improved anti-theft device

52 For people who tend to be rather absent minded the anti-theft circuit published in the April issue of Elektor has one drawback. It only works when it is switched on! With the modification shown here the circuit will operate the other way round. In other words, the anti-theft device will be activated all the time until it is switched OFF by pressing a (concealed) pushbutton upon entering the car. Electronically speaking, the simplest method of switching the system off is to "block" the circuit by using a thyristor. The accompanying circuit diagram shows all the modifications required to the original design. Switch S1 of the original circuit has been omitted and a push-button Sx has been included. The only other extra components are a few resistors and a thyristor. Resistor R1 can be replaced by a preset potentiometer and the value of C1 should be increased. The operating time of the circuit will then depend on the setting of the pot and can be varied over a wide range. When the ...