This circuit simulates an alarm to discourage thieves and it is originally designed to be used on bicycles but can be used for cars too. It blinks a LED on and off thus simulating a real alarm circuit.
The fake LED alarm electronic circuit consists of a square wave oscillator generating a 1 MHz signal with a pulse width of 10 milisecond. The LED lights u 10 miliseconds long. The circuit consumes 30 miliamperes when the LED lights and the average consumption is 300 microamperes.
To switch off the circuit automatically it has a built in LDR. When the LDR is exposed to light it becomes low ohmic and the output of U1 becomes 0. The oscillator stops and the LED turns off. The light sensitivity of the circuit is set with the potentiometer P1.
When there is no light falling on the LDR, the input of U1 is connected to the ground and its output becomes “1” thereby starting the oscillator which drives the LED to blink. If the circuit is powered with a 4.5 volts battery that has a 2 Ah capacity, it will work for aroung 1.5 years.