The Anti-Theft Alarm is a device that utilizes three switches, representing sensors that monitor the status of an item that the alarm protects. We designed the alarm to monitor four outputs, three lights and a buzzer. The first light will emit a green light if none of the sensors activate. After one sensor activates, the green light shuts off and the second yellow light turns on. The second sensor triggers a third red light to shine, but also turning off the yellow light. Finally, if the third sensor goes off, the red light shuts down and an alarm sound, signaling the device may soon be stolen.
Our experience began with a simple, flawed design. Using MOSFETs, we believed that we could easily create a circuit to turn on each light sequentially, but we also found that this design allowed every light to operate simultaneously if every switch remained on. Our next design had sixteen AND chips, but, after we had replaced a fried chip, we realized that this design also failed because it re-routed power inside the design chip. We changed the design and added four OR chips, two for both the yellow and red lights, to force the current to flow only through the LED and not backwards through other gates leading to the light. With a complete design in place, we had a circuit in which the process to activate the alarm instead turned on all of the lights. We finally identified an inverter chip that had no grounding wire; fixing this problem allowed the circuit to work.