# Need help building FM Transmitter

by Shelnutt2
Tags: building, transmitter
 P: 57 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 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 http://img523.imageshack.us/img523/2799/1005420wq0.jpg http://img405.imageshack.us/img405/3905/1005424hg7.jpg http://img523.imageshack.us/img523/716/1005426ca9.jpg (^^ 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;)).
 PF Patron Sci Advisor Emeritus P: 10,400 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
P: 57
 Quote by chroot 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 (the schematic of the circuit was on the page I linked)

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

PF Patron
Emeritus
P: 10,400

## Need help building FM Transmitter

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
P: 57
 Quote by chroot 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

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.
 PF Patron Sci Advisor Emeritus P: 10,400 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
P: 57
 Quote by chroot 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...
 PF Patron Sci Advisor Emeritus P: 10,400 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
P: 57
 Quote by chroot 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.
PF Patron
Emeritus
P: 10,400
 Quote by Shelnutt2 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
P: 57
 Quote by chroot 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 ). So it was a stereo singal from my iPod Shuffle playing into the base of the transistor.
 PF Patron Sci Advisor Emeritus P: 10,400 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
P: 57
 Quote by chroot 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.
 PF Patron Sci Advisor Emeritus P: 10,400 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
 P: 57 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.
 HW Helper Sci Advisor P: 1,572 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.
P: 57
 Quote by NoTime 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.
 P: 57 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).

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