@▷ Alarm Circuit Built for Motorcycles | Diagram for Schematic

Alarm Circuit Built for Motorcycles


The circuit was designed to create an early warning alarm system against any form of theft that is significant to the owners of motorcycles or bikes.

TerminologyMercury Switch – a switch constructed into a sealed glass tube or envelope containing two or more electrical contacts and a small amount or a bead of liquid metal mercuryBC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selectionsBC557 – PNP general purpose transistors used for amplification and switching due to its low current and low voltageCircuit Explanation

This alarm system can utilized any number of normally open switches like the mercury (tilt) switch. It functions by allowing or interrupting the flow of electric current in a circuit. The physical position or alignment of the switch in relation to the earth’s gravity is where the functionality of the switch depends a lot. No current will flow to the electrical contacts as long as the mercury stays on the lowest point of the glass bulb. However, once the switch is tilted at a certain angle or direction, the liquid mercury will be touching the set of contacts and will produce an electrical connection between the contacts. When the switch is tilted back to the opposite direction the electrical current stops immediately.

The mercury switch is positioned in such a way that they will close when the bike is lifted off its side stand, pushed forward off its center stand, or the steering is moved. The siren will then sound for at least one witch that will close. The alarm will reset, approximately after two minutes, when the switches have been opened again. Changing the value of capacitor C1 changes the time delay for the alarm to turn OFF. When the alarm is silent, no current is virtually used.

The circuit should be properly protected from condensation or dampness as these may cause the circuit to not to function properly. The components should fit the size of the enclosure so the wirings can be well protected. Alternatively, a hidden switch can be used instead of a key switch. Also, a relay’s normally closed contacts can be used while the ignition energizes the relay coil and the alarm will set itself upon turning OFF the ignition


In general, motorcycle alarm systems function in a different way as compared with those in automobiles. These alarms can be created in several forms such as a remote receiver where the alarm has the capability to be controlled via a remote control; as tilt sensors where a mercury balance sensor is utilized by being hooked up on the motorcycle which is a guaranteed way of interference detection; and as shock sensors where a loud distress signal is produced due to the signal that is transmitted to the alarm box when the motorcycle is tampered with, touched, or moved.

Choosing the appropriate alarm system should consider the following factors like the speaker alarm dimensions, main unit dimensions, working temperature, static current, alerting volume, working frequency, range of remote, and basic functions such as auto re-arming, mute alarm, sensitivity adjustment of remote, anti-hijacking, remote finding of motorcycle, remote arm and disarm, and remote cutoff engine.

The mercury switches are very useful in different types of applications, in spite of the hazardous threat of the chemical mercury, especially when permitted to vaporize. Although it can be handled safely, the mercury can cause death or nerve damage if skin contact or accidental ingestion occurs. This is why lots of manufacturers started replacing the mercury switches with other mechanical method of interrupting electrical current. On the other hand, they are still sought because of their accuracy like the water level sensing, open/close mailbox, room thermostats, motion-activated bomb detectors, car hood and trunk light activation, and home lighting circuits.

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