Detecting DTMF Tones From Scratch

October 20, 2011 by Brian Benchoff 9 Comments

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?

August 9, 2011 by Mike Szczys 13 Comments

[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

April 2, 2011 by Mike Nathan 28 Comments

dtmf_decoder

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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Free Home Phone Method Uses Parts We’re Familiar With

March 18, 2011 by Mike Szczys 19 Comments

[Headsheez] found a way to get his home phone service for free. He’s using a set of tools that we’re familiar with to route service from a typical analog phone system (which involves the extensions wired into your home) through a server to the Internet. On the hardware side of things this starts out with an Analog Telephone Adapter which translates the analog signal for use in a PBX system. He uses a copy of the open source PBX project called Asterisk which we’ve also seen used on devices like routers and the SheevaPlug. The actual telephone number comes from a Google voice account which for now is a free service but there’s no guarantee that it will remain that way in the future.

This should provide seamless service just like you’re used to with a traditional home phone line. There’s even caller ID for the number – but not the name – for incoming calls. The one big feature that is missing from this setup is the ability to call 911 for emergencies.

[via Reddit]

Remote Operated Security Gate Lets You Phone It In

March 2, 2011 by Mike Nathan 20 Comments

ring_detection_circuit

[Itay] has a friend who works in a rented office where the parking lot is secured by a remote-controlled gate. Unfortunately, while his friend shares an office with several people, they only received a single remote. To help his friends out, he built a small device that triggers the remote control whenever a phone call is received.

The remote modification was rather straightforward. He simply opened the device, adding a single wire to each button terminal. Rather than connect to the remote using wires, he decided to fit it with what looks like a scavenged DC power jack. The ring detector circuitry was constructed and stuffed in a small phone box, which is connected to the remote using a DC power plug. It’s a great solution to the problem, but let’s just hope no one gets a hold of the phone number they used for the trigger!

There are plenty of pictures on his site, as well as video of the ring detector being tested. Unfortunately [Itay] lost the original schematics for the circuit, so you will have to flesh that part out on your own if you wish to build a similar device.

Keep reading to see a few videos of the remote in testing and in use.

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Paging System For Your TV Remote

October 13, 2010 by Mike Szczys 16 Comments

[Matt] brought together a TV remote and cordless phone to add a locator system to the remote control. One of the best features of a cordless phone is the pager button on the base. When you press it the handset beeps until found. Matt gutted one and got rid of the unnecessary parts. He then cracked open his TV remote housing and inserted the telephone handset’s circuit board, speaker, and battery. The base station is used just like normal to locate the phone/remote combo, and has been modified with a charging cable to top-off the telephone battery which powers everything in the newly hacked unit. [Matt’s] demonstration video is embedded after the break.

It’s too bad that he got rid of the microphone. It would be interesting to take calls on this thing.

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Automatic Telephone Recorder

March 2, 2010 by Mike Szczys 40 Comments

[James Bond] types and those suffering from a hefty dose of paranoia can now record all telephone conversations. [Trax] built this module that monitors the phone line and starts recording when a handset is picked up. A computer does the actual recording, triggered by the microcontroller via a USB connection.

We like the use of an old PCB for a faceplate, we’ve certainly got some duds of our own sitting around. The three cords are a bit confusing though. One for the phone line, one for the USB, but what does the third do? Is it an audio-out connection?

We’ve asked [Trax] to post a schematic and source code if possible so that might clear up the mystery.

Update: [Trax] let us know that there is a red button with white lettering at the bottom labeled “download”. This contains schematic, code, pcb layout, and PC software. We just missed it the first time around.