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Some More Simple FM Transmitters

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gotcha, cheers!

Since you seem so clued up, hope you wont mind one more piece of re-assurance:

I'm being cheap and pulled some enamaled copper wire out of an old transformer. It's not 1mm diameter though, and my best estimate puts the diameter at .6-0.65mm (somewhere in there). I used a formula I found on another website (Low Power FM Transmitter) to figure out a number of spires needed coiling it round a 2.3mm(ish) core to give a 0.1uH inductance.
The formula was: L = N² * r² / (228r + 254l), where N is # of turns, r is average radius (approx 1.5mm in my case, by my estimates). I've ended up doing 9 turns.

To be honest I don't expect you to know, but in your experience, am I going to have any problems using these custom coils - or am I really going to need 1mm diameter wire (would the resistance affect things massively)?

Thanks in advance if you have anything to say about that.

Nigel Goodwin

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The thickness of the wire makes little difference, try them as the original design.

Bear in mind though that self supporting coils need to be just that, so the wire must be thick enough to be really solid.


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Depending on the stray capacitance of your layout, your coils might need to be squeezed together or spread apart for you to tune the transmitter at the correct frequencies.
Buy a transceiver. They're not expensive. Without a ham license making a transmitter is illegal anyways.
How would you go about debugging one of these circuits? I've finished soldering up a version of the transmitter from this forum by audioguru, but tuning to a slightly fuzzy spot on a radio beside the transmitter, I tried tuning the transmitter to see if I could get any sound, or any noticeable change in the radio's output, but none came. Short of getting at an oscilloscope, I don't really know what to do to check if the transmitter is doing anything?

The only test I did while building the thing was to check the 5V regulator after soldering it with it's capacitors, and it gave a 5V DC nicely. Once everything else was soldered, I guess I would not expect the 5V DC to still be there, because now there are resistors and capacitors running between the ground and regulator output, so I'm not really sure what I can check.

Is there something I can build to see if the circuit is oscillating at all?


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Of course the output of the 5V regulator should be 5V, even when it is loaded.

I tried tuning my FM transmitter with my Sony Walkman FM radio. The radio was badly overloaded and the signal was all over the dial, even when I switched the Local-Distant antenna attenuator to Local.

I made a field strength circuit:


my stripboard has a divide down the center, so it's relatively straightforward for me to remove half of the circuit and see what happens. On one side I have the voltage regulator and preamp, on the other I have the two tank circuit stages. When I disconnect the two tank circuits I get a good 5V reading. However, when I just tried hooking up the 9V supply to the RF amplifier (i.e. the Q3 and associated tank circuit part), I lost my 5V. Essentially the circuit is wired up but with a break above L1 and C6, and C3 completely absent (hence another break).

What could be causing my Q3 stage of the circuit to just drain almost completely to zero. The only part I have doubts about atm is my variable capacitor, which came with three legs, but two of which seemed to join to the same metal plate, so I have just got them on the same strip of my board, with the third leg forming the other end of my variable capacitor. (if needed, they're the variable capacitors available from Maplin Electronics ? Website- Ion Film and Slide Scanner)

Any ideas?


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You disconnected the oscillator's L and C so it draws almost nothing. The preamp transistor draws 0.3mA and the microphone daws 0.3mA. The regulator draws 0.4mA.

But look at Q3. If its hFE is 100 then its drain on the battery is 18mA. If the hFE is 180 then the drain on the battery is 32mA.
I think your 9V battery is dead since it has trouble supplying 30mA to 40mA.

My FM transmitter drew 53mA. A brand new name-brand (not Chinese) 9V battery dropped to 8V in 1 hour.

The trimmer capacitors have 3 legs (my tiny ones have 2 legs). Two legs on yours are the same connection. You connected yours correctly.
I need help building an AM receiver

Hi all!

I would like to work on a project this summer, I need to build an AM radio receiver using discrete components, no IC's. I kindda know about all the sub-systems of the project, except the local oscillator and the mixer. I know what they do and why I need them in my circuit, but I am still not sure how to start designing and building them. I am assuming I can build a local oscillaor using BJt transistors, not sure if I would be able to get a sine wave as my output though. On the other hand, I suppose the mixer is a multiplier that will multiply the signal, coming from my antenna after it was amplified, with the sine wave generated by my local oscillator. Now how I would implement this multiplier circuit, any hints or clues on this. Please relpy with any little info. It would all help, and thanks all in advance.

P.S. I wanted to post this as a new thread but could not as I dont have that prevelidge yet to initiate threads. If someone would post it as a new thread so that more members would see it that would be great.
I wonder is you could make an FM transmitter by using a coil as a microphone: as the coil vibrates the inductance will change causing the frequency to change.


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There are a lot of technical mistakes in the three FM transmitter circuits presented above.
Once you build them you will see how defective they are.
The optimum voltage for a simple FM transmitter is 6v. Above this, the voltage is wasted and below this, the output is not as high.
The secret to stability and high output is a tight circuit.
We have achieved 27km in a line-of-sight transmission from a 3v design (10mW output) so FM transmission is highly effective.

If you use a thin wire for the transmitting coil, it will vibrate when the bench is tapped and transmit the sound.
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Philips' datasheet for a 2N2222 in a metal package lists its minimum transistion frequency at 250mHz so it will oscillate at 100MHz perfectly.
A BC547 and BC548 are 150MHz so they are also fine.
A 2N3904 is 300MHz so it is fine.
Their current gain are almost the same so nothing in the circuit needs changing.
There are many more.
Hi. (Sorry for the late reply). So, any transistor that has the same specifications as the 2N2222 will work fine right?
thank you for your circuit. I am in the process of the completing the build today and my query related to the tuning of the transmitter. Can you please enlighten me as to what should be the tunning sequence as to VC1, VC2.

I plan to use it to transmit a 16Hz blip to a distance of 1.2KM as a status check of a mechanical device. No elec. interference is around and line of sighht is available for installation Do you forsee any problems with that!. FM would be below 89.9 or above 104 to 108 as these freq. are not used in the desert where it will be used.

Thank you again for any input that you or any of the others may have on this.


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You have too many posts about the FM transmitter. First you must fix its wrong transistor voltages.

Q2 is the FM oscillator and it must be tuned to the frequency you want to use. Q3 is the RF amplifier that has very broad tuning. it is tuned for a peak at the frequency you are using.

The range is far to a very sensitive high quality FM radio. If the radio is cheap and poor quality then the range is only across a street.

The audio response at 16Hz is poor.
Of course the output of the 5V regulator should be 5V, even when it is loaded.

I tried tuning my FM transmitter with my Sony Walkman FM radio. The radio was badly overloaded and the signal was all over the dial, even when I switched the Local-Distant antenna attenuator to Local.

I made a field strength circuit:

the diode does half wave rectification the 1nF does smothing and the 100K must be the output impedance...

but why you need the 22-100pF capacitor and the 10K resistor to the diode?
also can the diode be a BAT41?
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