optoisolator +ve polarity in - negative polarity out

This is an amateur application and I am just starting out with optocouplers (and electronics in general).

The optocoupler calculations in the circuit below were mirrored from an online tutorial, except the use of Vcc -5V. I am not completely sure they are correct for the application.

However, I cannot find a specific answer to my question...

Will the circuit below work in the configuration shown. Primarily, Vdd +5V and Vcc -5V, theoretically producing an output of ~-5V? (Though the circuit would be non-inverting if both sides were +ve polarity).

Is this a correct application of an optocoupler circuit, to separate sources of different polarity. It seems like an ideal application for the device.

The thought comes to mind, that Vcc could / should be connected to +ve ground. Guessing that the output would then need to be configured as an inverting optocoupler.

simulate this circuit – Schematic created using CircuitLab

Almost. You can indeed use a optocoupler but the photo-transistor is being reverse biased here, instead of forward-biased. The polarity of the photo-transistor side needs to be turned upside down.

So if you swap over the two connections (collector and emitter) from your photo-transistor, it'll work. This is shown below.

simulate this circuit – Schematic created using CircuitLab

That's assuming that your generic opto-coupler has a photo-transistor that can handle the load current and that the diode current is correct, but that's not your question here.

• Thanks. That's making sense. Just need to decide on a type of optocoupler. – George Cheshire Apr 7 '17 at 12:59
• @Olin Lathrop. The transistor in the common base application is interesting. I will run it in a simulator. – George Cheshire Apr 7 '17 at 13:02

It's not clear what polarity you want. If you want inverting, then your circuit would work if you flipped the collector and emitter of the transistor. With 0 V in you get 0 V out. With +5 V in, you get -5 V out.

If you want non-inverting, then reverse the transistor and resistor (after flipping the transistor to the right C-E polarity).

However, you don't need to use a opto-isolator at all since both sides are connected to the same ground. For a non-inverting level shifter, you can use a single transistor:

This is using the transistor in the common base configuration.

If you really do want inverting, then it takes two transistor, although there are then various options. Here are just two of them: