In this fun and easy project (which originally appeared in the Spy Tech edition of Make Magazine), you’ll connect a shirt-pocket “amplified listener” hearing aid with an in-car FM transmitter to create a wireless bug. Then you’ll tuck them inside a hollowed-out book with the microphone (mic) concealed by the dust cover, and you’ll have a covert listening device that you can leave lying around or on a shelf near a surveillance target. Then you (or an undercover agent) in the next room can eavesdrop on any devious plotting through an ordinary FM radio.
Step #1: Testing, One, Two, Three!Next
- Let's launch this mission. First, make sure you have all the required components; A - Hollow hardcover book with dust jacket, 1/2" dia hole drilled in spine; B - Miniature “amplified listener”; C - Miniature wireless FM transmitter; D - Miniature FM receiver with ear buds; E - AAA battery holder; F - 12” length of red hookup wire, 22 guage, stranded; G - 12” length of black hookup wire; H - heat-shrink tubing; I - hook-and-loop fasteners; J - 1/8" stereo phone plug (male); K - 1/8" stereo phone jack (female); L - 10 Ohm, 1/8 Watt resistor. You'll need (5) AAA batteries, as well.
- Next, we'll test everything that can be tested. Plug the headphones into the listener and turn it on. You should hear your surroundings like a bat.
- Next, plug the headphones into the FM receiver and tune it to a static-only spot on the dial.
- If you have an MP3 or audio cassette player, plug it into the FM transmitter. Turn the transmitter on and set the MP3 player to “play”. Tune the transmitter to match the receiver, and you should be able to hear the music of the MP3 player through the receiver. The signal should be strong, but it still might have some static.
- Test the components individually and together like this after each step of the assembly process. Delicate wires can snap and tiny switches can get switched at any stage, making troubleshooting difficult.
Step #2: Make the Interface CableNext
- In many cases, the amplified listener and the FM transmitter should be able to work together without any interface cable. In the case of the “Listen Up” amplified listener used in this project, an interface cable is needed.
- Locate the 1/8" stereo phone plug (male) and the 1/8" stereo phone jack (female). There are three solder terminals on each. These correspond to the parts of the plug and the jack that make the electrical connection that carry the audio signal. Identify the solder terminals of stereo phone plug (male) labeled “tip”, “ring” and “sleeve”. The solder terminals are the same for the stereo phone jack (female).
Step #3: Next
- Align the 1/8" stereo phone plug (male) and the 1/8" stereo phone jack (female) as they’re picture here. Notice the location of the tip, ring and sleeve solder terminals for each.
Step #4: Next
- Fold the 12” length of red hookup wire in half and cut with the wire cutters at the mid-point to create two 6-inch lengths. Do the same with the 12” length of black hookup wire. Strip ½” of plastic insulation off of both ends of all four of the hookup wires. Twist the metal conductors at each end so they’re compact and straight. Set one red and one black wire aside for later in the project.
- Slip one end of the red hookup wire through the hole in the stereo phone jack “tip” solder terminal and into the “ring” solder terminal and solder to both terminals as shown. Trim any execees bare hookup wire protruding from the ring solder terminal, leaving about 1/16”. Slip a length of heat-shrink tubing over the red wire. Solder the other end of the red hookup wire to the stereo phone plug “tip” terminal. Slip the heat-shrink tubing over the stereo phone plug “tip” terminal and shrink snugly over the solder joint. (You can see this in another view.)
Step #5: Next
- Slip one end of the black hookup wire through the hole in the stereo phone jack “sleeve” terminal and solder in place. Crimp the metal strain reliefs on the sleeve terminal around the black wire. Slip a length of heat-shrink tubing over the black wire and shrink snugly over the solder joint.
Step #6: Next
- Slip the other end of the black hookup wire into the hole in stereo phone plug “sleeve” terminal. Place one of the metal leads of the resistor over the black wire and crimp the metal strain reliefs on the sleeve terminal around both the resistor lead and the black wire. Solder both to the sleeve terminal.
- Slip the other resistor lead through the hole in the stereo phone jack “ring” terminal and solder in place.
Step #7: Next
- Here's the finished interface cable.
Step #8: Gut the Miniature “Amplified Listener” Circuit Board From its Plastic ShellNext
- Before we shed the extra bulk by taking out the listener’s guts, note the location and operation of the on/off/volume control wheel.
- Locate and remove the battery compartment cover of the miniature “amplified listener”. It should pop off easily. Take note of the proper positioning of the battery; the flat or “negative” end of the battery (-) is pressed against the cone-shaped wire coil connector; the pointy or “positive” end of the battery (+) is pressed against the flat-wound wire coil connector.
- Locate the (2) miniature Phillips head screws that hold the plastic shell to the circuit board and remove them. Remove the plastic trim that resembles the battery compartment cover. It should also come off easily. Locate and remove the (2) additional Phillips head screws. Remove half of the plastic shell. Locate and remove the final (2) Phillips screws and the remaining half of the plastic shell.
Step #9: Connect a New Battery HolderNext
- Recall the proper positioning of the battery. Mark the circuit board with a “+” in the area of the positive connector. Next, uncurl and then cut off both connectors leaving about 3/8” of wire protruding from the circuit board.
Step #10: Next
- Strip ½” of plastic insulation off the end of both the red and black leads of the battery holder. Slip a length of heat-shrink tubing over both battery holder leads. Wrap the red lead around the battery connector wire labeled “+” and solder.
Step #11: Next
- Wrap the black lead around the other battery connector wire and solder. Slip the heat-shrink tubing over these joints and shrink snugly in place.
- Test the amplified listener by putting an AAA battery into the battery holder. Plug a pair of ear buds into the jack on the listener. Turn the listener “on”. You should hear amplified sound in the earbuds.
Step #12: Extend the Leads of the Amplified Listener’s MicrophoneNext
- We’re going to move the listener’s microphone from its original position to the book’s outside spine, because the spine is generally exposed whether the book is in a bookcase or just lying around.
- To do this, locate the mic of the miniature amplified listener. The mic is connected to the circuit board with two wire leads. Depending on the model of listener, the leads may be short, bare wires or longer wires covered in plastic insulation.
- Select one of the two wire leads and mark a dot on the circuit board where the wire connects. Likewise, make a dot on the mic where the same wire connects. This will serve as a guide when re-attaching the mic to the circuit board after extending the mic’s leads.
- Cut both of the wire leads at the mid-point, leaving as much of each wire lead attached to the mic as to the circuit board.
- If the wire leads are insulated, strip at least ¼” of the insulation off the end of the leads connected to the mic and the circuit board.
- Locate the remaining two lengths of red and black hookup wire. Twist one end of the red wire around the lead on the mic marked with a dot and solder. Slip a length of heat-shrink tubing over the red wire and shrink snugly over the solder joint. Slip another length of heat-shrink tubing over the red wire. Twist the other end of the red wire around the lead on the circuit board marked with a dot and solder. Slip the other length of heat-shrink tubing over the solder joint and shrink snugly.
- Using the black wire, follow the same steps to connect the remaining lead on the mic and the circuit board. Again, test the amplified listener.
Step #13: Go “On the Air”!Next
- To insure that the components are working together, plug the male end of interface cable into the listener and the FM transmitter into the female end of the interface cable. Turn both components on. Listen wirelessly using the FM receiver. You may have to adjust the tuning of the receiver to match the transmitter. Also, set the volume level on the listener so the sound is strong but not overly distorted.
- Have a co-conspirator help with your test by talking near the mic while you walk away, listening through the receiver. See how far you can get before the signal is no longer intelligible. (For the best range, make sure to use fresh batteries.)
Step #14: Hook Up, Strap In, Turn On!Next
- Use the hook-and-loop discs to mount the three components (the FM transmitter, the amplified listener and the battery holder) to the floor of the hollow book compartment.
- Place the components first, before sticking them in. Slide the microphone into the hole in the spine. Inset the mic so it won’t touch the dust jacket. Tidy the wires so that they’re all well within the chamber and the book can be closed. Test the rig again, put the dust cover on, and you’re ready to save the free world!
Step #15: DebriefingNext
- Another cool use for this setup is as a covert, 2-way communication system. Assemble two of these devices. Then you and your fellow agent can conduct a whispered conversation in a noisy environment without sitting together.
Step #16: Limitations
- The problem with these in-car FM transmitters is that their range is only about 15'–20'. The listener can be in an adjacent room or the room above or below if there’s no metal blocking the signal. You probably can’t tune in from a car parked at the curb of the subject’s building. Sorry about that, Max!