Here is my homemade function generator that can produce a square, triangle, and sign wave. This is all done with a few bucks worth of parts. The heart of the function generator is a LM324N quad OpAmp. I got the schematic off of a site where they were also selling kits to build the signal generator.
Here is a link to the site where you can purchase the kit or just download the user manual complete with schematic.
Edit: the schematic may be gone now as the kit is discontinued. Here is a direct link for the PDF.
The box I put the generator in is an old radio transcever box that already had the BNC Bulkhead attached, this was good for me as I don’t have any BNC parts laying around. I took the old BNC male RF pigtail, cut the end of, and soldered on some shielded wire and some leads.
Parts include some resistors, two capacitors, four diodes, an LM324N IC, 1 Meg pot, LED and two switches.
The thing works amazingly well for how simple it is and how quickly I put it together! It is by no means a high quality machine but it works for the quality of work I am used to.