Line Powered Phone Messager (PIC16C819)

  
The phone company provides 3 levels of power. With no devices 'off hook' the line rests at 48 volts DC. If you draw any more than a fraction of a milliamp or so, you get the phone company's attention and a dial tone. When the line rings, you get about 100 volts (rms) of AC signal on top of the 48 volts DC for 2 seconds on and 4 seconds off. The peak current is about 120 volts/600 ohms = 200 ma. The circuit here takes advantage of the available power to put some audio message onto the phone line without the need of batteries or external power. This allows it to run completely isolated (floating) with regard to earth ground which makes the phone company's system very happy.
Line Powered Phone Messager (PIC16C819) - schematic

This hardware design can be used in several ways. It breaks down into the following functions: 1) 4 power diodes connect it to a phone line of either polarity. 2) The NE1/R1/CR1/C1 combination allows it to power itself up from the incoming ring signal. Alternativly, S1 can be used to grab the line manually at any time. 3) The Q1/LED/Resistor/Q2 circuit lets the processor grab the line (go offhook). 4) The MCP602 (rail-to-rail) op amp couples audio into and out of the circuit. 5) The MCP41010 8-bit potentiometer is a convenient D/A device. 6) The 24LC512 stores 8 seconds of 8-bit audio at 8k bytes/second. My current use of this device does not have the neon bulb installed, so it is activated manually. A british-sounding female voice talks on the line when I push S2, S3, or S4. After picking up the regular phone, I hit S1 to grab the line and S2 to say 'hello'. If I dont want to talk to the person, I wait until they ask for me, hit S3 to say 'no' and hang up. Otherwise I hit S4 to say 'just a minute' and then start talking myself. The software is rigged to hang up the device after 20 seconds of inactivity or when S2 and S3 are pressed simulaneously. Here is the current state of the Source File allong with the Board Layout I'm still thinking about other uses for this device.



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