Simplex repeater for PMR446 and CB-phones




Schematic of this repeater is quite simple.
PIC-chip is operating at 4Mhz clock-speed.
Timers of the PIC-chip are adjusted for 4Mhz.
You have to adjust program, if clock-speed is different.
Source programs are adjusted for ISD2575, but it is possible use example ISD2560.
When using different chip than ISD2575,  you must change RX and TX watchdog times correspondingly with program.
Transistor TR1 with its componets make VOX-circuit,
which sends receive enable voltage to the PIC:s RB0-pin.
Transistor TR2 with R7 switches phone to transmit.
VR1 is volume potentiometer, which is outside the box..
VR2 is 100k trimmer potentiometer, which is on the PCB.
With it, receive treshold is being adjusted. Without received audio,  there is no +5V at the PIC:s RB0-pin, when properly adjusted.
When RB0 has voltage, green LED is on.
LED1 is green led and LED2 is red led.
R2, R3, R4, D3 and X4 are for programming PIC. You don´t need these at the normal operating.
It is possible to program PIC elsewhere and then put it in to the repeater.
I tested microphone- and speaker plugs with Cobra MT-925 and MT-725.
Plug wirings could be different at other models.
SPK  is 3,5mm and MIC  is 2,5mm stereo plug.
If transmit doesn´t work, if red LED is lit, you can try decrease R7 value to the 2,2k ohms.
When using CB-phone, R7 is removed and connected directly to the microphones PTT-pin.


Simplex repeater schematic

Click picture to zoom.


Component Position Quantity  
Metal case   1 L 115mm x W 65mm H 30mm
Printed circuit boad / vero board   1  
Regulator 78L05 TO-92 IC1 1  
Voice record/playback chip ISD2575P IC2 1  
8-bit microcontroller PIC16F84A-04/P IC3 1  
Chip base 28-pins PDIP IC2 1  
Chip base 18-pins PDIP IC3 1  
Potentiometer10K ohm log. VR1 1  
Knot VR1 1  
Trimmer resistor 100K ohm lin. VR2 1  
D9-jack female    runko X4 1  
Light emitting diode green LED1 1  
Light emitting diode red LED2 1  
Transistor BC549 TR1,TR2 2  
Diode 1N4001 D1 1  
Diode 1N4148 D2,D3 2  
Resistor 1/4W 100 ohm R10 1  
Resistor 1/4W 330 ohm R5 1  
Resistor 1/4W 1K ohm R9 1  
Resistor 1/4W 2,2K ohm R3 1  
Resistor 1/4W 4,7K ohm R7,R14 2  
Resistor 1/4W 10K ohm R1,R2,R6,R8,R13 5  
Resistor 1/4W 22K ohm R4 1  
Resistor 1/4W 330K ohm R11,R12 2  
Ceramic capacitor 22 pF C4,C5 2  
Capacitor 0,1 uF C1,C2,C6,C7,C8,C9,C10 7  
Electrolytic capacitor 10 uF/16V C3 1  
Crystal 4Mhz XTAL1 1  
Power switch panel mount ON/OFF S1 1  
DC-in, SPK/MIC-plugs and wires      
RS-232 programming cable   1  
Programming software IC-Prog   1 download IC-PROG
Firmware with roger beep and
sound beacon (5 min. beacon)
Firmware without any roger beep and beacon     repeater076.hex
Firmware with roger beep and morse beacon (10 min. beacon)     repeater08.hex



First mount resistors, capacitors, diodes, transistors and last microchips.
Do not assembly PIC and ISD chips yet.
Switch power on, and check with multimeter that there is +5V at PIC-chip VDD, MCLR-pins and
ISD-chip VCCD, VCCA-pins.
Switch power off, and assembly PIC and ISD chips.
It is good idea to mount chips to the IC-bases so you can replace them later easily.
Use only gold coated IC-bases.
Re-check circuits before applying power.
If PIC-chip is programmed, blinking green LED shows that program is operating.

Operation conditions at operating voltage of 12.88V:

Current consumption standby : 2.56mA, with firmware toistin1-076.hex less
Current consumption recording : 29mA
Current consumption playback : 27mA
Current consumption beacon : 10.6mA
Measured with Fluke 179 multimeter.

It is possible use repeater without regulator, but supply voltage must not exceed 5.5V DC.
Check polarity before applying supply voltage!









© 1997 - 2005 Petri Tarkiainen