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FM transmitter unstable when holding ipod

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spitso

Member
Hi

I have made a FM transmitter named "The Wasp" located here THE WASP Page1.
Instead of conecting a microphone ive conected an ipod on low volume. It works perfectly untill i hold the Ipod to change song and major distortion as well as frequency changes occur.

Any ideas on how to eliminate this?

attached is schematic

thankyou Spitso
 

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Hero999

Banned
All sorts of things could be happening when you hold the Ipod. For example you could be acting as an aerial and coupling RF to the ipod or you could be acting as a capacitive ground plane which would enhance the effect of the RF injected into the ipod.

This isn't a very good circuit because it doesn't have a voltage regulator its frequency will drift as the battery discharges and it doesn't have any pre-emphasis so will sound like an AM radio.
 

RadioRon

Well-Known Member
I suggest that you add a ceramic capacitor, 100pF, from the IPOD music input of the transmitter to ground and place it with shortest possible lead length right where the ipod connects to your board. In addition, you might want to try winding the cable that connects the ipod to the transmitter into a coil of about 8 turns with 1 inch diameter (very approximate) with this coiled section being closest to the transmitter, not the ipod.

Both of these things are meant to keep the transmitter RF energy away from the ipod.
 

chemelec

Well-Known Member
I just bought a POS-150 from Electronic Goldmine. Part Number "A20392"
(On Sale at $4.00 each This Month.)
Output is a Voltage Controlled Frequency from 75 to 150 Mhz and Stable.
Modulating it is Easy, and I added a small output RF Amp to it and I'm getting a really good range. (Over 1 Mile with the Antenna Tuned up.)

Probably Not Legal, But I live FAR Away from any FM Transmissions, so I am not worried about that.
Actually NO Radio Reception here at all, AM or FM. (In a Valley, with mountains on 4 sides)
 

Hero999

Banned
I suggest that you add a ceramic capacitor, 100pF, from the IPOD music input of the transmitter to ground and place it with shortest possible lead length right where the ipod connects to your board. In addition, you might want to try winding the cable that connects the ipod to the transmitter into a coil of about 8 turns with 1 inch diameter (very approximate) with this coiled section being closest to the transmitter, not the ipod.

Both of these things are meant to keep the transmitter RF energy away from the ipod.
All good ideas, what about adding a ferrite bead?
 

spitso

Member
also with relation to varying frequencys. I read about how a circuit from this page Long range FM transmitter Does it by adjust voltage through a diode which changes capacitance of it acting as a variable capacitor. Any ideas how i could incorparate this into my transmitter?

Attached is the circuit which uses a variable resistor to change capacitance of diode therefore changing frequency to change.
 

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audioguru

Well-Known Member
Most Helpful Member
Make the Long Range FM transmitter if you want. It also does not have pre-emphasis so it will sound lousy on an FM radio.
 
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