June Birth Magoncia

user Electronics Engineer

city Innovuze Solutions, Inc.

  • Anti-Theft Alarm for Motorcycles with Auto ON/OFF Feature

  • Created: Mar 30, 2016

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A motorcycle alarm is a device that minimizes the chance of thieves in stealing motorcycles. Though there is really no way to completely prevent a thief from stealing, an alarm gives the owner a chance to know that someone is trying to steal his motorcycle. In this project, a simple circuit is designed to trigger a siren whenever someone tries to move a locked motorcycle. The whole circuit is composed of passive components, circuit protection, diodes, transistors, relays, and normally open switches.

The normally open switches in this project (mercury/micro switches) are the components that trigger the siren to sound. For example, when the motorcycle is being pushed forward off its center stand, one of the mercury switches completes the loop that will energized the coil of the relay that drives the siren. If one of the normally open switches is closed, the other terminal of the relay coil will be connected to ground through diode D1. This will energized the coil of the relay and as a result, the siren will sound. Also, the PNP transistor Q1 conducts and starts to charge the capacitor C1. If the motorcycle is returned to its original position, the switches will be re-opened. The PNP transistor Q1 switches off and diode D1 opens again. The charge on C1 will make Q2 and Q3 (configures as Darlington pair) conduct and keep the relay energized. This will make the siren sound for some period of time.

The duration of the siren sound is dependent on the charge of the capacitor C1 and the input impedance of the Darlington pair Q2 and Q3. The transistor Q2 alone is enough to energize the relay RY1, but it was paired by another transistor Q3 to form a Darlington pair and increased the input impedance seen by the capacitor C1 to prolong the sound of the siren. The motorcycle ignition switch and the relay RY2 control the auto ON/OFF feature. If the motorcycle is locked and the ignition switch is open, the relay RY2 is not energized and its contacts by default connect the positive terminal of the battery to the power supply node of the alarm. So the alarm is active when the motorcycle is OFF and locked. If the motorcycle ignition switch is closed, the relay RY2 is energized directly by the battery and it automatically disconnects the alarm’s power source leaving it disabled during the time when the motorcycle is being used.



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