To protect the loud speaker from destructive signal, the most simple way is by placing a fast blown fuse between speaker and the amplifier output. This common solution is depicted in the figure 1 (a).

Figure 1. Fuse Placement to Preserve Good Bass Response

The fuse in the circuit has a significant resistance to generate heat, because without this the fuse couldn’t melt. The resistance affect the amplifier response in reacting to the speaker inductive load, but the worst is that this resistance varies with the temperature caused by the dynamic signal current. This resistance variation affect the linearity of the amplifier’s bass response. This situation can be solved by placing the fuse inside the amplifier’s feedback loop. Using this method, any nonlinear effect introduced by the fuse will be compensated by the amplifier via feedback mechanism. The resistor R3 is chosen to be much smaller compared to R2 but much higher compared speaker’s impedance. A value of 220? is suitable for most cases.

The idea that a half lie is much more dangerous than 100% lie is implemented in this design. Rather than showing off that our car is 100% protected by a sophisticated alarm and protection system, this anti burglar protection circuit prefer to lie that the car is not protected by any electronic alarm or any protection system. The circuit presented here is based on unusual method to protect car theft. Need not a long time to wait after the machine is started, some troubles will arise. Restarting the machine will produce a same trouble, even worst.

The circuit is very simple. A 555 timer provide a time delay about five seconds. A normally-closed relay is connected between ignition switch and the ignition coil. S1 switch that is used to activate this protection system should be placed in a hidden area. When the ignition switch is turned on to start the machine, the current will flow normally to the ignition system via the relay and the machine will start to work. After a time delay set by the 555 timer ( the time delay can be varied by selecting different value for R1 and C1) , the relay will be open, and the ignition coil will be disconnected, the machine will show something wrong by stop working. You’ll guess that the burglar will try for two or three more try before giving up.