Author Topic: KT0803L transmitter + ESP32 - take it apart please  (Read 542 times)

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Offline braincatTopic starter

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KT0803L transmitter + ESP32 - take it apart please
« on: April 21, 2024, 02:09:29 am »
Greetings all,

This goes into beginners since that's who I am.
I wanted to make a short range FM transmitter that is microprocessor controlled.

I know that these things sell for $3-5 on aliexpress, but I wanted to make it myself.  So... I have a two-sided PCB, etched with HCl + H2O2, it has a ground plane, a 3.3V plane, and a 5V plane.  The 3.3V is converted from 5V using a linear  regulator.   The source of 5V is an old HP3611A with original capacitors.  The FM transmitter chip is KT0803L, controlled via i2c from the ESP32 IC.  The source audio comes from a headphone jack.   I could not solder it on the board because I designed the board with a jack upside down  :)
I am attaching the schematic, the 3D view of the pcb from kicad, and the actual photos of the board. 

I know it's a hack job, but I am learning, and I'd appreciate suggestions about making this thing better next time.  I can receive the audio with a VX-6R handheld.    The audio has a low-frequency interference in it (you can listen to it in an attachment), which almost goes away if I use computer's USB as a power supply.  The same noise is present if I drive the transmitter using a sine wave from the ESP (there is a pair of jumpers on the board that connect the corresponding pin to audio input).   BTW the board is unfinished, I have not soldered a female pin connector on it (to connect an LCD), and didn't add a 12V to 3.3V DC converter.

I wrote the c++ code to drive the ESP32 and cross-compiled it on linux using esp-idf development framework.

Thanks All.
« Last Edit: April 21, 2024, 03:11:07 am by braincat »
 

Offline braincatTopic starter

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Re: KT0803L transmitter + ESP32 - take it apart please
« Reply #1 on: April 21, 2024, 03:22:07 pm »
That noise that I mentioned has fairly low frequency (I'd say about 10 mhz).  I disable both bluetooth and wifi radios on the esp32, that didn't help.

Edit: Turns out one of the pins on the transmitter IC that was supposed to be grounded was not connected.  After grounding it, the noise went away.  KT0803L is not great btw: if I program it to transmit on 100MHz, I can hear the signal at twice that frequency, too.
« Last Edit: April 21, 2024, 04:13:40 pm by braincat »
 

Offline aliarifat794

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Re: KT0803L transmitter + ESP32 - take it apart please
« Reply #2 on: April 21, 2024, 05:46:29 pm »
That noise that I mentioned has fairly low frequency (I'd say about 10 mhz).  I disable both bluetooth and wifi radios on the esp32, that didn't help.

Edit: Turns out one of the pins on the transmitter IC that was supposed to be grounded was not connected.  After grounding it, the noise went away.  KT0803L is not great btw: if I program it to transmit on 100MHz, I can hear the signal at twice that frequency, too.
It's possible that the KT0803L is a clone.
 


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