This is an IR beam breaking alarm ideal to use in entry or passages. It is based on the working of the popular IR sensor Module TSOP1738 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.
TSOP1738 IR sensor module responds to only 38 kHz 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 electronic ballasts.
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, CA3140, TL071 etc. But its pin connections and output are different from other Op Amps.
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.