I am attempting to make a code key practice unit.

Greg-

The circuits using a 555 may be the easiest to build, but here is another from
back in the 60s. Some of the parts were obtained from an old transistor radio
and mounted in its plastic case, which was mounted to a piece of wood that also
held the key.

Use an audio transformer with a center-tapped primary, with its secondary
connected to a speaker. Connect the top of the primary to an audio
transistor's collector. Connect the bottom through a parallel 100K Ohm
resistor and 0.02 MicroFarad capacitor, to the transistor's base.

The key is in series with a 9 volt battery, connected between the transistor's
emitter and the transformer's center tap. A 0.2 microfarad capacitor is
connected across the transformer's primary, and a 1 microfarad capacitor is
connected between the center tap and transistor's emitter.

If an NPN transistor is used, the battery positive terminal is connected to the
center tap, and vice versa for PNP. The 1 microfarad capacitor polarity should
match.

Parts values are not critical, and you may get better results or a more
pleasing tone by experimenting.

73, Fred, K4DII