I am building a simple FM radio transmitter

[qoo]

Well, its not so simple for me actually. Thats the project I have
chosen for this semester. The most basic circuit I have found on
Internet is this:

http://www.electronics-project-design.com/FMTransmitter.html

Please tell me what does BC547 doing here and what is its alternative?
and please do tell me more about that electret microphone. Thanks

 

[Michael Black]

Well, its not so simple for me actually. Thats the project I have
chosen for this semester. The most basic circuit I have found on
Internet is this:

http://www.electronics-project-design.com/FMTransmitter.html

Please tell me what does BC547 doing here and what is its alternative?
and please do tell me more about that electret microphone. Thanks

The description actually is descriptive, even though I likely would
have used other words.

It's pretty common with schematics to start at the left and move right,
so any input or generation is at the left, and any output is at the right.

Hence you have the electret microphone at the left, "mic", and the output
of the transmitter, "aerial", at the right.

The microphone picks up the sound, its audio output gets amplified
by the first transistor (it's first because of that norm of reading
from left to right). The second transistor, ie the one on the right,
is an oscillator, generating a signal in the FM broadcast band.

But you want to send that audio signal out on the airwaves, so there
is that 100n capacitor between the output (ie the collector) of
the first stage and the base of the second stage (ie the oscillator
stage). That audio applied to the oscillator causes it to shift
frequency, following the audio signal as it varies in amplitude
and frequency.

The first stage, ie the audio amplifier stage, just about any
small signal NPN transistor should work there, it's not critical.
Transistors with a "BC" prefix are from the UK and/or Europe, and
you don't generally see them here in North America. The second stage
is more critical, since it has to be able to oscillate at 108MHz,
but a lot of general purpose transistors will still work there. Note
they use the same transistor for both, since it is a cheap transistor.
SOmeone else is bound to post about specific replacements for the BC457.

I thought the description on that page of the electret microphone was
fairly good, though I'm not sure an internal description was needed
for the purpose of the FM transmitter. Most consumer electronics
nowadays uses electret microphones when they have built in microphones,
so a cordless phone, some sort of tape recorder, cellphones, walkie
talkies, baby monitors and answering machines (and I'm sure I've forgotten
some) will have one that you can pull out, so watch those garage sales.
You do have to wire them in properly, they are polarity sensitive but
most I've seen either have color coded leads (red being positive) or
have a "+" sign to mark the positive lead. That positive lead would
be the side of the microphone that goes to the 22K resistor.

Michael

 

[Michael Black]

Actually, for all concerned (regardless of the homework aspect), I would be
interested in hearing this circuit being explained and discected too?

I've tried building these things in the past and they just don't work.
Stage by stage explanation would be brilliant, as well as design caveats
such as grounding. The issue is that almost all of the radio explanations
out there explain in terms for the complete layman, rather than how to
practically do it right there in front of you and why.

The description that page is fairly good, if it's not enough then
you should ask specific questions based on what doesn't make sense
to you.

The first thing I tried to build, when I was 11 years old, was an
"AM wireless microphone", actually it sent morse code. It never
did work, but then I didn't know enough about anything to figure
out where I went wrong. IN retrospect, it might have been as
simple as my lack of soldering skill, or the parts they offered
at the parts store as substitutes (I wouldn't have known whether
they were suitable replacements or not), or the coil had the pinouts
different from the magazine article so I never wired it in properly.
Or I may have simply made a wiring error, and just too inexperienced
to see the error when I looked over the wiring, and not having a clue
about the theory to work through what might be wrong.

In some ways you have to throw away a few early projects, but you
do learn from them. At least I did.

"FM wireless microphones" have the further disadvantage of being
higher in frequency, where wiring becomes a little more of a concern.
Keep the leads short, or don't expect it to work. Also, most people
don't have receivers that cover a lot of the spectrum up there,
so if the thing is mistuned (because the coil isn't done properly
or a capacitor is wrong, or some other factor), the transmitter
might be working fine but the signal not be in the FM broadcast band.

If all you've tried to make are "FM wireless microphones", it makes
sense to start with something simpler. Make a simpler audio oscillator,
so you can work on soldering skills and get a feel for everything. The
wiring and parts won't be so crucial, and the success will likely help
with the next step.

That circuit offered up, you might connect an audio amplifier (a boombox
with external inputs, or your stereo system) to the collector of the
first stage (on the left, where the 10K resistor connects to the
transistor), and see if you hear something when you speak into
the microphone. Simplifying limits the variables, and if you have
success, then you can deal with the second stage. If you get no
audio, then you have to make sure the wiring is right and the microphone
is good, etc.

Michael

 

[Rich Grise]

Actually, for all concerned (regardless of the homework aspect), I would be
interested in hearing this circuit being explained and discected too?

I've tried building these things in the past and they just don't work.
Stage by stage explanation would be brilliant, as well as design caveats
such as grounding. The issue is that almost all of the radio explanations
out there explain in terms for the complete layman, rather than how to
practically do it right there in front of you and why.

Well, we were all laymen once.

The first BC547 is the audio amp/modulator, and the second is the
oscillator/output. The audio is fed to the oscillator's base, which
changes the interelectrode capacitance, which changes the resonant
freq. of the tuned circuit on the fly, frequency modulating it,
albeit poorly. ;-)

The electret mic needs bias because it's essentially a capacitor.

Hope This Helps!
Rich

 

[Michael Black]

The electret mic needs bias because it's essentially a capacitor.

Well no, capacitance microphones need bias because they are a capacitor
and need the bias.

Electrets are basically capacitors, but are permanently biased, that
process of heating it up and placing it in a magnetic field while it cools
(I think that description is wrong, the make your own electret in Popular
Electronics decades ago used High Voltage from a TV set while it cooled).

Electret micrphones need an external source of voltage because
the all have a built in FET buffer, to take the audio signal
off the actual microphone element, which is a very high impedance
point, and transform it down to an impedance that is workable. The
external voltage is used to power the FET, not the actual electret
element. (Capacitor microphones needed a buffer too, but when
they were common there weren't FETs so a vacuum tube had to be
used and those were too bulky to be internal).

Electret microphones are readily available in junk equipment
as I pointed out in another post. It actually is worth sacrificing
one to see what's inside. I took one apart a few years ago, and
had expected to see nothing much. But there was the element, and
there was a plastic encased FET in a small case but still recognizable
as a transistor (I had expected a blog of epoxy) and a resistor.

Michael

 

[David L. Jones]

Well, its not so simple for me actually. Thats the project I have
chosen for this semester. The most basic circuit I have found on
Internet is this:

http://www.electronics-project-design.com/FMTransmitter.html

Please tell me what does BC547 doing here and what is its alternative?
and please do tell me more about that electret microphone. Thanks

Get the Talking Electronics FM bug books:
http://www.talkingelectronics.com.au/shop/index.php/cPath/40?osCsid=bd4ec6d84ef8236ccb8112b5bc31837c
They are not expensive, and explain everything very well.

Dave.

 

[Bob Monsen]

Thanks Michael and Rich

Interesting reading.

In some ways I come from the digital camp where everything is in logical
blocks. Linear stuff just always get's me confused.

Just one question;

The oscillator is the LC tank isn't it?

The following is a short explanation of the circuit given here:

http://www.electronics-project-design.com/FMTransmitter.html

The electret microphone, when 'biased' (essentially, powered) with a
resistor connected to a stable voltage, outputs a small voltage waveform at
the resistor/microphone junction, which is proportional to the pressure
waves in the sound it 'hears'. That microphone can't put out much
voltage/current, though, so the leftmost transistor is used to take this
tiny output (through a capacitor) and amplify it to a larger inverted
waveform.

That larger waveform is then used to control the frequency of an oscillator,
which is initially tuned to output a 'carrier' wave at some high frequency,
possibly between 88MHz and 108MHz. The frequency depends on the inductance,
and capacitance of the variable capacitor. It also depends on the
capacitance between the three terminals of the transistor in a complex way.
Finally, it also depends on that 5.6pF capacitor that goes from collector to
emitter, and the antenna. The transistor is used to add a tiny bit of energy
to the resonant circuit on each cycle to keep it resonating. It resonates
whether or not there is any input from the microphone.

The input waveform (through the amplifying transistor) modifies the
capacitance of the resonant circuit by changing the biasing of the base
terminal. This affects the frequency, since the resonant frequency is
inversely proportional to the square root of the capacitance.

So, this causes the frequency to vary from the tuned frequency by some small
amount which depends on the input sound. This variance can be picked up and
output by an FM receiver as sound.

There are lots of problems with this circuit. It is often difficult to get
it to oscillate at all. The capacitance of your hand near the circuit can
affect the frequency it is tuned to, so it can interfere with other
stations. They often oscillates at the wrong frequency, and instead
generates a harmonic in the FM band rather than oscillating at the desired
frequency. It undoubtedly generates a lot of stray noise into other bands,
but the power is so low that that usually does not matter.

There are much better circuits. For example, here is one:

http://www.jameco.com/webapp/wcs/st...toreId=10001&catalogId=10001&productId=117604

You can find the circuit it uses around the net, and build it yourself, or
just buy the kit, which is very cheap. It is very stable, and can be built
in about an hour. It will nearly always work the first time, unless you
can't follow instructions.

Regards,
Bob Monsen

 

[Rich Grise]

Well no, capacitance microphones need bias because they are a capacitor
and need the bias.

Electrets are basically capacitors, but are permanently biased, that
process of heating it up and placing it in a magnetic field while it cools
(I think that description is wrong, the make your own electret in Popular
Electronics decades ago used High Voltage from a TV set while it cooled).

OK, I stand corrected on this point, but it _does_ still need a power
supply - I at least got that part right.

Thanks!
Rich