darkness activated alarm


Posted on Feb 6, 2014

Most darkness activated alarms employ opamps and some logic ICs. Here, a less expensive approach is shown based on the eternal 555, this time in monostable multivibrator mode. Components R2 and C1 represent a one-second network. When the LDR (light dependent resistor) is in the dark, its resistance is high, pulling pin 2 of the 555 to ground. This


darkness activated alarm
Click here to download the full size of the above Circuit.

triggers the monostable and the (active!) 6-volt piezo buzzer will sound. Preset P1 is adjusted depending on ambient light levels. The circuit may be fitted on a wall in your home. Assuming P1 has been set for the existing ambient light level, the shadow cast by anybody entering the room or hallway will trigger the alarm.




Leave Comment

characters left:

Related Circuits

  • New Circuits

    .

     


    Popular Circuits

    Battery Current Sensor
    Broadcast-band-rf-amplifier
    muRata Manufacturing Measurement-Based SMT Capacitors
    Quiz Project
    Light sensor using Op Amp
    laser diode driver circuit
    RS 422 Compatible Indicator
    Metal detector circuit diagram 3
    Low-noise differential amplifier circuit composed PGA203 and OPA27
    Application Circuit CW4960 CW4962 dual voltage + 5V 1.5A -12V 100mA output
    Unidirectional thyristor dimmer lights triggered ordinary transistor circuit



    Top