Cheap Motorcycle Alarm

This is simple to build and cheap motorcycle alarm circuit which could be fitted in motorcycles to take care of them from getting stolen. The tiny circuit may be hidden anyplace, without having any difficult wiring. Practically, it fits all motorcycles as long as they’ve a electric battery. It is not going to drain out the battery though because the standby current is zero.

The hidden switch S1 could be a small push-to-on switch, or perhaps a reed switch with magnet, or any other identical simple arrangement. The circuit is made around a few lowvoltage MOSFETs configured as monostable timers. Motorcycle key S2 is an ignition switch, whilst switch S3 is a tilt switch.

Motorcycle key S2 gives power supply to the gate of MOSFET T2, when switched on. If you turn ignition off making use of key S2, you have about 15 seconds to get off the motorcycle; this function is executed by resistor R6 to discharge capacitor C3. Thereafter, if anyone tries to get on the motorcycle or move it, the alarm sounds for about 15 seconds and also disconnects the ignition circuit.

During parking, hidden switch S1 is normally open and will not permit triggering of MOSFET T1. But when somebody starts the motorcycle by using ignition switch S2, MOSFET T2 triggers through diode D1 and resistor R5. Relay RL1 (12V, 2C/O) energises to switch on the alarm (designed close to IC1) as well as to disconnect the ignition coil from the circuit. Disconnection of the ignition coil avoids generation of spark from the spark plug. Generally, you will find there’s wire running from the alternator towards the ignition coil, which needs to be routed through one of the N/C1 contacts of relay RL1 as shown in the circuit diagram.

Also, on disconnection of the coil, sound generator IC UM3561 (IC1) gets power supply through N/O2 contact of relay RL1. This drives the darlington pair built around T3 and T4 to produce the siren sound through loudspeaker LS1.

To start the vehicle, both hidden switch S1 and ignition key S2 should be switched on. Otherwise, the alarm will start sounding. Switching on S1 triggers SCR1, which, in turn, triggers MOSFET T1. MOSFET T1 is configured to disable MOSFET T2 from functioning. As a result, MOSFET T2 does not trigger and relay RL1 remains de-energised, alarm deactivated and ignition coil connected to the circuit. Connection to the ignition coil helps in generation of spark from the spark plug. Keeping hidden switch S1 accessible only to the owner avoids the motorcycle from pillaging.

Tilt switch S3 prevents attempt to move the vehicle without starting it. Glass- and metal-bodied versions of the switch offer bounce-free switching and quick break action even when tilted slowly. Unless otherwise stated, the angle by which the switch must be tilted to ensure the contact operation (operating angle), must be approximately 1.5 to 2 times the stated differential angle. The differential angle is the measure of the “just closed” position to the “just open” position.

The tilt switch has characteristics like contacts make and break with vibration, return to the open state at rest, non-position sensitivity, inert gas and hermetic sealing for protection of contacts and tin-plated steel housing. If you find difficulty in getting the tilt switch, you may replace it with a reed switch (N/O) and a piece of magnet. The magnet and the reed switch should be mounted such that the contacts of the switch close when the motorcycle stand is lifted up from rest.

Note. Please ensure that while driving, the two internal contacts of the Tilt switch don”t touch each other.

SCR BT169, MOSFET BS170 and transistor BC548 pin configurations

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