The simplest way is probably to just use a PIC, assuming you already have a 5V supply this could be as simple as a PIC16F628 and a single resistor!.
One of my RS232 PIC tutorials does pretty well just what you want, they actually use a MAX232 for the RS232 interface, but a simple resistor can be used instead - particularly for just one way.
If you wanted to use a UART, the old 6402 was commonly used for these sorts of purposes - but it's considerably more complex than a PIC solution.