It looks to me that the first circuit exploits a "parasitic" rf-oscillation to make it a transmitter. I cannot figure out what the role of the reverse-biased LED is, unless it is being used a small value capacitor. Gawd only knows where the thing is oscillating. You are likely tuning a harmonic that happens to land in the FM band, so the signal would be very weak.

On the second one, the coil should be fabricated as a single winding with a tap ~1/3 of the way from the bottom end (autotransformer). If you built it as two separate randomly placed coils, it will not oscillate. Google "Hartley Oscillator".

Read this current thread in these forums.

Hi,

thanks for your answer. You're right, the barrier layer capacitance of the LED is used as a 10 p capacitor. If this circuit doesn't oscillate between 88 and 108 MHz (you said it's only a harmonic), at which frequency is it approximately oscillating? Is there a solution for this problem without much more components?`

In the second schematic aren't actually two coils, it's one coil with a tap

I said that before. In the second schematic it's drawn as two coils because my programm cannot handle taps.
I've got this like an autotransformer. There is a picture in the attachment. Is this coil ok?
What about the other transmitter in the attachment? Better than the first?

Greetings

BC548C

If you built it as two separate randomly placed coils, it will not oscillate. Google "Hartley Oscillator"

I have a 1Mhz Hartley that uses two seperate coils and works, though it's a common emitter type.

Is this coil ok?

I have a similar transmitter working and the coil is 10 turns of #22 hookup wire wound on a 1/4" diameter. I used a machine screw to wind it on, and since the coils don't touch, no insulation is necessary. Never tried to wind one with the plastic insulation, but that would be interesting.

hey can anyone tell me why we use C7 capacitor (in BC548C's attatchment)????

It injects a portion of the L/C signal into the transistor emitter, which is needed for sustained oscillations.

I'll try to make a schematic that I can upload, if I don't forget. If I do forget, then PM me.

Thank you!

Originally Posted by rahul.sadtm

hey can anyone tell me why we use C7 capacitor (in BC548C's attatchment)????

It's the feedback for the oscillator, which runs in common base.

These circuits are not impressive. The last one especially. Minimum parts count does not improve reliability or performance...if it will work at all.
Take a look at Audio Guru's proven design for transmitting music to an FM radio.
You can find it on this forum. Pictures and all :-)

I designed my FM tramsmitter to fix problems complained about a "Simple FM Transmitter" that did not work;
1) I added an RF amplifier to the output so that the power is increased and as a buffer beween the oscillator and the antenna so that the frequency does not change when something moves towards or away from the antenna.
2) I added a low-dropout voltage regulator so that the oscillator's frequency does not change as the battery runs down, so that the preamp transistor works perfectly when the battery is new or is old and so that the transmitter still works when the battery voltage is very low.
3) I added pre-emphasis (treble boost) like FM radio stations have so that the sound heard on a normal FM radio (that has de-emphasis) is perfect and not muffled like simple FM transmitters produce.

But it is illegal because it output power is too high and its range to a sensitive hi-fi or good car radio is over 2km.