Circuit Operation:
In operation, the switch closure is detected on pin 7 of the processor which activates the reed relay and takes the line off-hook for 3 seconds to establish the dial tone. The processor then dials the number by opening and closing the relay a number of times for each digit. When dialing is complete, the processor waits 3 seconds and then transmits a steady tone of about 300Hz for 30 seconds through the modem transformer. The call is then terminated and the processor waits for the switch to open before resetting.
Design Considerations:
The PIC16F628 (18 pin) processor was selected because I had a few on hand and my homemade hardware programmer only accepts 18 pin devices. A smaller 8 pin device could have been used since only three I/O lines are needed, but the difference in cost is only about $1.50. One of the I/O lines (RA5) is used for programming and is always an input, but can used as a functional input so the switch closure could be detected on this line thus eliminating the need for one pullup resistor. But I elected to use 3 consecutive I/O pins (7,8,9) of the 8 bit port B and leave RA5 pulled up with a extra 10K resistor. The output pins (8,9) that drive the relay and transformer are limited to 25mA of current each, so an extra transistor (2N2222A) was needed to supply additional current to the relay coil.
The transformer resistance is around 100 ohms, so an additional 330 ohm resistor was added in series with pin 9 to limit the transformer current to around 10mA. An LED indicator and 330 ohm resistor were used on pin 8 to observe the dialing activity and indicate the line status. Several of the parts (relay, transformer and blocking capacitor) were obtained from an old 56K modem card. The schematic shows a 47uF / 50 volt non-polarized capacitor used to block DC current to the transformer, however a regular polarized 50uF cap could be used if correct phone line polarity is observed. The modem was probably designed to work with unknown polarities at different locations, so a non-polarized cap was used.
It's possible the cap and 470 ohm resistor can be replaced with a single resistor in series with the line to set the "off hook" line current to around 20mA. This may cause partial saturation of the transformer and reduced audio level, but might work well enough. The power supply voltage is not critical and a 4.5 volt supply from three AA batteries should work. Or a switching type regulated 5 volt wall transformer can be used. The problem is insuring the relay gets enough voltage to operate. The rest of the circuit should run on reduced voltage. I used a 4.2 volt cell phone charger that worked well.
Software:
The program (listed at bottom of page) is written in PIC assembly language which can be edited and compiled using Microchip's development software (MPLAB) available from www.microchip.com. The development software will generate a compiled HEX file to be loaded into the processor. Also needed is a hardware programmer to download the HEX file into the PIC processor.
I used a homemade programmer and DOS software by David Tait available from http://www.nomad.ee/PIC/. Filename (PIC84V05.ZIP). The only program changes needed are the entries for the phone number and the total digits to use.
The assembly code phone number table looks like this which represents a dummy number of 656-7459. Note the digits are in reverse order (top to bottom).
Phone ; Phone Number Table, in reverse order
addwf PC,1
nop ; No operation
retlw d'9' ; Last digit
retlw d'5'
retlw d'4'
retlw d'7'
retlw d'6'
retlw d'5'
retlw d'6' ; First digit
The total digits used (7 in this case) are stated in the Dial section.
To use a 3 digit area code, a 7 digit number, and a "1" to begin,
the entry would change to ( movlw d'11' ).
Dial ;------- Change this value to indicate total digits -----
movlw d'7' ; Use 7 digit phone number
movwf COUNTER
The call duration is assigned on the line (movlw d'100 ; 30 second tone timer)
just above the "Timeout" section. Increase the "100" value for longer durations
(i.e. 200 = 2 minutes).
Parts List:
PIC 16F628 microcontroller - Allied 383-0398
5 Volt Reed relay - Allied 681-0156
Modem transformer - Mouser 838-TTC-5023
5 Volt wall transformer - Jameco 320303PS
470 Ohm Resistor (1) - Allied 296-4682
680 Ohm Resistor (1) - Allied 895-3150
330 Ohm Resistor (2) - Allied 895-3145
10K Resistors (2) - Allied 895-0633
Small Signal Diode (1) - Allied 431-0618
Red LED (1) - Allied 670-1224
2N2222A transistor (1) - Allied 248-1005
;********************* AutoCall.asm 12/25/07 **********************
;********************************************************************
LIST P=16F628 ; Device number (PIC16F628)
ERRORLEVEL -224 ; suppress annoying message because of tris
ERRORLEVEL -302 ; suppress message because of page change
;--------------------- Configuration ---------------------------------
_BODEN_OFF equ H'3FBF' ; Brown out detection off
_CP_OFF equ H'3FFF' ; Code protection off
_PWRTE_ON equ H'3FF7' ; Power-on reset enabled
_WDT_OFF equ H'3FFB' ; Watch dog timer off
_LVP_OFF equ H'3F7F' ; Low Voltage programming off
_INTRC_OSC_NOCLKOUT equ H'3FFC' ; Use Internal RC Oscillator
_MCLRE_OFF equ H'3FDF' ; Use RA5 as functional input
__CONFIG _CP_OFF & _WDT_OFF & _INTRC_OSC_NOCLKOUT & _PWRTE_ON & _LVP_OFF & _BODEN_OFF & _MCLRE_OFF
;--------------------- Define Variables -----------------------------
INDF equ 00h ; Indirect file
FSR equ 04h ; File select register
CMCON equ 1Fh ; Comparator Control Address
INTCON equ 0Bh ; Interrupt control register
OPTION_REG equ 81h ; Option register
STATUS equ 03h ; Status register
TRISA equ 85h ; I/O control for port A
TRISB equ 86h ; I/O control for port B
PORTB equ 06h ; Address of port B
PORTA equ 05h ; Address of port A
PC equ 02h ; Program counter
COUNTER equ 20h ; General purpose Counters
COUNTER1 equ 21h
COUNTER2 equ 22h
COUNTER3 equ 23h
COUNTER4 equ 24h
TEMP equ 25h ; Temporary register
bsf STATUS,5 ; Select memory bank 1 (01)
bcf STATUS,6 ; Select memory bank 1 (01)
movlw b'00000010'
movwf TRISB ; Set port B as output, RB1 = Input
movlw b'00100000' ;
movwf TRISA ; Set port A as output, RA5 = Input
bcf STATUS,5 ; Reset to bank 0
bcf STATUS,0 ; Clear carry bit
bcf STATUS,2 ; Clear zero flag
bcf STATUS,1 ;
movlw 07h
movwf CMCON ; Comparators off
clrf PORTB
goto Loop ; Main loop
Phone ; Phone Number Table, in reverse order (656-7459)
addwf PC,1
nop ; No operation
retlw d'9' ; Last digit
retlw d'5'
retlw d'4'
retlw d'7'
retlw d'6'
retlw d'5'
retlw d'6' ; First digit
Loop ;------------------- Main Loop -----------------
movfw PORTB
btfsc PORTB,1 ; Read pin 7, skip if clear
goto Loop
bsf PORTB,2 ; Close relay
movlw d'3' ; 3 sec Delay
movwf COUNTER4
Off_Hook_Delay ; 3 sec delay
call Digit_Delay
decfsz COUNTER4,f
goto Off_Hook_Delay
Dial ;------- Change this value to indicate total digits -----
movlw d'7' ; Use 7 digit phone number
movwf COUNTER
Next
movfw COUNTER
call Phone ; Get Digit from table
call Output ; Dial the digit
call Digit_Delay ; Wait 1 second
decfsz COUNTER,f
goto Next ; Do next (1 of 7 digits)
call Digit_Delay ; Wait 1 second
movlw d'100' ; 30 second tone timer
movwf COUNTER
movwf COUNTER1
movwf COUNTER2
Timeout ; Generate tone for 30 seconds
movfw COUNTER
movwf COUNTER1
Tone
bsf PORTB,3 ; Set pin 9 high
call Tone_Delay ; Wait 1.5mS
bcf PORTB,3 ; Set pin 9 low
call Tone_Delay ; Wait 1.5mS
decfsz COUNTER1,f
goto Tone
decfsz COUNTER2,f
goto Timeout ; Repeat until COUNTER2=0
bcf PORTB,2 ; End call, On Hook, Done
Switch_Open ;------------------ Wait for switch open
movfw PORTB
btfss PORTB,1 ; Look at pin 7
goto Switch_Open ; Loop until pin 7 is high
call Digit_Delay ; Wait 1 second
goto Loop ; Restart
;------------------ End sequence, restart from top ------
Tone_Delay ; 300 Hz Tone
movlw d'255'
movwf TEMP
Loop_4
nop
nop
nop
decfsz TEMP,f
goto Loop_4
return
;-------------------------------------------------------------
Output ; Open relay for each pulse
movwf COUNTER1 ; Number of pulses
Next_Pulse
bcf PORTB,2 ; Set Pin 8 low
call Pulse_Delay
bsf PORTB,2 ; Set pin 8 high
call Pulse_Delay
decfsz COUNTER1,f
goto Next_Pulse ; Repeat until COUNTER1=0
return
Pulse_Delay
movlw d'150' ; 60 mS Delay
movwf TEMP
Loop_1
movwf COUNTER2
Loop_2
decfsz COUNTER2,f
goto Loop_2
decfsz TEMP,f
goto Loop_1
return
Digit_Delay
movlw d'16' ; 1 sec Delay
movwf COUNTER3
Loop_3
call Pulse_Delay
decfsz COUNTER3,f
goto Loop_3
return
end
