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
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.
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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