This project is different from other general detective Project. In a special Direction two LDR give a different type of sound but in an opposite direction it produces a sound for a lesser duration. You can use this thief way very effectively. Shop keeper , Factory owner can very well fix it up on their main entrance gate and know that such and such person is coming or going out of their premises since the Beep sound differs in duration in both the directions and it can be easily heard.


This project has been developed and tested O.K. in our TV Engineers Technical Lab.  The components and PCB used in the projects published in our books are available for sale at resonable price.

To assemble this project you need two IC555 and two LDR.  We can call this electronic age a magical age. The project being given here is used as a detective.  It is a such type of project that you and your family members will get surprised.  It may be pointed out here that only those people will be surprised who do not have the knowledge of electronics.  You can get the project fixed-up at your gate.  Whenever a person or any thing will move from LDR no. “2” to no. “1”/, then a long beep will be heard.

From this it becomes clear that a beep coming from one side is different from the other side. This is very useful for the securities purpose. Its working principle is very simple. When LDR 1 is glowing 1st IC 555 is meant in ON position and as long as it is ON position the other IC is in a trigger position . Due to this pin no 3 becomes high and it reaches buzzer positive terminal. On account of this other IC becomes negative and buzzer ring up. In case of 1st IC 555 becoming off the other IC555 comes in a trigger position and buzzer stops ringing. In case LDR 2 the LDR is not getting the light and after that LDR 1 is getting the light then a Big Beep is heard. In case the process is reverse the beep is heard for a shorter duration.


  1. Please take care of polarity of the condenser connected at pin no. 6 of IC 555.
  2. Do not solder IC on the PCB directly since there is possibility of parts being damaged due to leakage current in the soldering Iron.
  3. Fix the buzzer after checking the polarity, otherwise the buzzer will get damaged.


PCB [Code 5TP03=1]


100K=2, 100E=1, 100K [TONE]=2, LDR =2


0/01pf=1, 4.7mfd/16V=2


IC555 = 2, IC Base [8 Pin]=2, BUZZER=1


Cabinet =1, Supply 9V DC =1