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I have a device that is sending 9600baud using only Tx, Rx, and Gnd. I am not sure, but I think the voltage is either the 3v or 5v level.

I want to receive that serial data on my mobile phone.

I have found AFSK code for decoding on the mobile phone, however I am concerned about sending the 3 or 5 volts into the microphone circuit.

Can anyone help me understand what I need to do to the RS-232 voltages in order to step them down properly for the microphone circuit on the phone?

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    \$\begingroup\$ You say you have FSK code for decoding, but it doesn't sound like you have anything in place to convert the RS232 to FSK, am I understanding that right? \$\endgroup\$ – Kellenjb Jan 26 '12 at 19:27
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    \$\begingroup\$ Perhaps my question should be "How can I make my mobile phone headset jack into a serial port to send/receive RS-232?" \$\endgroup\$ – qxotk Jan 27 '12 at 1:15
  • \$\begingroup\$ @Kellenjb - I have now followed links to your other well-written answers, and I believe I should be referencing AFSK or Audio Frequency Shift Keying - as is used in old telephone modems. \$\endgroup\$ – qxotk Jan 27 '12 at 1:53
  • \$\begingroup\$ @Kellenjb You are correct, and I am looking for something to translate the RS-232 signals to AFSK to input to the audio jack on the mobile phone. I am currently investigating the audio jack modem on the SparkFun site here: sparkfun.com/products/10331 - except I am not sure what is passing over the FSK in and out ports of this board. Any insight on that? \$\endgroup\$ – qxotk Jan 27 '12 at 2:08
  • \$\begingroup\$ Some mobiles actually have a serial port in the accessory connector - it might be worth researching this possibility. \$\endgroup\$ – pjc50 Apr 16 '12 at 14:31
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  1. Microphone inputs on phones are designed to be compatible with electret microphones which produce very small ac voltages. A signal at about 5mV peak to peak should work. The phone may be looking for a DC load to detect the presence of a mic - so have a 1k dc impedance to gnd. Be careful, iPhones and Android phones have different pin outs on the headphone jack.
  2. RS232 signals are not dc balanced and therefore cannot be captured properly by an audio input. That is why the serial data must be FSK modulated (or similar algorithm) or Manchester encoded.
  3. Some (or most?) phones have a ~100Hz high pass filter on the input which not only blocks dc but also causes phase distortion so that an input square wave will not look much like a square wave after capture.
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For mobile applications, 3V voltage level signaling is OK instead of 5V, you may use 3V without any doubt, or you may try 5V you will not get any error or bad response as all mobiles applications are designed for 3 to 5 V signaling

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  • \$\begingroup\$ Have you considered the OP was talking about audio levels? And have you tried yourself what you've suggested? (without frying your audio input circuitry of course.) \$\endgroup\$ – user59864 Apr 30 '16 at 19:26

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