Homemade Phone PBX

Posted on Feb 7, 2014

Voicemail capacility (although not implemented, there are four ADCs that can be used to sample and record the phone audio) A storage volume (SD card) would need to be connected. This project was inspired by these other homemade exchanges: Markus Wandel and Andrew Holme. My project is not a kit, not from instructions, and not a copy of the

Homemade Phone PBX
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above people`s designs. I actually designed and am building all of it from scratch using parts I own and parts I could obtain. Power Supply box: Conatins a 60VDC power supply that will be used to generate high voltage 20Hz AC ringing. Also contains a 7VAC transformer for 5VDC. Fuses and switches. Talk Supply filtering: A salvages switching power supply provides 42VDC for phone talking current. This supply is noisy so extensive filtering is used. Here, and inductor (really half a transformer) and a capacitor remove some of the noise. The rest is filtered out on the main board using RC low pass filters set to 2. 34 Hz. Atmel AVR Atmega328 microcontroller. This chip handles all tone generation with the help of a DAC chip to the right. These two parts generate all busy, dial, and ringing tones by reading WAV files stored in the flash memeory of the AVR. The AVR also plays DTMF tones to the outside line to dial on it. The AVR is overclocked to 25Mhz so it has enough time to transfer data over a serial shift register interface to the DACs. MT8870 DTMF decoder. There is one of these and associated circuitry for each extension. The DTMF chip is connected through a four bit data bus shared among the other DTMF chips with CS lines to select what chip is being read. The Std (tone detect) pin is connected to the Atmega644 so it can wait for digits, select the MT8870 where the signal originated, and read off a four bit code refering to...

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