This is the lowest cost dialing alarm on the market and shows what can be done with an 8-pin microcontroller. The complete circuit is shown below. You cannot see all the features of this project by looking at the circuit - most of them are contained in the program. So, read on and see what we have included. Dial Alarm-1 has a single input (although a number of sensors can be placed in parallel on the same input line). The circuit requires a trigger pulse to turn on a BC 557 transistor. This delivers power to the microcontroller. The micro starts to execute the program and outputs a high on GP2 to keep the `turn-on` circuit active. It also turns on the LED in the opto-coupler and this causes the line to be `picked up` via a high-gain Darlington transistor. The micro then dials two phone numbers and executes a series of events to alert the called party of an intrusion.
DIAL ALARM-1 - schematic

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