Digitally controlled FM transmitter with 2 line LCD display

This is the front side of the controller card.

Digitally controlled FM transmitter
with 2 line LCD display. (Part I)
This transmitter is PLL controlled and still very easy to build.
Frequency is set with 4 buttons and a LCD display show you the frequency.
Print your own display text on first line.
The smallest step size is 50kHz and the transmitter will work from 88 to 108 MHz.
I will guide you all the way through this project.
All contribution to this page are most welcome!

Background
All over the net are descriptions of different types of FM transmitters.
You can find all kinds of constructions. Some are more cryptical than others.
One common thing about them is that they have poor explanation about function and assembly. Here is the backside of the controller card with all components added. The display is placed on the other side.

Here is the backside of the controller card with all components added. The display is placed on the other side.

Most of them don't explain the involving coils or how to make them.
How about performances and stability?

Why can't there be a construction that are easy to build and gives great performance at the same time?
We do have been able to send people to the moon!

I finally got tired of all crap and created this page myself. A page I always wanted to find out there.
I will show you a construction which will have great performance and still is very easy to build.
Even if you are a fresh born homebrewer, you will be able to follow my footsteps in this project.
I will support you with schematic , descriptions, software, components and PCB….how about that!
Enough blurring, let's get down to business.

Project start
A transmitter needs several units to work properly, so instead of mixing all into one huge project, I will divide the project into 4 steps.
Each step will result in a self functional unit. In the end of this journey, you will be able to link the steps together into a very powerful FM transmitter.

Part I (Digitally controlled FM transmitter with 2 line LCD display)

Part II (FM PLL controlled VCO unit)

Part III (1.5W power amplifier)

Part IV (7W power amplifier under development)

Part I (one)

This part will explain the main controlling unit for the FM transmitter. This part is very important since the transmitter frequency is digitally controlled and thereby very stable.
The hart of this unit is a PIC processor called PIC16F870, a 2 x 16 char display and four buttons.
Let's look at the schematic now.
As you can see I have chosen a PIC16F870 because it is easy to find and cheap to buy.
If you want more info about programming, you can look at my link:
Introduction to microcontroller PIC16F870 and PIC16F84.
If you can't find this circuit or have difficulty to program it, I can support you with this circuit pre-programmed. Mail Me for further information.
At the schematic you will also find a 2 line 16 char display.
This display is a " HD44780-based LCD Modules" which is very common.
Most LCD displays are based on this circuit. You will have no problem to find it or get if from me at my component page.

The display will show you information about the transmitting frequency.
Four wires goes to the transmitter board to control the PLL circuit called "INTERFACE OUT", (coloured in green left side).
When you press any of the four buttons the PIC16F870 will update both the transmitter board and the display with new data.

The frequency of this transmitter can be changed in +/-50kHz steps with two buttons and also in +/-1MHz steps with two other buttons.
The frequency limit is not set by software, only by the VCO of the transmitter board.
This means that you can use this software from 0 to 999MHz!

What else do we need to say?
Well, at pin 15 you will find a piezosummer that give a short "beep" when ever you press a button.
P1 will need to be adjusted to give good contrast of the display. The voltage at pin 3 of the LCD should be about 0.70V-0.75V.
The PIC16F870 runs with a crystal which should be from 2 to 13 MHz….not critical at all..
I have two different hex software below adjusted for different frequencies of the PIC.
The PIC16F870 will always remember the last frequency you used and when the power is connected, the first thing that happens is that the transmitter PLL and display will be uppdated.

PCB

part1u_2.pdf

PCB file for LCD controller (pdf).

Above you can download a (pdf) filer which is the black PCB. The PCB is mirrored because the printed side side should be faced down the board during UV exposure.
To the right you will find a pic showing the assembly of all components on the same board.
This is how the real board should look when you are going to solder the components.
It is a board made for surface mounted components, so the cuppar is on the top layer.
The hole mounted component should be soldered directly to the layer with the smd components.

The connection of this PCB is matched to the 16x2 LCD display I have on my component page.
The LCD is places on the backside of this PCB (See photos). A jumper J1 is added to choose if you want strong backlight or not.
If jumper J1 is disconnected the LCD will have soft backlight because a low current will pass through R6.
If jumper J1 is connected you will have strong backlight.

The component are not critial at all, the Pot (P1) can be from 1k-22k and (R1-R5) resistor can be changed to 1k-10k.
The orange squars are the input from the buttons to select frequencies. The green squars are connection to the PLL at the VCO part II.

Grey area is cuppar and each component is draw in different colours all to make it easy to identify for you.
The scale of the pdf is 1:1 and the picture at right is magnified with 4 times.
Click on the pic to enlarge it.
Click here to see photo and read how to solder smd components.

Custom made display text

The display has 2 lines with 16 Chars.
The first line of the display show "FM Radio station", next line will show the frequency.
If you want, you can write your own text on the first line.

To modify the test, you press Inc 50 kHz button or Dec 50kHz button during power up.
You can change the chars up/down with the two buttons.
When you find the char you like you press Inc 1Mhz button to go to next char.
When all 16 Char is entered, the unit restart with the new text.
All frequency register will also be reset back to factory settings 90.00 MHz.
Pretty simple.

Download PIC16F870 programs (INHX8M format)
The zip file contains hex file made for this project.
I have made two programs, each one are made for different crystal frequency of the PLL of the FM PLL controlled VCO unit (Part II).
You should use the first hex file, when you drive the PLL with a VCTCXO of 16.8 MHz.
You should use the second hex file, when you drive the PLL with a crystal of 13 MHz.

16_8.zip	PLL software to FM transmitter 16.8 MHz VCTCXO with own made text line 1 (file is zipped!).
13mhz_new.zip	PLL software to FM transmitter 13 MHz crystal (file is zipped!).

After you have loaded the PIC with one of the hex files above your unit will be functional and you don't have to worry more about it.
If you like software details, you can keep reading the section below.

When you program the PIC16F870 there are 3 bytes in the EEPROM memory which are interesting for you.
You can find them in EEPROM address 0, 1 and 2.
These 3 bytes set and remember the last used transmitting frequency.
If you change the frequency with the buttons, these 3 register will be updated.
In the software (transm_LCD.hex) I have made the initial frequency is set to 90MHz.

If you wish the initial frequency to be something else than 90MHz you can easy change the 3 bytes during programming, se table below.

Frequency (MHz)	Address 0	Address 1	Address 2
88.0	00h	37h	00h
89.0	00h	37h	a0h
90.0 (default)	00h	38h	40h
91.0	00h	38h	e0h
92.0	00h	39h	80h
93.0	00h	3ah	20h
94.0	00h	3ah	c0h
95.0	00h	3bh	60h
96.0	00h	3ch	00h
97.0	00h	3ch	a0h
98.0	00h	3dh	40h
99.0	00h	3dh	e0h
100.0	00h	3eh	80h
101.0	00h	3fh	20h
102.0	00h	3fh	c0h
103.0	00h	40h	60h
104.0	00h	41h	00h
105.0	00h	41h	a0h
106.0	00h	42h	40h
107.0	00h	42h	e0h
108.0	00h	43h	80h

Option:
(If you never will change the frequency with the buttons and don't want a LCD display, you can set the initial frequency and still use this controling circuit.)

Component support
This project has be constructed to use standard (and easy to find) components.
People often write to me and ask for components, PCB or kits for my projects.
All component for Digitally controlled FM transmitter with 2 line LCD display. (Part I) are included in this KIT (Click here to download component list.txt).
In the KIT you will get a high quality PCB for the Digitally controlled FM transmitter with 2 line LCD display. (Part I)

In the KIT you will get a high quality PCB for the Digitally controlled FM transmitter with 2 line LCD display. (Part I)

The kit cost 35 Euro (48 USD) and includes:
1 pcs	PCB (etched and drilled)
1 pcs	16x2 LCD w/ HD44780 Controller & Blue Backlight
1 pcs	PIC16F870 PIC16F870 pre-programmed with software (DIP)
1 pcs	78L05 (V1)
1 pcs	13.000 MHz Reference crystal (very accurate and match PCB)
1 pcs	100 ohm (R6)
5 pcs	3.3k ohm (R1, R2, R3, R4, R5)
1 pcs	20k ohm (P1)
2 pcs	22pF (C1, C2)
2 pcs	100nF (C5, C6)
1 pcs	2.2uF (C3)
2 pcs	220uF (C4, C7)
1 pcs	Piezo (PZ)
1 pcs	36 Way Pin header 2.54mm pitch
Order/question Please enter your email, so I can reply. Please type your Order/Question You can also mail me.

Final word
In this part, I describes the digital controlling unit of the transmitter.
The current consumption is 15mA-25mA at +5V, depending on J1 (strong backlight) is connected or not.
Again, this is a strictly educational project explaining how a RF amplifier can be built.
According to the law it is legal to build them, but not to use them.

Part II
Next part will be the RF transmitter unit.
Click here to go to FM PLL controlled VCO unit (Part II)

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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