I regularly practice Morse with a friend on 2m NFM by holding our tone generators, which we also use for personal practice, close to the microphone. We send text by CW, read it back, and discuss how we are doing and how can improve our "fists". However we have to turn the volume up on the tone generator so loud it is annoying to others in the house.
We would like to build a circuit that allows us to inject the tone directly into the microphone lead, and use earphones to listen to both CW and voice. I have seen circuits for this but they all include a tone generator, we would like to keep that separate.
Please has anyone advice on how to go about this, the design parameters to work to, and what pitfalls there might be?