Before I discover DIA, I used Microsoft Visio for schematic drawing to be included into my documents. The schematic drawn has to be exported into a graphic file, in which I prefer .png format. Then only it can be included in .tex document as a graphic.
Then I found DIA. You can search the web for more info on the software itself. In short, it is an open source software Lyx as well. What I found to be helpful is that, DIA has an extension to convert .dia files, where you save the schematic drawing, into graphic files in the format of .png which can be called directly by Lyx.
What I have to do is, draw whatever schematic that I want, save it in .dia format. Then in Lyx, insert the graphic as usual. At the graphic file name field, insert the .dia file name. Lyx will treat that as the graphic file where you can resize etc. similar to any graphic files included using the method.
Another setting to be done is that to let Lyx know where to fine the DIA extension to convert .dia file to .png file so that Lyx can call it upon generating final document. The setting can be found under menu Tools > Preferences and under File Handling > Converters tab. Under From format pulldown menu, search for DIA and under To format pulldown menu, search for PNG. In Converter field, insert this
dia -e $$o -t png $$i
and click Add button. That is it. Now you can use .dia file directly to include graphic into you document. Happy trying. See you soon.