Darkness Activated Alarm

25 February 2016 User Review

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Dark Alarm Light Detector is a simple DIY electronics project used for both light and dark detection. As a light detector, the project detects sunlight and light from filament bulbs. But it is not good for CFL and tube lights. This is a simple and funny DIY project for students. The demonstration of the kit is easy and the students can use it as a mini project.

Working of dark alarm light detector:

This simple electronics DIY project creates an alarm sound in darkness. In the presence of light, no sound is created by the alarm buzzer. This board is powered from a 9 volt battery. The main components of the kit are simple resistors and capacitors, a transistor, a phototransistor, a 555 I.C, a LED, power supply, PCB board and alarm buzzer. The phototransistor in the Dark Alarm Light Detector Project is used to sense the light. Here the transistor Q1 will act as a switch. As a light detector, the project will allow the phototransistor to sense light and convert it to current .At this stage, the transistor Q1 will not conduct, thereby the timer and alarm will not get activated. No sound is created by the buzzer, thus proving the presence of light in the Dark Alarm Light Detector Project.

darkness activated alarm electronics projects

When no light is detected (dark detection) by the phototransistor, its resistance will increase thereby turning ON the transistor Q1 and current starts passing through it, hence activating the 555 I.C. The activated timer IC in astable mode will cause the alarm buzzer to generate a beeping sound. By adjusting the variable resistor in the kit we can change the sound. The phototransistor should be used with at most care in this simple DIY electronics project while mounting and soldering since they contain optical filters, built with lenses. Also, the transistor and timer I.C should be used with proper Vcc and ground. The PCB used must be of good quality and all the necessary precautions while soldering must be undertaken to prevent the damage of the PCB. The simple components and low cost hardware will allow all the beginners to do their first electronics project cheaply.


  • The kit is good as a light detector.
  • This simple DIY electronics project is good for dark detection applications.
  • Slight modifications in the circuit will allow creating a shadow sensing alarm.

Dark Alarm Light Detector is a simple DIY electronics project for the students and all beginners to DIY electronics projects.

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