Need help building FM Transmitter

[chroot]

Lets assume that the oscilloscope is bad. I tried and couldn't seem to get the oscilloscpe to jump up or down at all when I attached a AA battery. What I did was I had a multimeter reading the coltage of the battery then I hooked the oscilloscope probe up to the positive side of the battery. It didn't seem to move the flat line at all.

Wait, an oscilloscope probe has two connectors -- one for the signal, and one for ground. How are you grounding the oscilloscope's probe? You need to connect it to both ends of the battery. And make sure the oscilloscops is on DC coupling.

- Warren

 

[Shelnutt2]

Wait, an oscilloscope probe has two connectors -- one for the signal, and one for ground. How are you grounding the oscilloscope's probe? You need to connect it to both ends of the battery. And make sure the oscilloscops is on DC coupling.

- Warren

There is a ground on the Oscilloscope. The manual said to connect the groudn to that. I assumed that's what I was suppose to do for everything..if that's not the case then tomorrow I'll hook it up correctly, and see if that changes anything.

 

[chroot]

Well, when you're measuring a battery, you need to connect the ground of the oscilloscope to the negative end of the battery. Otherwise you're not really measuring anything.

- Warren

 

[Dick Cappels]

I think you guys were on the right track when you were discussing the battery's internal resistance, the area of focus is spelled out in the original post:

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? ...

If it works on the power supply, it should work on the battery if, to the transmitter, the battery looks like the power supply. Clearly the battery does not look like the power supply. The two ways it might differ are in the output voltage and the internal resistance.

Did you look to see if the voltage drops when you connect the transmitter to the AA cell? How much did it change? If its more than 01. volt, then your AA cell is effectively dead (I guess that if internal resistance is 100 Ohms, the cell is dead.)

1.3 volts is pretty low for an AA cell, except in the case of ni-cads (in which case, it would be a little high). If its a fresh carbon-zinc, zinc-chloride or alkaline cell, the voltage should be 1.6 volts without a load.

And I really don't know how well this circuit works at 1.3 volts. Try a different cell anyway.

The question of whether you can see the audio on the scope is largely irrelevant if you could hear Meatloaf while running off the power supply. The circuit achieves frequency modulation by varying the voltage between the base and the collector of the transmitter. Since the collector voltage is constant with respect to ground, it is only necessary to vary the base voltage. Using a 2N4401 in this circuit, it only takes a few tens of millivolts peak-to-peak on the base in order to get a large enough capacitance change to appear to be a fully modulated FM signal to FM broadcast receivers (about +/- 75 kHz.)

There should be no audio signal on the collector of the transmitter. If you see one, then either the battery or the inductor have a very high resistance at audio frequencies.

My suggestion is to make the AA cell look more like the power supply. Get a fresh alkaline cell and give it a try, and let us know.

(BTW, sorry for the late reply. I was traveling without access to email.)

 

[Shelnutt2]

If it works on the power supply, it should work on the battery if, to the transmitter, the battery looks like the power supply. Clearly the battery does not look like the power supply. The two ways it might differ are in the output voltage and the internal resistance.

It did not work with the Power Supply. What I was reading on the Osciloscpe was a frequency coming from the Power Supply unit itself. I've come to distrust the oscilloscopes I ahve access too. Well and distrust myself with being able to get them to work correctly.

I will get some fresh AA tomorrow morning and post back up if I get anything.

Thanks for the reply Mr. Cappels.

Oh and I no longer have access to the oscilloscopes readily. My semester in school is done and I was unable to convince the physics department to let me take one home. I offered to sign my life over, but they said that still wouldn't pay for the Oscilloscope if something happened..so if I really need to use oen I might be able to use one during summer school. I think that they will be there for summer school.

 

[Surrealist]

It would be far more convenient to just use an on-chip, voltage-controlled oscillator. You could probably get a free sample from MAXIM IC.

 

[3demax]

http://anarchy.translocal.jp/radio/micro/howtosimplestTX.html
Extract from Q&A, the end of the page:
"Q: When I connect my iPod to the audio input of this transmitter, it suddenly stops to work. What's happening?

A: The ealier model of iPod has an automatic shut-down system in it. When you pull out the plug of your headphone, it automatically stops working. When the left and right channels are short-circuited, the same thing happens. Presumably, you use a stereo cable and solderd the left (white) and right (red) lead together. Please use the left channel lead (white) only. Usually the left output is compatible with mono audio. If you, however, insist to mix the left and right output of your audio source into the transmitter, make a simple "interface" like this by two sets of 1 kiro ohm registers."

I found this article in 2010, I'm a bit late with replying :) But hope that somebody, who will find this article like I did (it's on fifth position in http://www.google.com.ua/search?hl=uk&q=build++fm+transmitter+for+headphone+jack+curcuit"), will probably get an answer