This circuit is
presented as an alternative to the IR beam break detector featured in
the June 2009 issue (Silicon Chip). In order to make it relatively
insensitive to ambient light, it uses a standard IR receiver IC such as
the Jaycar ZD-1942. This has a high output (+5V) as long as a modulated
beam is detected.
The IR detector (IC3) controls an LM7555 CMOS
timer (IC2) which operates in monostable mode. When the beam is
broken, IC2 is triggered and its pin 3 output goes high for about half a
second. This extinguishes LED1 and turns on transistor Q1 to drive a 5V
The circuit is powered from six AA cells and a 78L05 5V regulator
(necessary for the receiver IC). The IR transmitter is also built around
an LM7555 (IC1), this time operating in astable mode at low duty cycle.
Its frequency is set to 38kHz with trimpot VR1. The IR diode was
salvaged from a defunct remote control but these are readily available
new. The transmitter is powered by four AA cells.
The system has a range of several metres and while it is insensitive
to the transmitter alignment, the detection window can be narrowed by
placing the detector near to the object to be detected and/or using some
form of baffle to restrict the window.
Source: Silicon Chip 14 March 2010