Mistery radio

Hello everybody

I'm tryng to build a working FM transmitter.I have found some schematics on the net, went shopping, got the parts, and nothing!! no design seems to work

https://www.forosdeelectronica.com/about3674.html

This is the last one. Is in spanish, but the schematic is easy to understand even without the notes.

Some techical data:

Running at 10-15V
2N2222 Transistors
Strange pot, no idea if is the required 5-60 pF (the man on the store said yes)
Electret mic
The transformer is an air coil, 5 turns of common wire (steel wire, no isulation)
The antenna is made on the same material, 30 cms (1 feet) long

This is the story, if you wanna read:
I turned on the radio, put it 1 feet away, and started talking while slowly turning the pot. Nothing happens. Aprox at 96 Mhz (a random frecuency I selected) when I touch the coil the scratch on the radio changed. More interesting, puting my hand closer to my failed transmitter and the radio starts to receive a portugese program!!! I think my circuit doing the job of the old vacuum tube radios, with BC, SW1, SW2, SW3 and those strange frecuencies.

But I dont care, I just wanna get this thing working, as a normal FM transmitter!! How can I fix it?

How have you built it?, layout is critical at VHF frequencies. Did you use his PCB layout?.

No I didn't, I used the same layout found on the schematic

**broken link removed**

**broken link removed**

I would suggest you look at Audioguru's improved FM transmitter (check the Electronics Projects forum), that gives a tested Veroboard layout.

Thank you vey much, Nigel Goodwin. Im gonna check it out

edit: amazing circuit, looks much more professional. Im puting it here to make a shortcut
**broken link removed**

The circuit you found is the same simple lousy child's toy like many others:
1) Its antenna is connected directly to its oscillator's tuned LC circuit so its frequency changes when someting gets near it and if something moves away.
2) It is designed to be powered by a 9V battery (not 10V to 15V) and the voltage drops as the battery is used. The oscillator's frequency changes when the supply voltage changes.
3) It doesn't have pre-emphasis (treble frequencies boost) like FM radio stations have. FM radios have de-emphasis (treble frequencies cut) then they sound normal when receiving sounds from an FM radio station. This transmitter will sound muffled on an FM radio.

1) My FM transmitter has an RF amplifier transistor between the oscillator and the antenna so its frequency doesn't change when something moves toward or moves away from its antenna. It also has much more output power.
2) It has a low-dropout voltage regulator for its mic preamp and oscillator so its frequency doesn't change when the 9V battery drops down to 5.4V.
3) It has the proper amount of pre-emphasis and a selection of the amount for the two different types in the world.

I used strip-board with a few jumper wires and mounted the parts as close together as I can, to reduce stray capacitance and inductance.
The coils are made from enamel insulated 1mm copper wire.

I found a simple FM transmitter project written in English that is nearly identical to the Spanish one you found:
**broken link removed**

The simple circuit Its horrible, what a waste of money!!! Sadly I didn't knew before. I will try again with your circuit, Audioguru.

Why do your coils look shiny and perfect? I can not get my coils well, as you can see, the spacing and overwall shape is horrible. Can I put something as a core, like plastic or paper?

I used enamel insulated 1mm copper wire for my coils. The enamel looks shiny. I wrapped them tightly around a 3mm screwdriver then removed it.
I scraped the enamel off the ends with a knife before soldering them.

My FM transmitter is too powerful so it might cause interference to a weak FM station on the same frequency.
Also it is mono, not stereo.

hello
can i input my mp3 player's headphone/out in place of mic and listen to the player on FM radio??

jawadlateef said:

hello
can i input my mp3 player's headphone/out in place of mic and listen to the player on FM radio??

Click to expand...

The MP3 player has an output level that is about 15 times higher than from an electret microphone. It also has a power voltage. The resistor that powers the mic must be removed and an attenuator must be installed. The attenuator can also convert the stereo to mono.