Bird Transmitter
This transmitter is custom made for bird listening.
It is a PLL controlled FM transmitter which deliver 100mW at 88-108MHz.
The transmitter is based on my FM transmitter and have micro-controller and
microphone implemented on the circuit board.
Since it use PLL, the frequency is very stable and can be set by 2 jumpers.
The antenna is only 30cm and transmitting distance is up to 500m.
The power supply is 9V and the current is about 90-100mA.
All contribution to this page are most welcome!

A mail from Spain started this project. It was Carlos who asked me if I could custom build a transmitter.
The idea was interesting and most of the work has already been done in my previous RF projects, so I had a go for it.
The main part of this transmitter is based on my FM PLL controlled VCO unit (Part II).
What I needed to implement is a PIC16F870 processor and a microphone amplifier.

One criteria of the construction was a short antenna which makes it more difficult. I can not emphasise how important a proper antenna is in a transmitter. If you are unlucky you might just get a few % of energy out into the air and the rest will reflect back and mess it all
up. Anyhow, the antenna of this construction is 30-35cm wire and it works..I will not say more..*smiling*

Hardware and schematic
Click to open in new window So, lets look at the schematic.
Most parts will be recognised from the FM project, but two parts are different.
The microphone can be found to the top left corner.
A NPN transistor amplifies the signal and a potentiometer set the input signal to the FM modulator part.
If you look at the PIC you will see it controls the PLL and two jumpers SW1 and SW2 will select the frequencies.

The table show you the actual frequencies for this project.
All you need to do is to change the jumper and the PIC will update the PLL with new frequency.
Freq (MHz)

Field test
My test was performed in an open area and I used my car to measure the transmitting distance.
I places the transmitter about 1 m above ground close to some trees with birds. I then went to my car and turned on the radio.
This transmitter works perfect and the audio quality was beyond my expectations.
So I drove a way 100m, still with perfect audio..then 200, 300, 400 and at 500m I was not "line in sight" and at that point I started to get some noise and sparks. What a performance!

At 200-300m away I could sit in my car and listen to perfect audio from birds singing.
It sounded like I had them in the backs seat of my car.

Some photo

Download PIC16F870 programs (INHX8M format)
The zip file contains hex file made for this project. Bird software to FM transmitter (the hex files are zipped!).

Final word
This project describes the FM PLL controlled VCO unit working with a micro-controller and a microphone.
Again, this is a strictly educational project explaining how a RF amplifier can be built and used.
According to the law it is legal to build them, but not to use them.

You can always mail me if there is anything unclear.
I wish you good luck with your projects and thanks for visit my page.

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Copyright © Last modified on 20 july 2005.