Simple Guitar Transmitter (EE Tip #102)

You need a guitar amplifier to play an electric guitar. The guitar must be connected with a cable to the amplifier, which you might consider an inconvenience. Most guitar amplifiers operate off the AC power line. An electric guitar fitted with a small transmitter offers several advantages. You can make the guitar audible via an FM tuner/amplifier, for example. Both the connecting cable and amplifier are then unnecessary. With a portable FM broadcast radio or, if desired, a boombox, you can play in the street or in subway.

Source: Elektor 3/2009

Source: Elektor 3/2009

stations (like Billy Bragg). In that case, everything is battery-powered and independent of a fixed power point. (You might need a permit, though.)

Designing a transmitter to do this is not necessary. A variety of low-cost transmitters are available. The range of these devices is often not more than around 30′, but that’s likely plenty for most applications. Consider a König FMtrans20 transmitter. After fitting the batteries and turning it on, you can detect a carrier signal on the radio. Four channels are available, so it should always be possible to find an unused part of the FM band. A short cable with a 3.5-mm stereo audio jack protrudes from the enclosure. This is the audio input. The required signal level for sufficient modulation is about 500 mVPP.

If a guitar is connected directly, the radio’s volume level will have to be high to get sufficient sound. In fact, it will have to be so high that the noise from the modulator will be quite annoying. Thus, a preamplifier for the guitar signal is essential.

To build this preamplifier into the transmitter, you first have to open the enclosure. The two audio channels are combined. This is therefore a single channel (mono) transmitter. Because the audio preamplifier can be turned on and off at the same time as the transmitter, you also can use the transmitter’s on-board power supply for power. In our case, that was about 2.2 V. This voltage is available at the positive terminal of an electrolytic capacitor. Note that 2.2 V is not enough to power an op-amp. But with a single transistor the gain is already big enough and the guitar signal is sufficiently modulated. The final implementation of the modification involves soldering the preamplifier circuit along an edge of the PCB so that everything still fits inside the enclosure. The stereo cable is replaced with a 11.8″ microphone cable, fitted with a guitar plug (mono jack). The screen braid of the cable acts as an antenna as well as a ground connection for the guitar signal. The coil couples the low-frequency signal to ground, while it isolates the high-frequency antenna signal. While playing, the cable with the transmitter just dangles below the guitar, without being a nuisance. If you prefer, you can also secure the transmitter to the guitar with a bit of double-sided tape.

—Gert Baars, “Simple Guitar Transmitter,” Elektor,  080533-1, 3/2009.

Great Plains Super Launch

Contributed by Mark Conner

The Great Plains Super Launch (GPSL) is an annual gathering of Amateur Radio high-altitude ballooning enthusiasts from the United States and Canada. The 2012 event was held in Omaha, Nebraska from June 7th to the 9th and was sponsored by Circuit Cellar and Elektor. Around 40 people from nine states and the Canadian province of Saskatchewan attended Friday’s conference and around 60 attended the balloon launches on Saturday.

Amateur Radio high-altitude ballooning (ARHAB) involves the launching, tracking, and recovery of balloon-borne scientific and electronic equipment. The Amateur Radio portion of ARHAB is used for transmitting and receiving location and other data from the balloon to chase teams on the ground. The balloon is usually a large latex weather balloon, though other types such as polyethylene can also be used. A GPS unit in the balloon payload calculates the location, course, speed, and altitude in real time, while other electronics, usually custom-built, handle conversion of the digital data into radio signals. These signals are then converted back to data by the chase teams’ receivers and computers. The balloon rises at about 1000 feet per minute until the balloon pops (if it’s latex) or a device releases the lifting gas (if it’s PE). Maximum altitudes are around 100,000 feet and the flight typically takes two to three hours.

Prepping for the launch – Photo courtesy of Mark Conner

On Thursday the 7th, the GPSL attendees visited the Strategic Air and Space Museum near Ashland, about 20 minutes southwest of Omaha. The museum features a large number of Cold War aircraft housed in two huge hangars, along with artifacts, interactive exhibits, and special events. The premiere aircraft exhibit is the Lockheed SR-71 Blackbird suspended from the ceiling in the museum’s atrium. A guided tour was provided by one of the museum’s volunteers and greatly enjoyed by all.

Friday featured the conference portion of the Super Launch. Presentations were given on stabilization techniques for in-flight video recordings, use of ballooning projects in education research, lightweight transmitters for tracking the balloon’s flight, and compressed gas safety. Bill Brown showed highlights from his years of involvement in ARHAB dating back to his first flights in 1987. The Edge of Space Sciences team presented on a May launch from Coors Field in Denver for “Weather and Science Day” prior to an afternoon Colorado Rockies game. Several thousand students witnessed the launch, which required meticulous planning and preparation.

EOSS ready for launch – Photo courtesy of Mark Conner

Saturday featured the launch of five balloons from a nearby high school early that morning. While the winds became gusty for the last two launches, all of the flights were successfully released into a brilliant sunny June sky. All five of the flights were recovered without damage in the corn and soybean fields of western Iowa between 10 and 25 miles from launch. The SABRE team from Saskatoon, Saskatchewan took the high flight award, reaching over 111,000 ft during their three-hour flight.

The view from one of the balloons. Image credit: “Project Traveler / Zack Clobes”.

The 2013 GPSL will be held in Pella, Iowa, on June 13-15. Watch the website superlaunch.org for additional information as the date approaches.