FM Transmitter

Thread Starter

ngroves

Joined Apr 21, 2022
2
Does anyone have a reliable circuit for a Short range FM transmitter please?, i just need to cover about 40ft in range.

Thanks

Neil.
 

Audioguru again

Joined Oct 21, 2019
5,413
I fixed a simple FM transmitter circuit with 4 steps:
1) The mic preamp did not work when the 9V battery was new at 9.5V and also when the battery was run down to 6V.
A re-design with a 5V low-dropout voltage regulator fixed it.
2) The radio frequency changed as the 9V battery voltage ran down.
Connecting the RF oscillator to the 5V regulator fixed it.
3) The radio frequency also changed when something moved toward or away from the antenna.
Adding an RF amplifier between the oscillator and antenna fixed it.
4) The audio received with an FM radio was muffled with no high audio frequencies.
Adding pre-emphasis (treble boost like all FM stations use to match the de-emphasis in all FM radios) to the mic preamp
fixed it.
But it is illegal because it is powerful enough to cause interference to many neighbors' FM radios.
A kit for an FM transmitter used an attenuator at the antenna to reduce the range.

Here is my circuit:
 

Attachments

MrSalts

Joined Apr 2, 2020
2,597
I fixed a simple FM transmitter circuit with 4 steps:
1) The mic preamp did not work when the 9V battery was new at 9.5V and also when the battery was run down to 6V.
A re-design with a 5V low-dropout voltage regulator fixed it.
2) The radio frequency changed as the 9V battery voltage ran down.
Connecting the RF oscillator to the 5V regulator fixed it.
3) The radio frequency also changed when something moved toward or away from the antenna.
Adding an RF amplifier between the oscillator and antenna fixed it.
4) The audio received with an FM radio was muffled with no high audio frequencies.
Adding pre-emphasis (treble boost like all FM stations use to match the de-emphasis in all FM radios) to the mic preamp
fixed it.
But it is illegal because it is powerful enough to cause interference to many neighbors' FM radios.
A kit for an FM transmitter used an attenuator at the antenna to reduce the range.

Here is my circuit:
AG,
What does the back side of that strip board transmitter look like? Can you please post a photo? I'm looking for an example of strip board assembly to show a young man with his own idea. He's not so sure this is the assembly method for his prototype. I'm betting your skills will show him, it can be a neat and tidy method.
 

Audioguru again

Joined Oct 21, 2019
5,413
I designed and made my FM transmitter 17 years ago and used it only for a few days. Today I cannot find it.
Like hundreds of other stripboard circuits I made, the strips are cut to length with a drill-it at perforated holes on the stripboard form more than half the wiring of a pcb and the parts and a few short jumper wires form the remainder of the wiring.

Many stripboard projects I made in my career are so neat and tidy that my prototypes were the final sale and installation. Some digital ones had thousands of parts and worked perfectly. Everything was soldered but if a part needs to be replaced then my solder-sucker could easily slurp away the solder for a new replacement part.

If the circuit is built on a solderless breadboard then the high amount of stray capacitance and inductance from the rows of intermittent contacts and long wires all over the place would prevent it from working.

Here is an example of the strip cuts:
 

Attachments

DickCappels

Joined Aug 21, 2008
9,508
This is the simplest version.
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You can use almost any transistor with an Ft of 200 Mhz or higher (2N3904 for example). The transmitter is tuned by deforming the inductor.


Notice: Before operating a radio transmitter, find out what kind of transmitter operation, if any, is permitted in your locality. Radio transmitter operation is a serious legal matter. In the United States, operation of unlicensed intentional radiators is covered by Part 15 of Title 47 of the Code of Federal Regulations. This design can be readily adapted to different frequencies and different power levels. If you choose to build and operate the transmitter described here, you do so at your own risk. I'm only publishing this as an example of what can be done.

This implementation is adapted to rebroadcast the output of a CD player, television receiver, or radio receiver. I use it so that I can move about the house and listen to my favorite programs without disturbing others. Within and the house, I find that I can get 10 to 20 meters away from the transmitter with the small pocket FM receiver I carry in my shirt pocket. Your mileage may vary. The transmitter as built and pictured below (the transmitter is in the blob of hot melt glue on the end of the battery holder) does not have an on-off switch. I put a 1.5 AA cell that was run down too far to run my CD player in this transmitter and it ran for over a month before I replaced it. The one in the transmitter at this moment has been running it continuously for over three months. Current draw is only about a milliamp with a new battery (assuming you don't have a super-high beta transistor in which case the theoretical limit is about 2.5 ma). An on-off switch is not necessary, though it may satisfy an emotional need.

Tips to get it working: Wind the coil on a 4 or 5 mm diameter Philips blade screwdriver or similar form then slip it off. I used some vinyl insulated #24 hookup wire as well as #30 enameled wire. In both cases, I played with the length of the coil to tune the transmitter to a dead spot on the FM band. The coil is held in place with hot melt glue. If you don't have a spectrum analyzer or frequency meter, use a good-quality FM receiver to make sure its tuned where you think it is. While adjusting the coil, keep in mind that all superheterodyne receivers have images. If you find that two or more adjustments make the transmitter show up on the same spot on the receiver, it might be necessary to take a short walk and find out which adjustment drops out first -this would be the image, because the receiver's front end (if it has a tuned front end) will reduce its sensitivity to the image.

Many kinds of transistors will work fine in this application. After all, its only an oscillator (frequency modulation is obtained my modulating the base-collector voltage, thereby modulating the depth of the depletion layer of the reverse-biased base-collector junction, which results in a change in capacitance at the collector, which results in a change the resonant frequency of the collector circuit.). I used an 2N4401 because I have a lot of them. I like 2N3904 and MPSH34 for this too.

Variations The objective of this design is to provide a simple, appliance-like low-power transmitter to rebroadcast audio. This transmitter does not have pre-emphesies, so it is not high-fidelity. It has sort of an "AM" sound for music and is fine for speech. Since there is no audio level control on the input, the audio level out of the CD player (or whatever you are driving it with) needs to be adjusted. Or, you can just add a 10k to 50k as an input level control, its no big deal. This transmitter, as built is not tunable once assembled, the coil tweaked to the desired frequency, and everything glued down. If you want to make one that's tunable, it might be easiest to reduce the 18 pf capacitor and put a small trimmer capacitor in parallel with the inductor (across the reduced value capacitor). Voltage variable capacitors would be an nice alternative to a mechanical variable capacitor but they don't offer much tuning range with only a 1.5V power supply.

Need more range? Don't make the antenna longer, instead raise the battery voltage.
 

bassbindevil

Joined Jan 23, 2014
631
Buy a car iPod/etc FM transmitter at a thrift store for a couple of bucks? If necessary, remove or jumper the choke that limits the output power. (That seems to be a common fault/feature of the various Belkin Tunecast transmitters.) You'll find discussions about this topic at forums dedicated to holiday light displays.
 

DickCappels

Joined Aug 21, 2008
9,508
Further to @Audioguru again 's comment, I use an FM transmitter as the output of my computer and I intentionally keep the transmitter power low to minimize the chance of interfering with FM reception in my neighbor's homes and also for my own privacy. I don't want them to know I watch old science fiction and Steve Martin movies.
 

bassbindevil

Joined Jan 23, 2014
631
Ramsey used to sell kits and finished low-power FM transmitters; you can find some on ebay. And very very rarely, for $5 at recycling yards. Panaxis, Veronica, and Broadcast Warehouse FM transmitters may have been better quality, but much rarer. There's one Veronica on ebay right now, as-is.
Low-Power FM Transmitter Circuits:
https://www.qsl.net/n9zia/lpfm/index.html
 
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