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Topic: Short-range (10') AM-band transmit from 3.5mm Android headphone jack (Read 676 times) previous topic - next topic


Yes that circuit is basically correct although you don't draw connections going into a capacitor like that. Just have one wire out of the capacitor and connecting to the other components. However the capacitor needs to be a lot bigger something like 47uF.

Now if you read the analogue input with no audio you will get a reading around 512 and with audio it will be above and below this. If you don't get that then you have something wrong or the Audio from your phone is not loud enough.


Ok I'm using 47uF capacitor interfacing with 100K resistors as you suggested & A0 pin input and getting 485 reading with audio off, and 520 - 460 range with audio on. Still no music reception on AM 530 although pin 9 is definitely transmitting something as proximity to antenna is affected only at that radio frequency (static / hiss). Here's updated code. (I commented out Serial.println to test radio reception.)

Code: [Select]
int audioPin = A0; // positive lead from 3.5mm phone headphone jack; other lead to GND

const byte ANTENNA = 9;
int val = 0;

void setup() {
 // set up Timer 1
 TCCR1A = _BV (COM1A0);  // toggle OC1A on Compare Match, Timer Interrupts
 TCCR1B = _BV(WGM12) | _BV(CS10);   // CTC (clear timer) no prescaler
 OCR1A =  14;       // 530 kHz


void loop() {
  int sensorValue = analogRead(audioPin);

  digitalWrite(ANTENNA, sensorValue);


and 520 - 460 range with audio on.
That is only a small range and I would suggest it is not going to do much. That signal needs amplifying.

Still no music reception on AM 530
Not surprised because, as I have said before, you are not actually generating AM but pulse width modulation, and most radios are not equipped to demodulate that.


The first link I posted is of a sketch being able to generate an AM modulated signal. But I don't remember if I have tested it like I did on FM example.


Have you any idea about what that sketch actually did?
You might have got some tones but that could not be considered to be AM. It transmitted all over everywhere and the FCC would not have considered that to be legal. As the OP has found while it will transmit tones it can not transmit speach.


zoomx yes I got that digital tone sketch working before starting this project. That thread is really interesting!

Mike I got this working somewhat by changing antenna to digital pin 2 (from PWM pin 9), and removing 5V-connected 100K resistor (no detected signal when this was connected), while keeping the 100K-to-GND resistor (for some reason keeping this helped signal). Suddenly radio received my audio input, so success. I experimented with capacitor values & keeping 47uF seemed best.

The "reception" (a mix of screech and music from input) was effective within 12" (1 foot) radius in proximity to Uno board, and from an AM loop antenna wired up across the room, with no reception in-between the Uno & the antenna. This short-range noise was being received all over AM kHz spectrum (whereas the post zoomx linked previously is very tight on a single frequency). My experiment was not being transmitted / received at all on FM--I checked.

Any suggestions to clean up signal & focus it on desired frequency would be appreciated!

I previously linked to FCC rules on Unlicensed Operation of Low Power Radio for the benefit of those of us in the US; I'm posting Part 15 Devices here, so low-power hobbyists in the US are aware of the law:

Part 15 Devices
Unlicensed operation on the AM and FM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules.  On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters).  See 47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice Opens a New Window. (still in effect).  On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters).  See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221.  These devices must accept any interference caused by any other operation, which may further limit the effective service range.


This short-range noise was being received all over AM kHz spectrum
From the FCC Document
Section 15.209 contains general radiated emission (signal strength) limits that apply to all Part 15 transmitters using frequencies at 9 kHz and above. There are also a number of restricted bands in which low power, non-licensed transmitters are not allowed to operate because of potential interference to sensitive radio communications such as aircraft radionavigation, radio astronomy and search and rescue operations.
How are you going to verify that? Crossed fingers do not stand up in a court of law.

Further from the document:-
Home-built transmitters, like all Part 15 transmitters, are not allowed to cause interference to licensed radio communications and must accept any interference that they receive. If a home-built Part 15 transmitter does cause interference to licensed radio communications, the Commission will require its operator to cease operation until the interference problem is corrected. Furthermore, if the Commission determines that the operator of such a transmitter has not attempted to ensure compliance with the Part 15 technical standards by employing good engineering practices then that operator may be fined up to $10,000 for each violation and $75,000 for a repeat or continuing violation.

Any suggestions to clean up signal & focus it on desired frequency would be appreciated!
You have had it. To stop screeching reconnect the 100K and amplify the audio signal to get a bigger signal. To get it on frequency use an AM modulator and so you do not spread harmonics all over the place.


Those nuances of low-power transmission regulations are good to be aware of & follow. I'll experiment with LM386 amp module & voltage bias if/when confirmation of exact transmission frequency (which may not be possible given possibility of harmonics).

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