The circuit consists of a small PIC microcontroller, assembly program,
and a few other parts to detect a switch closure from an open door, window,
or manual push button and then dial the cell phone number, and transmit a
steady tone to indicate the source of the call. The circuit uses the pulse
dialing system to interrupt the line connection a number of times to
indicate each digit. Pulse dialing (the oldest form of dialing) works by
actually disconnecting or `hanging up` the phone line a number of times
to indicate each digit. For example, the digit `5` would be dialed by
disconnecting and reconnecting the line 5 times in short intervals of
about 100mS.
Click here to download the full size of the above Circuit.
There is about a 1 second pause (with the line connected)
between each digit. The timing is not critical and I was able to dial
411 and connect to the local information service just using a momentary
push button switch in series with the phone line.
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)...
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