Step 1: What You Need
A vintage Mr. Microphone FM modulator. These were designed to transmit your voice over the low end of the FM band, around 88MHz. These are not easy to come by but you can find them at garage sales and occasionally on Ebay.
Step 2: Other Required Items....
To do it right, you need to replace the existing microphone with a better quality one. I used an Audio Technica lavalier microphone. YOu will also need a button cell 3V battery, some magnet wire and a container of your choice to mount everything in.
Step 3: Step by Step
1. Remove the windscreen and remove the one screw that holds Mr. Microphone together. Once apart, remove the circuit board, microphone, battery connection wiring and the antenna.
Step 4: Remove the Old Stuff
De-solder the connections to the microphone and antenna.
Step 5: Solder the New Stuff in Place
Solder the new microphone to the existing connections. Using magnet wire, make a coil antenna by wrapping it around a bolt. NOTE: Be sure to measure the length of the Mr. Microphone antenna and make sure the magnet wire is that exact length. Solder the end of the coil antenna to the existing antenna connection. Test it by turning it on and adjusting the frequency on your FM radio and on the small potentiometer on the circuit board.
Step 6: Modified Power Source
To save space, the 2 AA batteries have to be changed out to one 3V button cell battery. The run time will be less but it will power up the circuit and run fine for our needs. Just use tape (NEVER SOLDER) to make the positive(+) and negative(-) connections on the battery.
Step 7: Pick a Container
I chose Air Wick Stick Ups because they are the perfect size, come apart easily, unscrew open so the mic can "hear" and are ready to be stuck to just about anything. Behind furniture, under a table or chair...use your imagination. Take the newly modified Mr. Microphone circuit and conceal it in the container. You can use your car FM radio, an MP3 player that has FM and recording ability (great if you want to record the conversation) or any FM radio. Have fun and always use this responsibly.