This is an Infrared beam
breaking alarm ideal to use in entry or passages.It is based on the
working of the popular IR sensor Module TSOP 1738 which senses 38 kHz Infrared pulses from the IR LED of the transmitter. Range of the circuit is about 5 meters if the transmitter and receiver are properly aligned
TSOP 1738 IR sensor module responds to only 38kHz pulsed infrared rays. It will not sense continuous IR ray from the IR LED.So
a transmitter circuit(as one in TV remote handset) based on 555 IC is
required. Any standard transmitter circuit based on 555 IC can be used.
But its output should be 38kHz exactly. TSOP 1738 gives 5 volt output and 5mA current in the off position.
That is when IR rays are not available.Its output is current sinking so
that when it receives 38kHz IR rays, output becomes zero.Pin 2 of the
module should get a supply voltage between 4.5 to 6 volts.Higher voltage
above 6 volts will destroy the device. The module is generally immune
to ambient light, but may responds to sources of noice such as
Out put from the IR module is given to the inverting input of IC1.
LM311 is a precision voltage comparator . It looks like the common Op
Amps like LM741, CA3130,CA 3140,TL071 etc.But its pin connections and
output are different from other Op Amps.
Pin 2 Non inverting
Pin 1 Ground
Pin7 Current sinking Output
The non inverting input of IC1 is connected to a potential divider
comprising R1 and R2. When the IR sensor gets IR pulses from the
transmitter, output of IC1 remains high. When the IR beam breaks, output
from the sensor becomes high which triggers IC1. It then sinks current
to activate buzzer and LED.