PIC Security System Dials Your Cell Phone


Do-it-yourself phone dialer security system calls your cell phone, office etc.
whenever a door or window is opened, or panic button is pressed. Great Home Alarm.

PIC Security System Dials Your Cell Phone

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. 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.

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.


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