How to get 3.3V output from Vishay 6n137 high speed Opto Coupler?
As the datasheet says, the device has an open-drain NMOS output.

This means the output of the device looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Which means the output cannot drive itself to a voltage, but can pull the signal down to ground. It does have a small internal leakage of up to 1&mu;A so if it goes into a very high impedance input, even without a pull-up resistor it will eventually float up to a voltage between GND and Vcc.

But, since the device cannot drive a voltage on its output directly, you are free to pull-up to any voltage you require, just keep an eye on the fact that the output will only go to 0.3V to 0.6V if the resistor is small (hundreds of Ohms), so for very low voltage signals, like 1.2V that may not be low enough to be considered "off".

The value of your resistor will depend on how fast you want your signal to go back up. If you have fast signals, in the MHz range, or need fast flanks, you'll probably want less than one kilo Ohm. For slower stuff and slower edges, say 10~100kHz range or even lower, probably 3.3k to 5.1k will work fine. But it does depend on the amount of trace or wire connected to the output as well. Bigger trace makes it slower, because those have more capacitance.