# Need help building FM Transmitter

## Main Question or Discussion Point

So I'm trying to get my fm circuit to work. I've taken the design from the website below.

http://cappels.org/dproj/FMdist/fmdis.htm

http://img217.imageshack.us/img217/6839/fmdis3un4.jpg [Broken]

My current issue is that the circuit works, but only when hooked to a DC transformer. I get a signal of ~ 102 megahertz, but thats with a DC transformer hooked to it. Note the DC transformer is one we use at school, it has a varriable setting of 1-6 volts, and then 50/100/150/250/500 volts. I used the 1-6 connetions, and experimented between 1 and 1.5v, and same minimal difference.

Doesn't work:
AA battery, 1.3 volts, .0010 amps
9volt battery 8.8volts .03 amps

Works:
DC transformer 1-1.5v, .0015 amps

WHY? Why do I get no frequency with the battery but with a DC transformer I get a frequency?

Only thing I can remotely think of is maybe something having to do with the transformer converting the AC current to DC and causing a ~100 MHz frequncy? But I don't know as AC current is only 60 herts here in the USA...

Pics of circuit, and in these pictures the transistor is flipped. I made a mistake, and since then the transistor was repositioned and soldered intot he correct spot, so don't worry about that. (I had flipped the B and C of the transistor. I had E-C-B, its suppose to be E-B-C, and right this moment and during my test with the DC transformer and batteries it was E-B-C.)

http://img527.imageshack.us/img527/5135/1005418qb9.jpg [Broken]

http://img523.imageshack.us/img523/2799/1005420wq0.jpg [Broken]

http://img405.imageshack.us/img405/3905/1005424hg7.jpg [Broken]

http://img523.imageshack.us/img523/716/1005426ca9.jpg [Broken]
(^^ the gray stereo cable is for input, the to wires stuck to the ductape are unitl I go get a AA size battery holder, and the long black wire thats curled is the antenna)

Thanks for the help guys. I've been looking for a Engineering forum for a while, this beats posting on ocforums.com (computer forums;)).

Last edited by a moderator:

Related Electrical Engineering News on Phys.org
chroot
Staff Emeritus
Gold Member
Why don't you give us a schematic of your circuit? Your pictures are useless.

Also, there's no such thing as a "DC transformer."

- Warren

Why don't you give us a schematic of your circuit? Your pictures are useless.

Also, there's no such thing as a "DC transformer."

- Warren

http://img217.imageshack.us/img217/6839/fmdis3un4.jpg [Broken] (the schematic of the circuit was on the page I linked)

DC generator then?..You know what I am talking about.

Last edited by a moderator:
chroot
Staff Emeritus
Gold Member
No, I have no idea what you're talking about. Is this "DC Generator" just a variable-voltage DC source? How exactly are you "hooking" this into your circuit?

- Warren

No, I have no idea what you're talking about. Is this "DC Generator" just a variable-voltage DC source? How exactly are you "hooking" this into your circuit?

- Warren
It a unit, it converts AC current to DC current. I plug it into a wall outlet. On the front of the unit I hav several choices of were I can plug wires into (several female jacks) to get different voltages. 1-6v(its adjustable with a knob) and then it goes 50/100/150/250/500, and those are not adjustable, they are just straight connetions.

What I did was plug hook the unit up to my circuit where the battery would go. So instead of the battery I used wires that have alligaotr clips on one end and male connection on the other, to attach the DC power unit to my circuit.

DC Power supply is what I should have called it.

Look at this:
http://electronickits.com/kit/complete/powe/xp720.jpg [Broken]

Thats not the unit I have, and the one I use only has DC options and other stuff that is different. But that unit is the same idea as mine.

Last edited by a moderator:
chroot
Staff Emeritus
Gold Member
Okay, it's just a variable-voltage DC power supply. If you have at one time applied 9V across your transistor, though, you might have damaged it. There should be no difference between a 1.5V battery and a 1.5V power supply, given that each can supply the same amount of current.

- Warren

Okay, it's just a variable-voltage DC power supply. If you have at one time applied 9V across your transistor, though, you might have damaged it. There should be no difference between a 1.5V battery and a 1.5V power supply, given that each can supply the same amount of current.

- Warren
I'm pretty sure the transistor is fine, as after I ran 9v through it I tried the AA and ths Power Supply again. With the Power suply working and the AA not. I'll swap out the transistor again tomrrow though, just to check.

I know, it make no sense, I can not figure out why it works with the Power Supply and not the AA. I mean as I already psoted there is a .0005 difference in the amps, but is the really enough to make a difference? I doubt it, I'd be willing to bet that the +/- % of the Digital multimeter would neglect the .0005 difference...

chroot
Staff Emeritus
Gold Member
Well, the AA battery definitely has a higher series resistance than does your DC power supply, but it's not going to be anywhere close to the 330 ohm biasing resistor on the schematic.

When you say it "doesn't work" with a AA battery, what exactly do you mean? How are you verifying the functionality of the circuit?

- Warren

Well, the AA battery definitely has a higher series resistance than does your DC power supply, but it's not going to be anywhere close to the 330 ohm biasing resistor on the schematic.

When you say it "doesn't work" with a AA battery, what exactly do you mean? How are you verifying the functionality of the circuit?

- Warren
Doesn't work. Meaning that I get voltages everywhere, but I get no frequency on the osciliscope, no reading, just a flat line.

chroot
Staff Emeritus
Gold Member
Doesn't work. Meaning that I get voltages everywhere, but I get no frequency on the osciliscope, no reading, just a flat line.
Are you putting a signal into the "headphone jack," i.e. into the base of the transistor? What kind of signal is it?

- Warren

Are you putting a signal into the "headphone jack," i.e. into the base of the transistor? What kind of signal is it?

- Warren
I plugged in my iPod shuffle to it. I had my iPod shuffle playing every time I had the circuit hooked to the osciliscope, and I made sure the same song was playing ever time. (Meatloaf-I'd do anything for love (but I won't that), so its a nice long song:tongue: ). So it was a stereo singal from my iPod Shuffle playing into the base of the transistor.

chroot
Staff Emeritus
Gold Member
So, using the AA battery, you can see your Meatloaf waveform at the base of the transistor, but you get nothing but a flat line at the collector of the transmitter?

- Warren

So, using the AA battery, you can see your Meatloaf waveform at the base of the transistor, but you get nothing but a flat line at the collector of the transmitter?

- Warren
I've never measured to see the waveform/frequency, of the base. I never thoguh to check the it when it was comign off the iPod, I always just checked the at the antenna.

chroot
Staff Emeritus
Gold Member
Well, if you have a reasonable-looking waveform on the base -- with large enough amplitude to change the current through the transistor's collector significantly -- yet nothing changes on the transistor's collector, then it means you have a faulty transistor. I don't quite understand why it would work with any power supply at all, though, in that case.

What you should measure roughly are the average voltage and amplitude of the signal at the base of the transistor.

I assume that nothing changes when you vary the volume control on your iPod, right?

- Warren

Ok, tomorrow I will use the osciliscope on the base of the transistor. If it appears fine, then what? I guess I'll swapp out the transistor, and if after I change the transistor and I am in the smae boat what else should I try? Or what other information would you like me to collect?

You are correct, when I alter the iPod volume nothing changes by my readings. That was the only input/suggestoin that my physics teacher had. He is a drop out civil engineer who doesn't like math..and thats why he is teaching physics now..lets not get into him. But anyway no the volume doesn't change a thing.

NoTime
Homework Helper
How exactly are you determining it ever worked?

One thing is that component layout is critical in the FM range.
For example the 5pf should go C E with as short of leads as possible.
Best layout is probably just like the schematic pic keeping the leads short.
With the components spread out like your pictures you can get very odd operation.

Also, It looks to me to be an AM transmitter.
Some cheap FM demodulators will work with an AM signal by slope detection, but the better ones will not.

How exactly are you determining it ever worked?

One thing is that component layout is critical in the FM range.
For example the 5pf should go C E with as short of leads as possible.
Best layout is probably just like the schematic pic keeping the leads short.
With the components spread out like your pictures you can get very odd operation.

Also, It looks to me to be an AM transmitter.
Some cheap FM demodulators will work with an AM signal by slope detection, but the better ones will not.
Becuase I got a frequency of about 103Mhz from the osciliscope when reading off the antenna.

I'll double check all the connections and make them as short as possible.

I believe that this transimtter brodcast in mono, but it still si designed to broadcast inside the FM band. The designer of the circuit on his website said he used a walkman to pick up the station.

Ok, my circuit doesn't work. Somethingis wrong.

The frequency is coming from the DC power unit. It creates a ~100-110 Mhz frequency.

The osciliscope reads a small frequency coming off the input stereo jack, from my iPod. I get ~15-20Mhz frequncy when I have the osciliscope set to .005v (50 milivolts). The iPod gives off .003volts over the stereo cable.

the transistor collect, is on the positive side, (side of the inductor and antenna)
the base is conected to stereo cable (via capacitor and resistor)
the emit part of the transistor is hooked ot the 330 ohms resistor and ground.

(I first had the collect and emit parts flipped, until I read online that the collect was the positive side.)

Whats wrong with the circuit? I have everything laid out the way it is supose to be, and I just don't know why I don't get a transmition.

BTW I have had the osciliscope set to 2v, 1v and .5v trying to see if I get a frequency and I don't. (I have also tried tunning on a walkman just incase my osciliscope is setup wrong or something..but still a no go).

chroot
Staff Emeritus
Gold Member
Ah... you're trying to use automated frequency measurements of a signal that isn't a pure sinusoid. That's never going to work. Are you actually looking at the waveform to see whether it makes sense, or just looking at the frequency measurement?

Your iPod should be producing something more like a complex waveform (not of any specific frequency!) with a peak-to-peak envelope of a couple of volts, depending upon its volume control. If you're only seeing a 3 millivolt (!) signal at the base of the transistor, then something is definitely wrong with your wiring. Are you sure you wired up your stereo cable the right way? There are three conductors in there, two for signal and one for ground.

- Warren

Ah... you're trying to use automated frequency measurements of a signal that isn't a pure sinusoid. That's never going to work. Are you actually looking at the waveform to see whether it makes sense, or just looking at the frequency measurement?

Your iPod should be producing something more like a complex waveform (not of any specific frequency!) with a peak-to-peak envelope of a couple of volts, depending upon its volume control. If you're only seeing a 3 millivolt (!) signal at the base of the transistor, then something is definitely wrong with your wiring. Are you sure you wired up your stereo cable the right way? There are three conductors in there, two for signal and one for ground.

- Warren

The waveform is the onlything I ahve to look at, the osciliscopes I have at school don't give any readout but a waveform on a small screen. The waveform looked like the one linked below (the one on the bottom of the graph).
http://www.stereophile.com/images/archivesart/1006Sonfig17.jpg

The iPod itself gives off 3milivolts, but there is 1volt going into the base of the transistor, 1.5 on the collect part and only .3volts from the emit part going to ground.

Warren I really appreciate your help. Your the first person who has been able to help me. Thank you for the help so far.

(and NoTime, thanks for the help too)

chroot
Staff Emeritus
Gold Member
Right -- 3 mV from the iPod is wrong. You need to figure that problem out before you can go any further. Your iPod should be producing a waveform of a couple of volts in amplitude. 3 mV is so small that it could never, for example, drive a set of headphones.

Try disconnecting your cable from your circuit board and see if you can measure the iPod's headphone output directly. It should not change when you connect it to your board. If it does, you've made some kind of bad connection and are grounding it accidentally.

- Warren

NoTime
Homework Helper
The iPod itself gives off 3milivolts, but there is 1volt going into the base of the transistor, 1.5 on the collect part and only .3volts from the emit part going to ground.
Hmmm.
.3mv may not be that unreasonable if measured at the transistor base, but you should see a bigger signal on the emiter.
As Chroot says the iPod itself should measure around 1v.

Your coil does not look, from the picture, like it matches the winding instructions, being a few turns short. Check the tank circuit resonant frequency with a grid dip meter.

Don't know what your scopes bandwidth is, but you may be above it.
Try a demod probe or frequency counter to see if its working.

Well I took the stereo cable off, and measured the voltage of the iPod without it attached to the circuit. It measured only 3milivolts again. The iPod works fine, if I unplug it and put it in headphones I can hear music. So I'm not sure whats up.

I measured 3milivolts from the stereo cable when it was attached to my circuit, before it went through the resistors, but the fact that I get the same voltage outside the circuit...

grid dip meter? Is that different than a regular oscilloscope? I'm not sure if I have a grid dip meter.

chroot
Staff Emeritus
Gold Member
Are you measuring this 3 millivolts between the right and left signals in the headphone cable? Perhaps that's the confusion -- the difference between the left and right channels, with normal audio, is energetically insignificant, on average. The difference betwen the channels should go way above 3 mV though, even with normal music.

Try measuring the potential between one channel and ground -- it should be significant.

You are, by the way, connecting the ground from your stereo cable to the ground of the rest of your circuit, aren't you? Are you sure you haven't accidentally shorted some parts of your stereo cable together? It really sounds like you're essentially tying the two audio channels together (or to ground) somehow by accident, thus getting basically zero signal out of the iPod. Try using an ohm-meter to make sure that all three conductors in your cable are distinct, and are not accidentally connected to any other conductors.

It's quite likely that your RF signals are too high for your oscilloscope to display, but your audio signals should not be.

- Warren

Last edited:
ok, I just cut a new stereo cable. So I have about 3 inches of cable and then the jack. When I hook my iPod to it, I measured from the red wire and the ground I got .003 volts. Then I measured from the white wire and ground and got .003 volts too. The volume is on max, its louad enough to hear with the headphones away from your ears. Why am I getting .003 volts? I am completely disconted from the circuit, so the iPod is giving off .003 volts. I can take a picture of what I'm doing if you want. I just not sure what else I can do to check the voltage.

The oscilloscope I am using says on the box 20Mhz, but the display gives a graph from -40 to 150..so I thought that was -40Mhz to 150Mhz? The manual doesn't tell me anywhere what the -40/150 mean, so I just assumed they are Mhz.
And when I hook my circuit up to the oscilloscope now, I just get a flat line at the 2v setting. at the .5volt setting I get about a 20Mhz waveform, but its weird because If I hook a AA up to the oscilloscope I get a 20Mhz waveform on .5v setting..and its set to DC voltage. Why would I get a waveform on a DC battery???