This circuit will explain the PLL controller unit for the FM transmitter. It is very important since the transmitter frequency is digitally controlled and thereby very stable. The heart of this unit is a PIC processor called PIC16F870, a 2 x 16 char display and four buttons.
Circuit Schematic
You will also find a 2 line 16 char display based on " HD44780-based LCD Modules" which is very common. Most LCD displays are based on this circuit.
Printed Circuit Board (PCB file for LCD controller pdf)
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.
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.
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 PIC.
- When you drive the PIC with a VCTCXO of 16.8 MHz. (PIC HEX)
- when you drive the PIC with a crystal of 2-5MHz. (PIC HEX)
- When you drive the PIC with a crystal of 13MHz.(PIC HEX)
100 = R6
3.3k = R1, R2, R3, R4, R5
20k = P1
22pF = C1, C2
100nF = C5, C6
2.2uF = C3
220uF = C4, C7
X1 = 13.000 MHz
PZ = Piezo
V1 = 78L05
IC1 = PIC16F870
LCD 16x2 Char Blue type (VCO circuit)
Source
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