Circuit knowledge

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.

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 such a  way as to faithfully pick up vibration from the  motorbike frame due to tampering.

One set of contacts on relay RE1 is used to  effectively disconnect the ignition coil to pre-vent the bike from functioning when some-one tries to steal it. Usually, there is a wire  running from the alternator (point A) to the  ignition coil (TR1), which has to be routed  through the N/C (normally closed) contact  of the relay. The hidden switch S1 is preferably a miniature type or its electrical equivalent. To deactivate the alarm, the hidden  switch should be flipped to the ‘off’ position  to disable the movement sensor and the siren driver/timer circuit when the ignition key is  turned…