I'm trying to build a FM transmitter, but without sucess. The current schematic I'm using is this one:


The alternating current at the (+) of the op amp is the P2 output of my phone.

V3 and V4 are 2 switching power supplies connected into the AC outlet.

I am trying to calculate the inductor with this site: https://m0ukd.com/calculators/air-cored-inductor-calculator/

The wire I'm using to build the inductor is from an UTP cable. The multimeter that I have is very simple, and I can't measure the inductance of the coil. I'm trying to tune this on the radio of my other phone, and can't get it to work. I'm building this on a breadboard. I would appreciate it if someone could help me.

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    \$\begingroup\$ It's always the same: breadboard + RF do not mix. Also, the 741 is the last opamp on earth that you should be using, your power supply is probably not even large enough for that. Also, you're missing decoupling caps on your Opamp's supply. Also, a transmitter needs an antenna, and you got none in your schematic. So, these are all blockers I can tell you before even trying to understand what your circuit does. \$\endgroup\$ – Marcus Müller Sep 5 '20 at 21:16
  • \$\begingroup\$ @MarcusMüller The 741 it's only part of a pre amplifier. The two power supplies can deliver 1.5A. The caps, i don't know, maybe it's a problem. The antenna, of course I tried with one, I just missed into the schematic. I'm thinking that's really the problem, the breadboard or the inductor, or... the power supplies, that's connected into the AC, and maybe it's causing some interference... I don't know. \$\endgroup\$ – Joab Sep 5 '20 at 21:27
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    \$\begingroup\$ I think @MarcusMüller is well aware that the 741 is the signal amplifier, but some versions of the 741 require more than +/-5V (even +/- 10V). Most require AT LEAST +/-5V, and unless your supplies are perfect they probably have some tolerance that could be problematic. You also have a capacitor on the output of the amplifier that can degrade phase margin and potentially cause oscillation. \$\endgroup\$ – John D Sep 5 '20 at 21:38
  • \$\begingroup\$ If the input of the old opamp is 0VDC then its output is also 0VDC and stops the oscillator. then connect a coupling capacitor (330nF) between the output of the opamp and R1, R2 and the baser of the transistor. \$\endgroup\$ – Audioguru Sep 6 '20 at 0:42
  • \$\begingroup\$ Actually, get rid of the 52 years old opamp and connect the output of your phone to the input of the coupling capacitor I added in my previous post. \$\endgroup\$ – Audioguru Sep 6 '20 at 0:46

Forget building this on a breadboard. The oscillator can't work on a breadboard.

Breadboards have capacitance between the pin rows that's about the size of the capacitors in the oscillator.

Breadboard pin rows have about the same inductance as the coil in your oscillator.

These inductances and capacitances are not some part you could remove from the breadboard. They are a side effect of the way breadboards are built.

Besides all that, the wires used to connect things on breadboards are usually long enough that they have inductances that are as high, or higher than, the inductors in the oscillator.

If your oscillator could work, you wouldn't need the 741. The 741 is there in the original circuit to amplify the (very weak) microphone signal to a level high enough to modulate the frequency of the oscillator. The output of your phone's headphone jack is already high enough to do that.

You'll need to build this on a piece of perf board, keeping the connections as short as possible.

You'll have to solder the connections.

Coils made of wire with thick insulation won't work. The insulation between the windings will act like a capacitor in parallel with the inductor. You need thin wire with enamel insulation. Random wire salvaged from whatever is at hand won't work.

If you should by chance get that circuit to work, you will find it to be very finicky and touchy. The transmit frequency will change depending how close you are to it, how close other things are to it, how hot or cold it is, whether you are looking at it cross eyed (or not) or for no discernible reason at all.

The circuit you have found is the simplest circuit that can be used to demonstrate the concept of an FM transmitter. It is not a practical circuit at all.

  • \$\begingroup\$ It's ok. So, where can I find a practical circuit to build on a perfboard? Because I researched I found a lot, but I don't know which one will work \$\endgroup\$ – Joab Sep 5 '20 at 21:56
  • \$\begingroup\$ Something like this. \$\endgroup\$ – JRE Sep 5 '20 at 22:16
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    \$\begingroup\$ @Joab the blunt reality is that this is a bad choice for a project. Broadcast receivers are designed to work with broadcast transmitters, not things that try to fit between tiny power limits for unlicensed transmitters, and not working at all. If you want to build analog voice radio gear, get a ham license and learn to do things appropriately. \$\endgroup\$ – Chris Stratton Sep 6 '20 at 0:06

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