger]

Engineers or designers are people. People make mistakes. This is clearly one of them.

An optocoupler, specially the cheaper ones, are not very good at transmitting information. Its current gain most times is below unity. The frequency response sucks. Actual output drive is limited.

The only reason one puts up with the limitations is because of isolation. But if you have common grounds, it is absolutely pointless.