Rotary and cordless phones mashup

This pile of hardware marries telecommunications hardware from distinctly different generations. [Andrew D. Farquharson] wanted the retro look and operation of a rotary phone, with the convenience of a modern cordless. He combined the two technologies to achieve his goal.

The first problem was to find a way to translate the rotary inputs to something he could use. There are already a bunch of projects that use rotary hardware so he didn’t have to reinvent the wheel. He followed this guide to connecting Arduino to a rotary phone.

The next step was to interface with his cordless phone. He ditched the case and soldered rainbow ribbon cable to the entire button matrix. An opto-isolator is used to protect the Arduino while making each connection. Finally, he patched into the mechanism which monitors the cradle to see if the handset has been picked up. It sounds like his code lets you enter the number on the dial, then pick up the handset to actual transmit it through the cordless phone.

Using old phones as an intercom in your VW bus (or anywhere else)

In case you haven’t noticed from my many comments on the subject, I drive a VW bus. It is a 1976 Westfalia camper with sage green paint and green plaid upholstery. I absolutely love it and so does the rest of my family. We go for drives in the country as well as camping regularly. We have found that the kids have a hard time communicating with us while we’re going higher speeds. These things aren’t the quietest automobiles in the world. Pushing this bread loaf shaped hunk of steel down the road with an engine that might top out at 75hp results in wind noise, engine noise, and of course, vibration.

I decided to employ a really old hack to put two functional telephones in the bus so my kids can talk to my wife (or whoever the passenger is) without screaming quite so loud. This hack is extremely easy, fairly cheap, and can be done in just a few minutes. The result is a functional intercom that you could use pretty much anywhere!

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Ethernet over telephone wire

When [Bobo1on1] upgraded his Internet connection from ADSL to Fiber he ran into an issue of actually getting that speed to his desktop computer though his LAN setup. Before he had been using a telephone extension wire which ran from where the DSL entered the house, through a splitter, to his computer where the modem was located. Now that the router used by the fiber system is located at teh entry point, he has no easy way to run Ethernet cable to his computer room. Wifi is predictably slower than the 50mbit WAN connection, and he was unable to use the telephone cable as Ethernet directly.

The solution turns out to be a pair of TP-Link home plug adapters. These are designed to use your home’s mains wiring for data transfer. But [Bob] rigged it up so that they can push 224 mbits/sec over the telephone wire. Since you can’t run mains voltage through the telephone wire he had to hack a method to separate power for the devices from the data I/O. This was done with an external power supply and some passive components for filtering. The drawback is that this is half-duplex so up/down communications cannot happen at the same time.

This toy intercom system is way better than a pair of tin cans and some string


On his blog, [Kenneth Finnegan] recently showed off a replica of a fun toy he used to play with as a kid, a telephone intercom system. The setup is pretty simple, requiring little more than a pair of analog phones, a battery, and a resistor.

The phones are connected to one another using a standard telephone cable, but [Kenneth] uses a 9v battery to introduce a small bias current into the loop, allowing the speakers at either end to hear one another. He also added a small LED into the circuit so that there is a visual indication as to when both handsets are off hook.

The setup is very simple at the moment, though [Kenneth] does have some ideas in mind to enhance his intercom system. He hopes to tweak the remote phone to ring when the local phone is picked up, among other things.

Telephone technology is nothing new, but for just a few dollars (or less) your kids can be entertained for hours as [Kenneth] was way back when.

Continue reading to see a short video overview of the phone system, and be sure to share your ideas for enhancing it in the comments section.

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Detecting DTMF tones from scratch

If you’ve ever wondered about the best way to detect dial and DTMF tones from a phone line, [Debraj] is your man.

[Debraj] built a DTMF detector using the Goertzel algorithm. Normally, when we think about detecting tones, we pull FFT out of our bag of tricks. The Goertzel algorithm isn’t as computationally complex as FFT and can be implemented on even the smallest microcontrollers.

For the build, the first thing to solder is a nice audio transformer and some protection diodes. The ring tone from a phone line goes from +35 V to -35 V – a bit more than a microcontroller could handle. A PIC18F4520 dev board was used as the brain of the system with all the code is available on [Debraj]’s site.

Although implementations of the Goertzel algorithm are a little uncommon, [Debraj] has seen a few interesting projects using this technique. [Debraj]’s build could easily be modified into a guitar tuner with a few changes in the code, for example.

This project was built as the command and control for a home automation system and from the video after the break, we can’t wait for [Debraj] to get annoyed at the phrase, “To turn on the kitchen lights, please press 1…”

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Ever wonder where cool interactive museum exhibits come from?

[Victor’s] girlfriend works at a museum and enlisted his expertise in designing an interactive detective game for kids visiting the museum. The vision was for the kids to discover phone numbers that they could call for clues. Originally he planned to display the clues on a character LCD, but obviously it’s much neater to hear the clues in the handset of the phone.

Quickly switching gears, [Victor] dropped the ATtiny2313 and started over with an Xmega chip — in fact, it was our recent Xmega post that inspired him to document his project. The microcontroller is responsible for a lot of goings-on. It scans the key matrix for inputs, simulates the DTMF touch tones, reads audio files from a FAT file system on an SD card, and plays them back over the hand set’s speaker. Since most of the hardware is already built into the phones, it was not hard to fit his add-ons inside the case. A simple audio amplifier circuit joins the microcontroller, which is patched into the rows and columns of the keyboard. Take a gander at the video after the break to see the device in action.

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Simple DTMF decoder pulls numbers from YouTube videos


While many of us have banished land line telephones from our houses, there are still quite a few people who utilize POTS lines today. These analog phone systems use Dual Tone Multi Frequency (DTMF) signals in order to audibly represent all of the keys on a telephone keypad and place calls. [Brad] over at LucidScience decided that it would be useful to have a DTMF decoder on hand, and got busy building one.

His DTMF decoder box uses a CM8870 DTMF decoder chip, which you might assume is all you need to get the job done. This chip performs its duties very well, outputting a 4-bit binary code for each button press it registers, but that doesn’t do a whole lot of good without being able to represent those codes in a meaningful fashion. He first built a breadboard decoder circuit that would light 1 of 16 LEDs depending on the detected button press. This was well and good, but he decided that an Arduino-driven LCD display would work far better.

When he was finished, he had a compact decoder box with an LCD display, which accepts input from either an RJ-11 cable or an audio jack. He says that the audio jack is particularly useful for decoding tones from computer audio, such as YouTube clips. [Brad] praises the CM8870 chip, stating that it can pull phone numbers from pretty much any audio or phone signal you throw at it, regardless of quality. We think it would make a great basis for a telephone-based security system, if that was something that appealed to you.

Be sure to stick around to see his DTMF decoder circuit in action.

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