Burglar Alarm System With High Sensitivity
Most
standard light sensors that used in ordinary burglar alarm systems
requires precise alignment of light beam to deactivate the alarm system
during standby mode. However this circuit of burglar alarm is so
sensitive that it will detect a moving person at a distance of few
meters in pure daylight or under electric lighting.It requires virtually
no set up, and may be simply placed within the line-of-sight of almost
any light source including ambient daylight or high end fluorescent
electric light. The alarm system uses a beep generated from the circuit
driven through a buzzer that is high enough to detect the entry of a
person inside the fence.
This
security system uses a op amp comparator 741 with two potential
dividers in its inverting and non inverting inputs. (about half of
supply voltage). The burglar detector LDR and its controlling preset VR1
from another potential divider to provide a variable input to NIV input
pin 3.
Adjust VR1 properly then output of 555 ic will be high because of the LDR effect. Ie when the shadow of a person come across the LDR light path, its light intensity level and the voltage at the non-inverting input of IC1 will drop momentarily.The output of IC1 suddenly becomes low, allowing T1to conduct. This triggers the monostable (IC2) and the alarm sounds.
You can easily set up the circuit in a common PCB and keep Keep LDR1 inside a black tube to increase its sensitivity. On testing with ideal conditions, the alarm system detected a person from a distance of about three meters.
Adjust VR1 properly then output of 555 ic will be high because of the LDR effect. Ie when the shadow of a person come across the LDR light path, its light intensity level and the voltage at the non-inverting input of IC1 will drop momentarily.The output of IC1 suddenly becomes low, allowing T1to conduct. This triggers the monostable (IC2) and the alarm sounds.
You can easily set up the circuit in a common PCB and keep Keep LDR1 inside a black tube to increase its sensitivity. On testing with ideal conditions, the alarm system detected a person from a distance of about three meters.