IR Beam Break Circuit: 38kHz Timer for Camera Control

Circuit diagram:

Description

This circuit has been offered as a replacement for the infrared beam interruption sensor detailed in the June 2009 edition of Silicon Chip. To achieve a degree of immunity to surrounding light, a commonly utilized infrared receiver integrated circuit, like the Jaycar ZD-1942, is utilized. This component provides a high output (+5V) provided a modulated beam is detected successfully.

The infrared receiver (IC3) governs an LM 7555 CMOS timer (IC2) which functions in a monostable configuration. Upon beam breakage, IC2 is activated, and its pin 3 output rises to a high level for approximately half a second. Consequently, LED1 is switched off, and transistor Q1 is activated to drive a 5V low-power relay.

The circuit is supplied with energy from six AA batteries combined with a 78L05 5V regulator (required for the receiver IC). The infrared transmitter is constructed using an LM7555 (IC1), operating in an astable mode at a reduced duty cycle. The circuit's frequency is set to 38kHz using a trimpot VR1. The infrared diode was recovered from a discontinued remote control; however, these are accessible for purchase new. The transmitter is powered by four AA batteries.

The system possesses a range of several meters, and despite its resilience to transmitter alignment, the detection window can be narrowed by positioning the detector in close proximity to the target object and/or by employing a baffle to restrict the window’s dimensions.

LM7555 CMOS Timer

The LM7555 is a versatile CMOS timer integrated circuit. It's commonly used in circuits requiring precise timing functions. This timer is key to the operation of the beam detection circuit, providing a reliable delay when the infrared beam is broken. Its ability to operate in a monostable mode is crucial for generating the brief high-level pulse used to trigger the relay and control the LED. The device’s core functionality is based on a capacitor charging and discharging process, which is precisely controlled by the IC's internal circuitry, ensuring accurate timing performance. It’s a robust and relatively low-power device, making it well-suited for a range of applications where precise timing is essential.