Motorbikes are often a target for thieves. Here is an alarm that’s loud, cheap and simple to build. Arming and disarming the alarm is done with a hidden switch, S1. This tiny circuit does not unduly load the battery, as it draws very little current in the standby condition. To activate the alarm, turn or press the hid-den switch S1 to the ‘on’ position. If anyone attempts to start the motorbike, +12 volts from the ignition switch (connected to ‘B’) causes transistor T1 to conduct and switch on T2. The siren (LS1) then sounds for about 20 seconds, the period being determined by FET T3 wired as a monostable timer. The siren is a high-power ready-made piezo horn of the self-oscillating type. Circuit diagram : Cut-rate Motorbike Alarm Circuit Diagram Another piezoelectric component in the cir-cuit has a different purpose — Bz1 detects attempts to tamper with the vehicle, or move it without starting the engine. The piezo trans-ducer element should be mounted in suc...