Replacing Electret Microphone With Audio Input?

On Feb 18, 2:24 am, default <defa...@defaulter.net> wrote:
On Sun, 17 Feb 2008 03:52:10 -0800 (PST), "Dave.H"



the19...@googlemail.com> wrote:
On Feb 17, 12:55 am, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 17, 12:52 am, default <defa...@defaulter.net> wrote:

On Sat, 16 Feb 2008 05:46:44 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
I've got a 10K audio taper pot. Will that also work.

It will
--

OK. I hope to get this transmitter up and running today. I will post
here with the results.

Built it and rebuilt it and it didn't work. I installed the 12 pF
instead of the trimmer cap, even took it apart and resoldered all
connections. Any suggestions why it doesn't work?

Two things come to mind assuming no mistakes in the construction.

I would disconnect everything west of the vertical 10K that joins with
the base of the transistor - it isn't necessary for oscillation.

Then I would replace the transistor with another type - I've always
had better results with the metal can version of the 2N2222 for FM
mics but change the transistor if you have another NPN good for high
frequency.

The other thing, assuming no oscillation, is the feedback cap. It
shouldn't be very critical but it can go a long way to getting a
transistor that won't oscillate to start. Too high and it won't work
and too low and it won't work . . .

Try a gimmick cap in there. If you have some wire wrap wire or cat 5
USTP those are probably ideal but any solid wire will work. Magnet
wire is another good choice. Vinyl insulation isn't ideal at that
freq so if you can get it to work with vinyl, change it for a lower
loss dielectric to get maximum range.

http://www.shol.com/kuggie/cttip/pix/gimmic.jpg

Two wires twisted together, but not connected, form a "gimmick" cap.

No magic formula for value of the cap - thinner insulation = greater
capacity, longer gimmick = greater capacity, dielectric constant of
the insulation determines capacity also, dissipation factor of the
insulation = Q (quality).

good insulations - Teflon, polyethylene, Tefzel, Delrin, enamel (low
dissipation factor)

Without modulation - it can be hard to hear the transmitter. It will
sound like a Thump when you tune through it on the receiver and it
should be able to kill the sound of a weak FM station when it is
tuned to the same freq.. On an analog TV with antenna, it will look
like wavy lines or a herringbone pattern.
--
Click to expand...
I have spaced the components too far apart, that could be the
problem. The only thing I've done other than rebuild it is check for
a signal on an FM radio, I will perform other tests later, I have
learnt to leave things like this for a few days then come back to
them.
 
On Feb 17, 12:55 am, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 17, 12:52 am, default <defa...@defaulter.net> wrote:

On Sat, 16 Feb 2008 05:46:44 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
I've got a 10K audio taper pot. Will that also work.

It will
--

OK. I hope to get this transmitter up and running today. I will post
here with the results.
Click to expand...
Built it and rebuilt it and it didn't work. I installed the 12 pF
instead of the trimmer cap, even took it apart and resoldered all
connections. Any suggestions why it doesn't work?
 
On Sun, 17 Feb 2008 03:52:10 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

On Feb 17, 12:55 am, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 17, 12:52 am, default <defa...@defaulter.net> wrote:

On Sat, 16 Feb 2008 05:46:44 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
I've got a 10K audio taper pot. Will that also work.

It will
--

OK. I hope to get this transmitter up and running today. I will post
here with the results.

Built it and rebuilt it and it didn't work. I installed the 12 pF
instead of the trimmer cap, even took it apart and resoldered all
connections. Any suggestions why it doesn't work?
Click to expand...

What method did you use to build it? It is a 100 MHZ circuit so
layout can be critical -

The next question would be what is the transistor doing? What is the
voltage from base to ground? What is the voltage from ground to the
emitter? (and if it were oscillating, taking a reading there would
stop it).

Can you / have you measured current consumption of the circuit?

What steps have you taken to verify it isn't working and what steps
have you taken to fix it other than checking your connections?
--
 
On Sun, 17 Feb 2008 03:52:10 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

On Feb 17, 12:55 am, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 17, 12:52 am, default <defa...@defaulter.net> wrote:

On Sat, 16 Feb 2008 05:46:44 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
I've got a 10K audio taper pot. Will that also work.

It will
--

OK. I hope to get this transmitter up and running today. I will post
here with the results.

Built it and rebuilt it and it didn't work. I installed the 12 pF
instead of the trimmer cap, even took it apart and resoldered all
connections. Any suggestions why it doesn't work?
Click to expand...
Two things come to mind assuming no mistakes in the construction.

I would disconnect everything west of the vertical 10K that joins with
the base of the transistor - it isn't necessary for oscillation.

Then I would replace the transistor with another type - I've always
had better results with the metal can version of the 2N2222 for FM
mics but change the transistor if you have another NPN good for high
frequency.

The other thing, assuming no oscillation, is the feedback cap. It
shouldn't be very critical but it can go a long way to getting a
transistor that won't oscillate to start. Too high and it won't work
and too low and it won't work . . .

Try a gimmick cap in there. If you have some wire wrap wire or cat 5
USTP those are probably ideal but any solid wire will work. Magnet
wire is another good choice. Vinyl insulation isn't ideal at that
freq so if you can get it to work with vinyl, change it for a lower
loss dielectric to get maximum range.

http://www.shol.com/kuggie/cttip/pix/gimmic.jpg

Two wires twisted together, but not connected, form a "gimmick" cap.

No magic formula for value of the cap - thinner insulation = greater
capacity, longer gimmick = greater capacity, dielectric constant of
the insulation determines capacity also, dissipation factor of the
insulation = Q (quality).

good insulations - Teflon, polyethylene, Tefzel, Delrin, enamel (low
dissipation factor)

Without modulation - it can be hard to hear the transmitter. It will
sound like a Thump when you tune through it on the receiver and it
should be able to kill the sound of a weak FM station when it is
tuned to the same freq.. On an analog TV with antenna, it will look
like wavy lines or a herringbone pattern.
--
 
On Feb 17, 12:52 am, default <defa...@defaulter.net> wrote:
On Sat, 16 Feb 2008 05:46:44 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
I've got a 10K audio taper pot. Will that also work.

It will
--
Click to expand...
OK. I hope to get this transmitter up and running today. I will post
here with the results.
 
On Sat, 16 Feb 2008 05:46:44 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

I've got a 10K audio taper pot. Will that also work.
Click to expand...
It will
--
 
On Sat, 16 Feb 2008 05:35:17 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

On Feb 17, 12:28 am, default <defa...@defaulter.net> wrote:
On Sat, 16 Feb 2008 03:01:28 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:

The website also mentions replacing a 10K resistor, with a pot. Would
I need this if I drive the transmitter from an ipod headphone output,
and if so, what resistor should be removed? There are 5 10k units
there.

You see the vertical resistor just on the input where the LR channels
come together? Put a 10K pot in place of it, with the junction of the
LR 10K's to the full clockwise position and ground on the CCW
position. Change the position of the 100 nf to the wiper of the pot
so it connects there and nowhere else.

--

OK thanks, I assume it doesn't matter if I use a linear taper or audio
taper? I want to use a linear taper.
Click to expand...
It doesn't matter to me . . . Seriously: If you anticipate using it
frequently to adjust volume you might be happier with an audio taper
pot. If you set it once and forget about it, it hardly matters.

The question is how constant is the volume from the 'pod? If you
think the control on your receiver is good enough to compensate use a
linear.

It doesn't matter one way or the other - it just "feels" right when
you use an audio pot for audio signals. I put a linear pot into a
stereo I built at my office/shop where I was working. It had 50 W a
channel and was too much for the little office but everyone who used
the control said it "didn't have enough volume." Never bothered me
though.
--
 
On Feb 17, 12:35 am, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 17, 12:28 am, default <defa...@defaulter.net> wrote:



On Sat, 16 Feb 2008 03:01:28 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:

The website also mentions replacing a 10K resistor, with a pot. Would
I need this if I drive the transmitter from an ipod headphone output,
and if so, what resistor should be removed? There are 5 10k units
there.

You see the vertical resistor just on the input where the LR channels
come together? Put a 10K pot in place of it, with the junction of the
LR 10K's to the full clockwise position and ground on the CCW
position. Change the position of the 100 nf to the wiper of the pot
so it connects there and nowhere else.

--

OK thanks, I assume it doesn't matter if I use a linear taper or audio
taper? I want to use a linear taper.
Click to expand...
I've got a 10K audio taper pot. Will that also work.
 
On Feb 17, 12:28 am, default <defa...@defaulter.net> wrote:
On Sat, 16 Feb 2008 03:01:28 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:

The website also mentions replacing a 10K resistor, with a pot. Would
I need this if I drive the transmitter from an ipod headphone output,
and if so, what resistor should be removed? There are 5 10k units
there.

You see the vertical resistor just on the input where the LR channels
come together? Put a 10K pot in place of it, with the junction of the
LR 10K's to the full clockwise position and ground on the CCW
position. Change the position of the 100 nf to the wiper of the pot
so it connects there and nowhere else.

--
Click to expand...
OK thanks, I assume it doesn't matter if I use a linear taper or audio
taper? I want to use a linear taper.
 
On Sat, 16 Feb 2008 03:01:28 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

The website also mentions replacing a 10K resistor, with a pot. Would
I need this if I drive the transmitter from an ipod headphone output,
and if so, what resistor should be removed? There are 5 10k units
there.
Click to expand...
You see the vertical resistor just on the input where the LR channels
come together? Put a 10K pot in place of it, with the junction of the
LR 10K's to the full clockwise position and ground on the CCW
position. Change the position of the 100 nf to the wiper of the pot
so it connects there and nowhere else.


--
 
On Feb 16, 12:32 am, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 16, 12:27 am, default <defa...@defaulter.net> wrote:



On Fri, 15 Feb 2008 05:21:19 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
On Feb 16, 12:16 am, default <defa...@defaulter.net> wrote:
On Fri, 15 Feb 2008 04:56:18 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
On Feb 15, 2:18 pm, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 15, 1:52 pm, default <defa...@defaulter.net> wrote:

On Thu, 14 Feb 2008 18:38:55 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
On Feb 15, 1:20 pm, default <defa...@defaulter.net> wrote:
On Thu, 14 Feb 2008 18:15:00 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:

default wrote:
On Thu, 14 Feb 2008 15:45:31 -0800 (PST), "Dave.H"
the19...@googlemail.com> wrote:

I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?

Of course. You wouldn't have stereo with a simple one or two
transistor transmitter - so you may also want to combine both channels
going in.
\

I understand that, but my radio receiver is a mono type anyway, so I'm
not looking for a stereo version.
--

sorry I somehow got the idea that the Ipod would be the music source.
--

iPod is the music source, it will be broadcasting to the mono radio. I
will use that circuit I linked to before, seems to have the audio
channels combined by 10K resistors so I don't have to worry about
that. Even though I've already got a device that combines left and
right with 1.8K resistors.

You have a better idea of what you are doing than I do. The Ipod
output has to (should) be mixed to drive the transmitter with the
contents of both channels. That can be as simple as two resistors
from each channel to a common input.

Not aware of the circuit you linked to before. You aren't referring
to the one where you ground the two signals to control volume are you?
--

No, the one @http://www.techlib.com/Karen/radio.htm#FM%20Transmitter

The article linked to previously mentioned using a 5-20 pF trimmer cap
instead of a fixed capacitor. What capacitor do I need to replace to
install a 4.2pF-20pF trimmer? This would make it easier to tune.

The 12 pf cap is the tank circuit tuning cap; spreading or
compressing the coil will also tune it.

The 3 pf is a feedback cap and gets it oscillating. The .01 is a
bypass cap and not critical to anything. The 1 nf on the base is a
bypass but will affect audio frequency's as well.
--

So if I got this right, I replace the 12 pF?

Yes, put a variable in place of it. The plate of the variable that
the adjustment screwdriver touches should be the ground side so the
adjustment tool doesn't cause it to change freq..

Just look at the cap to determine which side should be ground, It is
normally obvious.
--

Thanks, I don't really like the idea of fiddling with the coil, I like
to leave coils alone. The trimmer I'm going to use is a PCB mount
one, from Dick Smith of course Cat # R2945
Click to expand...
The website also mentions replacing a 10K resistor, with a pot. Would
I need this if I drive the transmitter from an ipod headphone output,
and if so, what resistor should be removed? There are 5 10k units
there.
 
D

Dave.H

Guest
I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?
 
On Thu, 14 Feb 2008 15:45:31 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?
Click to expand...
Of course. You wouldn't have stereo with a simple one or two
transistor transmitter - so you may also want to combine both channels
going in.

Electret's have built in preamplifiers or impedance matching amps and
they usually impress a DC voltage on the audio connection to the
electret mic.. You just need to disconnect that resistor and not use
it.

There also may be a need for a capacitor to couple audio in without
upsetting the DC bias of the transmitter.
--
 
On Thu, 14 Feb 2008 18:08:48 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

On Feb 15, 10:45 am, "Dave.H" <the19...@googlemail.com> wrote:
I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?

I found a circuit that does use a regular audio input, but I can't
find a couple of capacitor values at Dick Smith. The caps have to be
ceramic. I can find a 3.3 pF for the 3 pF, but I can't find a 10 nF
ceramic unit. Would it be OK to just parallel 10 1 nF caps, until I
can find a 10 nF unit?
Click to expand...
Yeah the 3.3 is good enough. Isn't 10 nano farad just another way of
saying .01 microfarad? .01 microfarad is very common.
--
 
On Thu, 14 Feb 2008 18:15:00 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

default wrote:
On Thu, 14 Feb 2008 15:45:31 -0800 (PST), "Dave.H"
the1930s@googlemail.com> wrote:

I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?

Of course. You wouldn't have stereo with a simple one or two
transistor transmitter - so you may also want to combine both channels
going in.
\

I understand that, but my radio receiver is a mono type anyway, so I'm
not looking for a stereo version.
--
sorry I somehow got the idea that the Ipod would be the music source.
Click to expand...
--
 
"Dave.H" (the1930s@googlemail.com) writes:
default wrote:
On Thu, 14 Feb 2008 15:45:31 -0800 (PST), "Dave.H"
the1930s@googlemail.com> wrote:

I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?

Of course. You wouldn't have stereo with a simple one or two
transistor transmitter - so you may also want to combine both channels
going in.
\

I understand that, but my radio receiver is a mono type anyway, so I'm
not looking for a stereo version.
Click to expand...
But if you don't combine the two channels at the receiver, you'll only
be getting some of the content.

IN order for there to be stereo, there has to be difference between the
channels. How it's arranged can be quite complicated and depends on
what was recorded and how the recording engineer felt things should sound.

If you only have one channel of a stereo signal, you will lose content.
So that trumpet over in the right channel that is only on the right channel
to give a sense of space, will be missing if all you feed into the transmitter
is the left channel. Other things may still be there, but lacking in volume
because the intent was to place it somewhere between the two channels.

Michael
 
"Dave.H" (the1930s@googlemail.com) writes:
On Feb 15, 10:45 am, "Dave.H" <the19...@googlemail.com> wrote:
I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?

I found a circuit that does use a regular audio input, but I can't
find a couple of capacitor values at Dick Smith. The caps have to be
ceramic. I can find a 3.3 pF for the 3 pF, but I can't find a 10 nF
ceramic unit. Would it be OK to just parallel 10 1 nF caps, until I
can find a 10 nF unit?
Click to expand...
10nF is .01.

1nF= .001
10nF=.01

Thus you need the extremely common .01uF capacitor. And I suspect
that's not even critical, such a large value is bound to be a bypass
capacitor and hence something around that value will do fine.

Michael
 
On Feb 15, 10:45 am, "Dave.H" <the19...@googlemail.com> wrote:
I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?
Click to expand...
I found a circuit that does use a regular audio input, but I can't
find a couple of capacitor values at Dick Smith. The caps have to be
ceramic. I can find a 3.3 pF for the 3 pF, but I can't find a 10 nF
ceramic unit. Would it be OK to just parallel 10 1 nF caps, until I
can find a 10 nF unit?
 
On Feb 15, 1:08 pm, "Dave.H" <the19...@googlemail.com> wrote:
On Feb 15, 10:45 am, "Dave.H" <the19...@googlemail.com> wrote:

I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?

I found a circuit that does use a regular audio input, but I can't
find a couple of capacitor values at Dick Smith. The caps have to be
ceramic. I can find a 3.3 pF for the 3 pF, but I can't find a 10 nF
ceramic unit. Would it be OK to just parallel 10 1 nF caps, until I
can find a 10 nF unit?
Click to expand...
Forgot the link: http://www.techlib.com/Karen/radio.htm#FM%20Transmitter
 
default wrote:
On Thu, 14 Feb 2008 15:45:31 -0800 (PST), "Dave.H"
the1930s@googlemail.com> wrote:

I am looking at a number of FM transmitter circuits, purely as a
project, but all of them use electret microphones as input. Would it
be possible to replace this with an audio input for my iPod?

Of course. You wouldn't have stereo with a simple one or two
transistor transmitter - so you may also want to combine both channels
going in.
\
Click to expand...

I understand that, but my radio receiver is a mono type anyway, so I'm
not looking for a stereo version.
> --
 
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