The door bell I have is from ebay (~$5, 100-meter range). There are some advantages using wireless door bell as sending/receiving unit:
A. Low cost and easy to find.
B. It has pretty good transmitting range.
C. The frequency of transmitter and receiver are preset, no adjustment is needed.
D. Low interference.
Just for fun, I make a telegraph key using a wooden spring type clothes clip, two M3 screw nuts, one M3X8mm screw and one M3x12mm screw, two M3 washers, a couple pieces of hook up wire, some double-sided tapes, and a piece of plastic board. (picture 1)
Pull the clothes clip apart, then place two wooden pegs together and dill a hole. The hole should be just a little bigger than the screw you use. Use a smaller drill bit to drill a hole near the corner of the plastic board, and use M3 screw tap to tap the hole. (picture 2 and picture 3)
Put a small piece of double-sided tape on one wooden peg. Bond the peg on the plastic board. Overlap the hole on the peg and hole on the board. Use the screw to clamp down a piece of hook up wire. (picture 4 , 5, and 6)
Similarly, clamp down another piece of hook up wire on the other peg with screw and nut. (picture 7)
Put the clothes clip back together. Now the telegraph key is finished. It is acting like a momentary push switch. (picture 8)
Open the back cover of the transmitter. Take out the circuit board. Drill a small hole on the side of the enclosure for the telegraph key and buzzer wires. (picture 9 &10 )
The transmitter is using 12VDC. I use RadioShack (073-259) 76dB Piezo Buzzer to add sound effect. Locate the power switch pins on the transmitter PCB, solder the telegraph key wires parallel to the power switch pins. Also, solder the buzzer wires parallel to the transmitter circuit after the power switch. (picture 11 &12)
Use double-sided tape to bond the transmitter and the buzzer on the plastic board. Now the modified transmitter is finished. Don’t forget to install the battery. (picture 13 &14)