light activated police siren using 555 timer

Posted on Feb 7, 2014

Here`s a simple light activated police siren using light dependent resistor and NE555 timer. Before you make this project, I would recommend you to make the following projects. The 555 timer with R1=68K(say timer1) is configured in slow astable mode and the timer with R4=10K(say timer2) is configured in fast astable mode. By applying voltage to t

light activated police siren using 555 timer
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he Pin 5 (control voltage input), we can change the timing characteristics of the timer. In the astable mode, control voltage can vary from 1. 7V to the input voltage level(i. e. 6V in this case) that produces output frequency lower or higher than the frequency produced by the network of R1, R2 and C1 for timer1 and R4, R5 and C3 for timer2. To know more about the output frequency of 555 timer, read this tutorial: ASTABLE MODE The police siren effect is produced because of the input voltage at pin 5 of timer2. It makes the timer2 to produce frequency modulated output. In this circuit, timer1 controls the output frequency of timer2, producing police siren effect. When you put light on LDR, the police siren circuit gets power from the transistor Q1. You can change the sensitivity of the circuit by varying the resistance VR1. If you swap LDR and VR1, the circuit works in opposite way. HOW TO MAKE A LIGHT SENSOR

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