Far as I can tell, opto coupler's are either on or off? Is this correct? I guess there are also Opto Amplifiers which are something else?

Thank You

I was asking mysef this a couple of days ago.

If I meassure the resistence of the output of the optocoupler, I can see a difference when applying 12v with different resistors on the input. So my conclution if this is, yes, it can be something in between.

You need to look at the CTR (Current Transfer Ratio) of your specific opto-coupler.

Here is a bit of reading http://www.renesas.eu/products/opto/technology/ctr/

// Per.

This depends on the opto-coupler. There are hundreds of models, some designed for digital applications, some for analogue. In general analogue opto-couplers are not super linear and critical applications will generally digitize somehow and use a digital encoding to send data over an opto-coupler. Something like PWM or PPM is common.

Looking specifically at the datasheet for the MOC3010M, this is a triac output part. I think it's going to be basically on on-off affair.

Good, that is what I'm looking for. Thanks for the link Zapro. I will check it out.