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n/a
DELETED (Inactive)
21 Posts |
 Posted - Mar 27 2003 : 01:15:11 AM
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I've read about how the phone rings, now I am curious about how I can answer it, I was thinking some sort of cmos switch with a sample and hold. I want to receive the tones over a phone line (part of a larger circuit), but I am not exactly clear how the phone line works. Any suggestions would be great, thanks
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YS
Nobel Prize Winner
USA
1132 Posts |
Posted - Mar 27 2003 : 09:04:51 AM
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To answer the phone, you need to connect a 1-transistor circuit to the phone line. It should draw about 26mA from the phone line, and as phone exchange senses this current, the line voltage will drop immediately from 42V to about 7VDC. Polarity is unknown, so please include the diode bridge. You will stay "off-hook" as long as you draw this current. Now, you need to receive/transmit AC (audio) signal in frequency range of 300Hz-3000Hz with amplitude about 1Vpeak-to-peak. You can use a transformer from the old modem, connected via 10uF capacitor to the line in parallel to your current-drawing device. On the other side of transformer, you have a funny mix of transmitted and received signal. People use opamps to separate them. To be exact, use opamp as output amplifier, connected to transformer via 600 ohm resistor; another opamp is a differential amplifier, calculating a difference between line signal (from trassformer) and transmitted signal (from output opamp). The difference is received signal amplitude will be about 100mV peak-to-peak or less.
Find an older modem schematic for illustration.
Good luck.
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Aaron Cake
Administrator
Canada
6718 Posts |
Posted - Mar 27 2003 : 10:26:41 AM
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You should really use a relay to actually connect to the phone like. A transistor is a bit fragile (in my opinion anyway).
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YS
Nobel Prize Winner
USA
1132 Posts |
Posted - Mar 27 2003 : 10:46:32 AM
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Actually both. I just did not mention a relay. A transistor circuit I spoke about is necessary to draw a certain current within all possible conditions. For an experiment, just a 270 Ohm (or about that) resistor would be OK to go off-hook. But it will mess up line impedance, which should be 600 Ohm in the US, so signal level will go down.
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Answering the phone
