DIY Cell Phone Security System: Dial Your Way to Safety!
Description
A self-built home security system initiates calls to your mobile phone, office, or other designated numbers whenever a door or window is opened, or a panic button is activated. This creates a robust Home Alarm system.
The circuit incorporates a small PIC microcontroller, an assembly program, and several other components to detect a switch closure from a door, window, or manual push button, subsequently dialing the cell phone number, and transmitting a constant tone to signify the call’s origin. The circuit employs pulse dialing, interrupting the line connection multiple times to represent each digit. Pulse dialing, representing the oldest dialing method, achieves this by briefly disconnecting and reconnecting the phone line a certain number of times to encode each digit. For instance, the digit "5" would be dialed by disconnecting and reconnecting the line five times within approximately 100 millisecond intervals, followed by a one-second pause with the line connected. The timing isn't crucial, and I successfully dialed 411 and connected to the local information service solely using a momentary push button switch in series with the phone line.
Circuit Operation:
During operation, the switch closure is detected on pin 7 of the processor, activating the reed relay and placing the line on-hook for 3 seconds to establish the dial tone. The processor then dials the number by opening and closing the relay multiple times for each digit. Upon completion of dialing, the processor waits 3 seconds before transmitting a constant tone of approximately 300Hz for 30 seconds through the modem transformer. The call is then terminated, and the processor awaits the switch to open before resetting.
Design Considerations:
The PIC16F628 (18 pins) processor was selected due to its availability and compatibility with my custom hardware programmer, which is specifically designed for 18-pin devices. A smaller 8-pin device could have been utilized since only three I/O lines are needed, but the cost difference was only about $1.50. One of the I/O pins (RA5) is reserved for programming and remains an input, yet can function as a functional input to detect switch closure, thus eliminating the need for a pull-up resistor. Instead, three consecutive I/O pins (7, 8, and 9) of the 8-bit port B were utilized, with RA5 connected via a 10K resistor. The output pins (8 and 9) driving the relay and transformer are limited to 25mA of current, necessitating an additional transistor (2N2222A) to supply the required current to the relay coil. The transformer resistance is approximately 100 ohms; therefore, a 330 ohm resistor was added in series with pin 9 to limit the transformer’s current to approximately 10mA. An LED indicator and 330 ohm resistor were used on pin 8 to monitor dialing activity and indicate line status. Several components – the relay, transformer, and blocking capacitor – were salvaged from an old 56K modem card. The schematic shows a 47uF / 50 volt non-polarized capacitor used to block DC current to the transformer. A standard polarized 50uF capacitor could be utilized if the correct phone line polarity is observed. The modem was likely designed to operate with undefined polarities across various locations, thus necessitating a non-polarized capacitor. It's feasible to replace the capacitor and 470 ohm resistor with a single resistor in series with the line to set the "on-hook" line current to approximately 20mA. This might result in partial saturation of the transformer and reduced audio levels, but could still function adequately. The power supply voltage is not critical; a 4.5 volt supply from three AA batteries should suffice. Alternatively, a switching type regulated 5 volt wall transformer could be employed. The primary concern is ensuring the relay receives sufficient voltage to operate correctly. The remaining circuit sections can operate on reduced voltages. I utilized a 4.2 volt cell phone charger, which performed reliably.