Most people hate unsolicited calls, and it’s worse in the dead of night when we’re all trying to sleep. Smartphones are easy to configure to block nuisance calls, but what if you need a solution for your Plain Old Telephone System (POTS)? [Molecular Descriptor] has built a system to invisibly stop landline phones ringing after hours.
The basic principle relies on an analog circuit that detects the AC ringing signal from the phone network, and then switches in an impedance to make the phone company think the phone has been picked up. The circuit is able to operate solely on the voltage from the phone line itself, thanks to the use of the LM2936 – a regulator with an ultra-low quiescent current. It’s important if you’re going to place a load on the phone line that it be as miniscule as possible, otherwise you’ll have phone company technicians snooping around your house in short order wondering what’s going on.
The aforementioned circuitry is just to block the phone line. To enable the system to only work at night, more sophistication was needed. An Arduino Mega was used to program an advanced RTC with two alarm outputs, and then disconnected. The RTC is then connected to a flip-flop which connects the blocking circuit only during the requisite “quiet” hours programmed by the Arduino. The RTC / flip-flop combination is an elegant way of allowing the circuit to remain solely powered by the phone line in use, as they use far less power when properly configured than a full-blown microcontroller.
It’s a cool project, with perhaps the only pitfall being that telecommunications companies aren’t always cool with hackers attaching their latest homebrewed creations to the network. Your mileage may vary. For more old-school telephony goodness, check out this home PBX rig.
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.
Continue reading “Simple DTMF decoder pulls numbers from YouTube videos”
Tinker.it has published plans for building a software synthesizer using an Arduino. The Auduino uses granular synthesis to create a truly unique sound. The grain is constructed from two triangle waves. Each one has adjustable frequency, decay rate, and the repetition rate can be changed too. The Arduino just needs five potentiometers attached to the analog inputs and an audio jack on the digital out. You don’t have to use pots; you can use anything that varies the analog input between 0 and 5 volts. A video of the device is embedded after the jump. Continue reading “Auduino software synth”