Will that help? I'm going off the top of my head but you'd have to use both outputs and invert the signal going into one of the inputs to get the voltage swing across the piezo (assuming using one output pin on the PIC)? You'd only get about 14VAC? What is the capactive loading spec of the 232 device; can it drive a cap load (piezo's are normally a "relatively speaking" large capacitive load).

Have a look at this website: http://www.discovercircuits.com/P/piezo.htm. There are a couple of circuits shown. All you really need is a small inductor, a transistor and the piezo. The PIC is your oscillator. I'd have to think this is cheaper and easier than a MAX232?

Keep in mind the resonant frequency of the device as well. Thinking back, one of the problems that the car alarms I worked on had when I was brought in was the guy before me was trying to brute force the piezo to get the output needed (+120db @ 3 feet). It was at around +97db. Tweaking the frequency made it jump to over +120db. The other thing that is more important than you'd think is in the way the piezo is held. I've done edge fixture mostly; the tighter this is clamped the better for output.

I invented the "Singing Balloon"; this uses a piezo attached to a "foam speaker". It is edge mounted as well and driven with PWM from the sound chip. The output is around 10VAC to the piezo (in this case it reproduces music). The piezo *really* needed about 30VAC but the product (disposable in every sense of the word) couldn't stand the cost of the added circuitry to make it happen. I think to date there's been about 30-35 million of them produced.

Mike Tripoli