Increase the range of your FM transmitter and broadcast audio around your home.
If you read “Broadcast Your iPod to FM Radio” [Hack #5] , you might have noted that the FM transmitters discussed seemed to work like very weak FM stations. Unsurprisingly, that is precisely the function of the mentioned FM transmitters. Their power, and thus range, is limited by FCC regulations. This hack details how to amplify the signal from an FM transmitter to cover a larger area.
Here’s what you’ll need to complete this hack:
An iPod
An FM transmitter
Various electronic components (listed later in this hack)
A soldering iron
Fearless voiding of the FM transmitter’s warranty
Strictly speaking, you don’t have to build anything, because there is a wide variety of available kits and prebuilt devices that will save you the trouble. One example is the FM100B Super Pro FM Stereo Radio Station Kit (http://www.ramseyelectronics.com; $269.95). This solution will provide much better quality than the following design, but it has a few drawbacks. The first is that it costs $269.95, and the second is that it is much more interesting (and cheaper) to build something like this from scratch.
Warning
Before I get to the specifics of this hack, let me note that if you’re living in the United States (or one of several other countries), this hack is illegal. That means you shouldn’t do it—rather, think of this hack as a Gedanken experiment. After all, no one has attempted Schrödinger’s cat. If you live outside the United States, check with your local authorities before starting to make sure you’re not violating local regulations. If you’re the sovereign of a small nation, by all means proceed with reckless abandon.
The circuit we’ll build is a basic design chosen specifically for simplicity and car/home compatibility. Think of it as a learning experience; once you’ve built this circuit, you’ll have the confidence to tackle more intricate designs and be able to tailor the performance of your FM amplifier to your needs. To get started, you’ll need the fairly specific parts outlined in the following list.
Tip
This list contains RadioShack part numbers and prices for convenience only. Any decent electronics shop should have the parts you need.
Transistor 2n4401 (part #276-2058; list price $0.96).
220-kΩ resistor (part #276-2058; list price $0.99).
10-kΩ resistor (part #271-1126; list price $0.99).
.001μF capacitor (part #272-126; list price $1.29).
100μH RF choke (part #273-102; list price $1.29).
30 inches of solid copper wire (alternatively, you could cannibalize a telescoping antenna from an unused radio, as long as the length is correct).
Breadboard (part #276-169; list price $22.99). This is not strictly needed (you could just solder everything together), but a breadboard makes the construction process much easier.
10-kΩ potentiometer (part #271-215; list price $3.29).
12V/500mA AC to DC power converter (part #273-1773; list price $15.99). Get the friendly folks at the electronics shop to mark the positive and ground connections for you; you’ll save time and money by avoiding the need for a multimeter.
Time to void the warranty. Crack open whatever transmitter you’ve chosen for sacrifice (for this project, choose a battery-powered or iPod-powered FM transmitter) and locate the antenna. In the Belkin TuneCast, the antenna is the blue wire inside the mini-jack cable.
For instructions on locating the iTrip’s antenna, see “Turn Your iPod Mini into a Radio Station” [Hack #13] . There is no standardized location for the antenna, so I can’t say exactly where you will find it on other FM transmitters. You’re looking for a length of wire without any obvious function (i.e., it’s not connecting the batteries or the headphone jack). If you’re having trouble finding the antenna, look carefully at the green circuit board; often, there is a small white ANT label where the antenna meets the printed circuit board, giving away the location.
Once you’re sure you’ve found the antenna, devise a way to expose a length of said antenna outside of the FM transmitter (one inch should be plenty) without destroying the connection to the internal circuitry. Reassemble the unit with the newly accessible antenna wire still proudly outside of the unit. Some FM transmitters use bare wire, while others use insulated wire. If your antenna falls into the insulated category, take this opportunity to remove the insulation from the exposed part of the wire. Congratulations; you’ve completed the modification the FM transmitter.
This is the fun or challenging part, depending on your perspective. While the schematic shown in Figure 1-6 might look complicated at first glance, it is much easier to build than a cursory inspection reveals.
Here are a few tips before starting: don’t power up the circuit before you’re done, don’t touch the circuit after you’ve inserted the plug in the wall receptacle (safety first), and don’t be afraid to ask for help. People who know how to construct circuits are generally proud of their knowledge and are usually happy to help. Once you’re assured the circuit performs correctly, you can remove all the excess material to make the device as small as possible (done carefully, the entire amplifier circuit could fit on a quarter).
If you bought a breadboard, building the circuit is pretty easy. If not, you can still get through it, but things will be a bit trickier. The instructions that follow are for building the circuit without the breadboard, but they are also applicable when using the breadboard.
Tip
A note on connecting the wires: this circuit won’t be permanent until you apply solder to all the junctions, but try to make the connections as secure as possible for testing purposes (or use a breadboard).
The potentiometer is a good place to begin. If you examine the potentiometer closely, you’ll note that it has three holes to receive wires (the holes are called lugs). Get three pieces of equal-length insulated wire (three inches is plenty) and strip off most of the insulation, leaving a one-inch strip of insulation in the middle of each wire.
Connect one wire per lug and trim any excess exposed wire next to the lug. At this point, turn your attention to the wire connected to the middle lug. Attach one end of the 220-kΩ resistor to the wire and trim any excess wire/ lead (the leads are the wires coming out of the components). Grab the RF choke and attach it via a wire to the unadorned side of the resistor.
Attach another section of wire to the unused end of the RF choke. Attach similar wires to the leads of the remaining resistor, the capacitor, and all three leads of the transistor. Hold the transistor with the flat side toward you and note the far-left lead (this is called the collector). Wire this lead to one wire of the 10-kΩ resistor, and similarly to the RF choke and capacitor (I did this by twisting the bare ends of the four wires together with a wire nut). Take the middle lead (called the base) of the transistor and wire said lead to the unused wire of the 10-kΩ resistor.
The project is nearly complete. At this point, we want to add a ground wire (it is probably wise to choose a different color to avoid later confusion). Choose a length of wire long enough to traverse the entire circuit, and strip about an inch of insulation off of each end. Wire one end of the ground wire to the remaining transistor lead (known as the emitter). Wire the opposite end of the ground wire to one of the remaining unused wires attached to the potentiometer (do not trim excess). Connect the capacitor’s remaining unused wire to whatever you’ve decided to use as your new antenna.
All the circuit lacks now is power and an input. First, add the input. Take the section of wire you exposed on your FM transmitter earlier and connect it somewhere between the base lead (middle lead) of the transistor and the 10-kΩ resistor.
To power the circuit, you’ll need the 12V DC power adapter. Leave it unplugged for the moment. If you’re using the RadioShack model mentioned earlier, you’ll see two small holes. Hopefully, you followed the earlier advice and had the electronics shop folks mark the positive and ground sides; if not, you’ll have to use a multimeter to deduce the orientation.
Insert the ground wire into the neutral (ground) side of the plug, and insert the last free wire on the potentiometer into the positive side of the plug (you may want to fold the ends of the wire for a more secure fit). Trim off any excess exposed wire, check to be sure all the connections are secure, and inspect the circuit carefully to ensure that there are no connections that aren’t in the diagram. Once you’re confident everything is ready, plug the DC converter into the wall and (the moment of truth) fire up your iPod and FM transmitter.
Tip
At some point, you’re going to want to solder the circuit together. The connections made by twisting wires together are fine for testing, but for daily use, you need a more permanent solution. You shouldn’t find soldering your masterpiece too difficult, but if you do, a nice primer is available at http://www.epemag.wimborne.co.uk/solderfaq.htm. There is one common mistake you need to keep in mind: the potentiometer can be destroyed by overheating with the soldering iron, so take special care when soldering that particular component.
The hard work is done, and now the fun begins. Before you start using your newly birthed amplifier in earnest, keep the following in mind: the circuit will certainly interfere with, and possibly override, any broadcast stations that share the same frequency. That means that if your neighbor is listening to NPR and you tune your transmitter to NPR’s frequency (90.9 MHz, for example), said neighbor is going to be listening to your iPod instead of NPR’s careful reporting. Minimize neighbor frustration by picking a dead spot on the FM dial (that is, a channel where you hear only static). Purposefully drowning out the annoying radio station your neighbor is listening to by broadcasting silence on the same frequency [Hack #13] is possible, but not recommended.
Once you’ve selected the frequency, power everything up and grab an FM radio. Tune it to your carefully chosen frequency and take a walk. You’re basically checking reception; ideally, what you want is full coverage on your bit of turf and zero coverage elsewhere. That is unlikely to happen, but you can fine-tune the result by adjusting the potentiometer. If the coverage is sub-par, rotate the knob on the potentiometer to decrease resistance, thus increasing the covered area. If the coverage extends beyond the limits of your personal fiefdom, rotate the knob in the opposite direction to decrease the range of your personal FM station. Experimentation yields the best results while providing the most entertainment.
The usefulness of your new amplified transmitter is limited only by your imagination. If you used a transmitter that attaches via the iPod mini-jack only, you can use your new transmitter with any device that the mini-jack plug fits. That means you can stream the music off your desktop Mac to all the radios throughout your house (try that with Airport Express!), broadcast from your portable CD player, use a camcorder as a microphone and broadcast your own monologue, and so on. The applications are limitless.
Also note that the circuit was designed with portability in mind. To use the device in your car, snip the end off a cigarette-lighter adapter and attach the power (usually a red wire) to the potentiometer and the ground wire (usually black) to the ground wire of your amplified transmitter. Your mini-station is now fully mobile.
—Chris Seibold
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