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crrimson
Apprentece
USA
5 Posts |
 Posted - Apr 25 2008 : 8:11:52 PM
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Hi, I'm a beginner at electronics, and my last 2 attempts at building an FM telephone circuit have failed. I tried the schematic on your webpage, and I have it all hooked up on a breadboard right now. I can't get a radio to tune in on it. All the parts are nearly identical to the ones on the list, the only difference being my capacitors are only rated for 50V, but I figured the phone line is 48-52V didn't think it would be a problem. Like I said though, I'm really new at this stuff.
Can anyone give me some specific details on how to troubleshoot this circuit and get it working? I would really appreciate it a lot. All I really have is an analog multimeter and my radio :). Thanks, I really hope someone can help. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Apr 25 2008 : 9:56:54 PM
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The circuit operates at the very high frequency of 100MHz. It won't work on a breadboard because the capacitance between the rows and wires is too high and each wire has series inductance that is too high. It needs to be made with very short wires on a pcb or on stripboard with the strips cut short.
It doesn't have any tuning capacitor like all the other FM transmitter circuits. So you must squeeze together or spread out the coil or add a turn ot two or remove a turn or two to tune its frequency. The coil should be made with enamelled wire so the turns do not short together. |
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crrimson
Apprentece
USA
5 Posts |
Posted - Apr 27 2008 : 7:09:49 PM
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| Thanks for the suggestion, I soldered all the parts together, and after expanding the coil a little bit, I actually found my signal on the radio on the low end around 88 MHz! I am so happy it works, usually my circuits fail, so this is awesome! Something kinda strange happens though, when I pick the phone up I can barely hear anything over the radio because it's broadcasting where some other station is, however when I hang up the phone that radio station that was coming in clear, completely disappears! I would put my hand on the circuits antenna and move it around and every so often I would find a spot where it would make the radio station disappear and my phone call come in clear. It's strange. Even the fact that it works at all makes me happy, but I'll keep trying to get it to work right. Maybe I need a longer antenna, or maybe just need to play with the coil some more? I'm not sure. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Apr 27 2008 : 8:10:10 PM
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The circuit is too simple to work properly.
Its antenna is connected directly to its tuned circuit so its frequency changes if something gets near or moves away. It meeds an RF buffer amplifier to isolate its antenna from its oscillator. |
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crrimson
Apprentece
USA
5 Posts |
Posted - Apr 27 2008 : 9:29:16 PM
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| Well it works for me and I'm just glad I had some success building it after a few failures at similar ones. :) |
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crrimson
Apprentece
USA
5 Posts |
Posted - May 01 2008 : 1:26:53 PM
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| What would be a way to improve range on this circuit without modifying the schematic? It says it's supposed to go up to 200 ft., I'm lucky if I can get it to go 50 ft. I have found another circuit that uses a 9V battery for a longer distance transmitter, but I'd like to know if theres any tweaking I can do to this one. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - May 01 2008 : 2:45:36 PM
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| The single red LED reduces its supply voltage to only 1.8V. Try two to five red LEDs in series. |
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crrimson
Apprentece
USA
5 Posts |
Posted - May 02 2008 : 01:25:38 AM
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That's exactly what i'm looking for. But just to boost the distance I added 22 LED's and a maglite in series. Seems to help I talked to my friend Carlos and now he can listen to my phone conversations in Mexico.
Thanks audioguru. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - May 02 2008 : 1:37:27 PM
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The amount of current given to the transmitter depends on how many LEDs are in series, thow much current the phone draws and how much current your telephone line gives (its distance from the exchange).
If the current is high then the LEDs will burn out. |
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FM telephone bug problems
