A compound device is a collection of more than one component. The components in a compound device are inter-related to one another and centrally controlled by a micro-controller unit. Depending upon the state of one component, the other components react.

For example, a fire alarm is a perfect example of a compound embedded device--as soon as fire is detected, the water sprinkler system and evacuation alarm system gets activated. In the case of the fire alarm system, a fire sensor attached with a micro-controller detects fire and then the micro-controller sends signals to the peripheral components for orchestrating various activities such as activating water sprinklers and fire alarms. Advanced fire alarm systems can automatically dial emergency services via a GSM module.

Building a certified fire alarm system is an advanced topic and would be beyond the scope...