oscillatorsoundspeakers
I've connected an 8-ohm speaker in series with coil. When I apply 5 volts / 500 mA to the combination (coil and speaker), I heard a very high frequency sound (something like whistling).
The coil is not air-cored (I think the core is made of ferrite or iron) and it has about 50 turns.
I would like to know why this circuit oscillates or produce this sound?
It's a phase shift oscillator.
Normally, feedback from the collector to the base acts "negatively" and this is quite important for some amplifiers. This is because the collector signal is the inverse of the base signal (also known as 180º out of phase). Anything fed back does so without causing oscillations. This type of feedback is also used in op-amps for controlling gain.
On the circuit in the question there are a bunch of components that take the collector signal and phase shift it enough so that at a particular frequency, it appears in phase with the base signal and reinforces it. This makes it oscillate.
On a more technical level, the feedback formed around R2, R3, R4, C1, C2 and C3 act as a "mild" notch filter. It should be said that the intent of a "good" notch filter is to totally remove one frequency (such as 50Hz or 60Hz when mains AC is a problem). The frequency which is notched out will be phase shifted by 180º and if it isn't totally notched-out (as in a good notch filter) what remains will feed back and reinforce the original base signal causing it to oscillate.
It doesn't matter that the signal might be attenuated by 20dB, there will still be enough signal left to be amplified and generate a sinewave.
It's a bit like asking how to build an internal combustion engine from a gearbox. It's just easier to get a receiver circuit and build it without reference to a transmitter circuit. AM or FM Receivers are usually more complex than an AM or FM transmitter. My advice is get a circuit by googling and disregard any commonalities with the transmitter you have built.
Why is it capable of being received on a whole load of frequencies even when you radically change tank capacitance and inductance?
Answer: because it's a badly designed transmitter with lots of harmonics being generated. You said you received it on 95MHz and 108MHz - it may well be transmitting on a couple of spaces in between that a digitally tuned FM receiver isn't able to perfectly tune to. FM receivers in the UK tune every 200kHz and conceivably there could be a couple of spots that they jump over.
You put a monstruous 1000uF cap across the coil - that cap at the sort of frequencies being transmitted probably looked like an inductor in series with several ohms and maybe moved things about a bit but, due to so many harmonics being generated you were still bound to find that an FM receiver could still pick-up the transmission. Here's a graph showing the impedance of an electrolytic capacitor changing to inductance above a certain frequency: -
How long is a piece of string or how much inductance has a piece of wire - wire has inductance and getting something like 0.1uH is trivial - 10 cm of wire has around 100nH = 0.1uH.
Best Answer
Modern cell phone chargers operate a very high frequency- well above audible even for small dogs.
However, what I suspect is happening is that your inductor is perhaps 1mH and your adapter is perhaps capable of 600mA before overcurrent protection kicks in.
So, when you connect a fixed voltage to an inductor in series with a resistor, the current increases with time constant L/R towards Vin/R = 0.65A (allowing a bit of resistance for the inductor- and the DCR of the speaker won't be exactly 8 ohms).
This time constant (based on my guess as to the inductance) is of the order of 125usec, so if the overcurrent protection kicks out then restarts, the frequency of restarting should be in the several kHz range.
You don't hear the sound when the speaker is connected directly across the power supply because the restart frequency is above the audible range.
To confirm this, look at the voltage and current waveforms with an oscilloscope.