Home | Site Map | Components | 555 | Symbols | Study | Books | Construct | Solder | Projects | FAQ | Links | Privacy
Electronics Club
electronicsclub.info

Light-sensitive Alarm Project

Download PDF version of this page

The circuit detects a sudden shadow falling on the light-sensor and sounds the bleeper when this happens. The circuit will not respond to gradual changes in brightness to avoid false alarms. The bleeper sounds for only a short time to prevent the battery running flat. Normal lighting can be used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor. Breaking this beam will then cause the bleeper to sound. The light sensor is an LDR (light-dependant resistor), this has a low resistance in bright light and a high resistance in dim light.

  • The light-sensitivity of the circuit can be adjusted by varying the 100k preset.
  • The length of bleep can be varied from 0.5 to 10 seconds using the 1M preset.
Using the 7555 low-power timer ensures that the circuit draws very little current (about 0.5mA) except for the short times when the bleeper is sounding (this uses about 7mA). If the circuit is switched on continuously an alkaline PP3 9V battery should last about a month, but for longer life (about 6 months) you can use a pack of 6 AA alkaline batteries.

This project uses an edge-triggered 555 monostable circuit.


Parts Required

Bitsbox
Hobbycraft
  • resistors: 10k, 47k, 1M ×3
  • presets: 100k, 1M
  • capacitors: 0.01µF, 0.1µF, 10µF 25V radial
  • transistor: BC108 (or equivalent)
  • 7555 low-power timer IC
  • 8-pin DIL socket for IC
  • LDR (light-dependant resistor) type ORP12
  • bleeper 9-12V
  • on/off switch
  • battery clip for 9V PP3
  • stripboard 12 rows × 25 holes


Stripboard Layout

Stripboard layout for light-sensitive alarm

Circuit diagram

Circuit diagram for light-sensitive alarm



© John Hewes 2014, electronicsclub.info (based in the UK)
No part of this website may be reproduced in any way commercially without my prior permission.

Site Map

Top of page

Home
Downloads
Books
FAQ
Contact
Discounts
Links

Construction
 - Breadboard
 - Stripboard
 - PCB

Soldering
 - Tools
 - Workbench

Components
 - Capacitors
 - Connectors
 - Diodes
 - ICs ('Chips')
 - CMOS logic ICs
 - 74 series ICs
 - Lamps
 - LEDs
 - Relays
 - Resistors
 - Switches
 - Transistors
 - Variable Resistors
 - Others
 - Starter Kit

555 Timer IC

Study
 - Block Diagrams
 - Circuit Diagrams
 - Circuit Symbols
 - Electricity/Electron
 - Series & Parallel
 - Voltage & Current
 - Meters
 - Multimeters
 - Resistance
 - Ohm's Law
 - Power & Energy
 - AC & DC Signals
 - Oscilloscopes
 - Power Supplies
 - Transducers
 - Voltage Dividers
 - Transistor Circuits
 - Analgoue & Digital
 - Logic Gates
 - Capacitance
 - Impedance
 - Counting
 - Quantities & Units
Projects
 - Dummy Alarm
 - Quiz
 - Traffic Light
 - Dice
 - Model Lighthouse
 - Simple Tester
 - Flashing LED
 - Electronic 'Lock'
 - Adjustable Timer
 - Light Alarm
 - Xmas Decoration
 - Railway Signal
 - Level Crossing
 - Lead Tester
 - 'Random' Flasher
 - Heart Badge
 - Valentine Heart

More Projects

Kits for Children

Electronics Club

Tsohost
Use coupon code
electron10 for 10%
discount off hosting
Privacy Policy & Cookies
This website does not collect any personal information unless you contact me by email. If you send me an email your name, email address, and any other personal information you supply will be used only to respond to your message. Your personal information will never be given to any third party without your permission.
This website displays affiliate advertisements. If you click on these advertisements the advertiser will know you came from this site and I may be rewarded if you become their customer. No personal information is passed to advertisers.
This website uses StatCounter cookies to estimate the number of unique visitors. No personal information is stored in the cookies. If you would like further information or wish to refuse these cookies please visit the StatCounter website.
To learn how to delete and control cookies from your browser please visit AboutCookies.org.