Controlling a negative voltage with Arduino GPIO

I feel this should be reasonably easy, but can't quite seem to figure it out. I'm trying to control an op-amp enable pin that's logic low at the negative rail (-15V) and logic high(enabled) ~2 volts above the negative rail. The +15/-15V power rails are there and accessible, but how do I use an Arduino GPIO to control the pin? TIA

Jon

Use a transistor for level shifting.

blckdmp: I feel this should be reasonably easy, but can't quite seem to figure it out. I'm trying to control an op-amp enable pin that's logic low at the negative rail (-15V) and logic high(enabled) ~2 volts above the negative rail. The +15/-15V power rails are there and accessible, but how do I use an Arduino GPIO to control the pin? TIA

Jon

Which opamp?

TI Opa1632.

That's standard TTL logic levels referenced to the negative rail. Its almost certainly designed to be used via an opto-isolator, which would be one way (so long as the isolator's output can cope with 30V).

You can also use a NPN transistor, emitter to -15V, but then you'd need a current source to drive that from the Arduino levels, which means a PNP as well. Optocoupler is simpler I think, you don't have to understand anything about current sourcing and level shifting that way, and more importantly you won't inject logic supply noise into your audio circuitry...

Thanks Mark, The optocoupler seems to make a great deal of sense. So how would that be hooked up to the Enable pin on the transistor end of the optocoupler? I'll take a crack here, (but unlikely I'll be right :)... Enable pin tied to the emitter pin of the optocoupler (-15V), and +15V through a high-value resistor on the Source pin of the optocoupler? Is that close? Other pull-up/down or current limiting resistors required? I always get confused when dealing with transistors and hi-Z inputs in terms of actual current flow.

cheers, Jon