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Simple GPS to PC idea

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Old 07-18-2004, 07:18 PM
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jheissjr
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Default Simple GPS to PC idea

How about using this very simple idea to get GPS data from the plane to a PC on the ground:

1. Hook the data out wire from serial out on the gps receiver directly to the mic input on the video transmitter.

2. Hook the audio output from the video receiver to the data-in pin on the COM port

3. Open Hyperterminal hopefully see the NMEA strings of data pouring in.

So basically instead of hooking up the gps receiver directly to the COM port on the PC, the transmitter and receiver are put in between the PC and GPS sending data not through the a wire, but through the air on the audio channel instead.

The only way I could see this idea failing is there needs to be two way communication back and forth between the PC and GPS (instead of one way for the above idea). I was not sure if this is the case but if its not, I don't see why the above would not work.
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Old 07-18-2004, 07:41 PM
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Default RE: Simple GPS to PC idea

This method has worked fine for others. It is easiest to use with serial interfaces that are TTL-like (0 to 5V) rather than true RS-232 (+12V/-12V). You should plan on AC coupling it and providing a bit of attenuation to prevent overdriving the mic input.

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Old 07-18-2004, 07:56 PM
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Default RE: Simple GPS to PC idea

So to condition the signal from the GPS to the mic-in, could I use a simple op-am with adjustable gain and if the GPS uses true RS232 an adjustable offset too. Do you happen to know if most GPS receivers use true RS232 or 0-5. I will have to find an o-scope so I can see which one mine is ( http://www.deluoelectronics.com/cust...4&cat=1&page=1 ). Do you know any websites that have done this that I could check out?
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Old 07-18-2004, 08:14 PM
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Default RE: Simple GPS to PC idea

The handheld GPS's use RS-232 or RS-232C, so you will need to condition it. You can use an OPamp or simple transistor sort of interface. The Audio-In of most Wireless Tx's is usually a 1V line level type, so some attenuation will probably be needed to prevent overdrivng the input.

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Old 07-18-2004, 09:11 PM
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Default RE: Simple GPS to PC idea

Is that 1V Peak-to-peak on the line input? I am glad it is not a low voltage 10mV mic input like I thought it was. 1V should make design and adjustment alot easier. Also, you suggested to AC couple it to keep them isolated incase anything happens, corret? If I were to do that, would you suggest using an opto-isolator after the opamp. Thanks for the help RC-CAM.
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Old 07-18-2004, 09:23 PM
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Default RE: Simple GPS to PC idea

If your Tx's input consists of an eletret mic on a pigtail, then it will be a mV sort of deal. If it is one of the usual designs that did not ship with a mic, and is just a wire harness, then it will usually be a standard 1V pk-pk line input. They often internally AC coupled so you will probably be able to skip installing your own cap. If you do, be sure to plan for low freq reponse to support the raw (unmodulated) 1200 baud serial data. You do not need an optoisolator, but if that sort of interface is easier for you to design then go for it.
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