With "land line" wired phone services making a graceful exit at stage right, I've accumulated an assortment of cordless phones. I've been mulling over ideas, and experimenting with ways to repurpose these still functional transceivers for a while.
I spent a lot of time experimenting with using a cordless phone as a wireless music system. I learned about and constructed a summing network to combine stereo audio into mono. I first tried using the handset as a transmitter, inputting the combined stereo output from my laptop into the auxiliary mic input. I discovered that even with the base unit powered by it's AC adapter, without an active phone line connected, it wouldn't communicate signal. I found that injecting 9 volts DC into the line connection would activate it. This is because wired phone systems have an off-hook constant DC voltage of around 9 volts, so that's what triggers the communication mode. With a current limiting resistor and an AC coupling capacitor, I connected the line jack of the phone base unit through an RCA wye connector, to the left and right channels of my receiver. This works, but the audio quality is very poor. Further research led to learning that the audio bandwidth of most cordless phones is limited to about 3khz. Due to this and the noisy nature of this technology, I wasn't satisfied with using it for recorded music.
I decided to try using a cordless phone as a wireless guitar system. A 3khz audio bandwidth is within the playable frequency range of a typical guitar, and adequate for most simple playing. Testing proved that again, this works, but the static noise is problematic for clean tone playing. It's not a problem for dirty crunchy fuzzy distortion tone though. Eureka!
What You Need to Make it:
- 1 - Working Cordless Phone
- 1 - Phone Cable
- 1 - RJ-11 Jack
- 1 - 2.5 mm Stereo Headset Cable
- 1 - 1/4" Mono Plug
- 1 - 1/4" Mono Jack
- 1 - N-Channel JFET
- 1 - 10K Resistor
- 1 - 1uf Capacitor
- 1 - 9V Battery
- 1 - 9V Battery Snap
- Boot Lace or Paracord
- Heat Shrink Tubing
- Small Project Enclosure or Material to Make One
- Soldering Iron & Solder
- Wire Cutters & Strippers
- Drill & Bits
- Assorted Tools if Making Enclosure
Step 1: Make an Active Patch Cable
This is the cable that will connect your guitar to the phone handset. This assumes that your handset has an auxiliary 2.5mm headset input jack. If not it is also possible to open the handset case and remove the mic and solder a 2.5mm jack in it's place.
After seeing a great Instructable article on making speaker cables I decided to mimic the technique of using the outer sheath of paracord as a wire covering. I didn't have any paracord, but I did have some old boot laces that gave me a similar result.
After sheathing the cable, I heat shrunk a tube around the sheath and strain relief of the 2.5mm stereo plug. Then I heat shrunk a tube around the sheath on the 1/4" plug end. I also heat shrunk a tube around a small compression spring to act as a strain relief.
My 1/4" plug was an old right angle one that was missing the back-shell. So, I made one out of some scrap cherry.
The active part of the cable is the N-Channel JFET. The electret mic in the phone handset has a built in JFET impedance converter. This effectively is a pre-amp. This is how we will create distortion of the guitar signal. I didn't have a JFET, so I filed the retaining flange off the back of an old electret mic, and disassembled it to remove the built in JFET impedance converter.
Referring to the schematic and images, solder the gate pin of your JFET to the signal terminal of your 1/4" plug. Solder the source pin of your JFET and the ground wire of your cable to the 1/4" plug ground. Solder the drain pin of your JFET to the signal wire of your cable. You can determine which of the two stereo signal wires you need by using your multimeter to check for continuity between the tip of the 2.5mm plug and a wire on the other end.
Step 2: Make a Phone Interface
Referring to the schematic and images, solder your 1uf capacitor (negative lead if electrolytic) to the signal terminal on your 1/4" jack. Solder one end of your 10K resistor & the red wire from your RJ-11 jack to the other capacitor lead. Solder the red battery snap wire to the other end of the 10 k resistor. Twist the black battery snap wire & the green RJ-11 jack wire together, then solder them to the ground terminal on your 1/4" jack.
I made a custom enclosure out of scrap cherry, acrylic, and ABS. I cut the blanks and grooves with my table saw. I sanded the wood parts and lightly chamfered the sharp edges. I drilled and countersunk screw through holes in the acrylic top and bottom. I Drilled a 3/8" hole in one ABS plate to accept the 1/4" jack. I also cut a square opening to accept the RJ-11 jack, which I glued to the plate. I drilled pilot holes and fastened the enclosure together with #4 wood screws.
Step 3: Hook It Up and Jam
Connect a 9V battery to the phone interface. Connect a phone cable between the interface and the phone base unit. Plug in the phones power adapter. Connect an instrument cable between the interface and your amp. Connect your active patch cable between your guitar and your phone handset. Push talk on the phone to activate.
Power is only drawn from the battery when a phone cable is connected between the phone base unit and the interface.
Rock on friends!