Infrared Beam Break Detector

This purpose of this article is to design a circuit using Infrared signals to detect a beam break which can be used in multiple real world applications. The IR receiver used is TSOP1738. Below are some of the main requirements of Infrared Transmiter signal properties as described in the datasheet

• Carrier frequency should be close to the  center frequency of the bandpass (38kHz)
• Burst length should be 10 cycles/burst or longer
• After each burst which is between 10 cycles and 70 cycles a gap time of at least 14 cycles is neccessary

Design

1. Carrier Frequency (f₁) : The center frequency of TSOP1738 is 38kHz

f₁ = 1.44/((R_a1+2R_b1)C)
f₁ = 38kHz

Let
R_a1 = 1k
C = 0.01uF

With that R_b1 = 1.394k or
R_b1= 2k variable resistor


2. Burst and Gap frequency (f₂) : Let burst cycle equals gap cycle be equal to 40 cycles (burst between 10 and 70 cycles and gap greater than14 cycles)

f₂ = 38k/ 40
f₂ = 950
f₂ = 1.44/((R_a2+2R_b2)C)

Let
R_a2 = 10k
C = 0.01uF

With that
R_b2 = 70.789k or
R_b2 = 100k variable resistor

Note:
1. BC547 is used at the output of transmitter as switching transmitter to boost the voltage increasing the range.

2. When the Infrared beam is broken, the output of TSOP1738 goes high. Using switching transistor 2N2222, the signal is inverted to High to Low which can be directly interfaced to External edge triggered interrupts of 8051 (EXT0 and EXT1)

Infrared beam break detector transmitter receiver circuit