I ran across some code for turning a raspberry pi into a wspr transmitter with nothing but the GPIO pins (and a filter). Here is the code repository.
I loaded up a raspberry pi with raspbian lite and compiled the WsprryPi code. From there, it was just a matter of adding a filter between the GPIO pins and the antenna.
I fired up the software and let it transmit for about a half hour or so and here is the result.
The raspberry pi was putting out about 10 mW into my 40 meter dipole and I was getting pretty good range with it.