This Simple circuit can be used to Guard your bike from theft. It gives a loud alarm tone if somebody tries to start the bike. The alarm disables only when the hide switch S2 is opened. The circuit has little component count and can be easily fixed in the bike.
Working of the circuit is simple. The alarm generator buzzer or Horn is activated by an SCR and its triggering is under the control of the transistor T1. In the Armed position, Switch S2 is closed and S1 (Key switch of the bike) is opened. In this state, T1 will not conduct and SCR and Buzzer remains idle in armed position. When somebody turns on the Key switch of the Bike using a duplicate key, Capacitor C1 charges through R1, D1 and R2. It will take a time delay of few minutes to attain full charge in C1. When C1 fully charges T1 conducts and triggers the SCR. Buzzer connected to the Anode of SCR gets electrical path and it sounds the alarm. LED also lights to indicate the theft. The time delay is added through C1 so that the alarm will sound only after the bike is started .This aborts the attempt of theft.
The unit should hide in a place like the Carrier Box. So that switch S2 can be protected. An additional bike horn can be connected in the place of the buzzer to get loud sound. Connection point from R1 should go to the key switch point that goes to the engine so that; the unit will be activated only if the key switch is closed. Time delay can be changed by changing the value of C1 or R2. Hide switch S2 should be kept closed for arming the bike only after removing the switch key. Power to the circuit is obtained from the 6/12 volt bike battery. High current type transistor T1 and SCR are used to handle the high power of bike battery.Use 1 watt resistors to handle high power.
Note: This circuit is not sufficient if the tricky thief knows some electronics.