Electronic Design

Add Position Detection Capability To Light-Curtain Circuitry

Light curtains use a linear array of LEDs and optical detectors to sense when an object breaks a light beam. Then they trigger an alarm output. However, light curtains typically don't provide spatial data to indicate where along the light curtain the light beam was broken. The design shown here adds that feature. It describes a circuit that senses where along the light curtain the beam was broken and transmits the location to a PC via RS-232.

The length and resolution of the light curtain is easily expanded, and it can operate with a wide window opening. The circuit uses an array of 16 multiplexed infrared (IR) emitters and IR receivers.

The receiver board incorporates 16 Vishay TSOP6236 IR receivers, U0-U15 (Fig. 1). They are designed for TV remote-control applications, which operate at a 950-nm infrared wavelength with a 36-kHz carrier. The IR receivers offer a wide transmission range with excellent ambient light rejection. TTL outputs from each IR receiver interface to a Maxim MAX7313 16-port I/O Expander (U16) configured as TTL inputs.

The IR emitters comprise an array of 16 Vishay TSML1020 950-nm wavelength pulse LEDs (D0-D15) (Fig. 2). The LEDs are pulsed at 36 kHz by an Intersil HA7210 crystal oscillator (U1) and an International Rectifier IRLML2402 MOSFET (U2). Individual LEDs can be switched on and off one by one by a MAX7313 (U3) operating with current-sink outputs.

Both the IR receiver's and the IR emitter's MAX7313s are I2C-addressable and interface to a microcontroller via a two-wire interface: data and clock lines. The light curtain's length or resolution is easily expanded via additional pairs of 16-channel IR receivers and IR emitters. Each MAX7313 unique address is set via J3, J4, and J5. An MC78LC33 +3.3-V dc voltage regulator powers the MAX7313s. Each MAX7313 is terminated with 4.7-Ω pull-up resistors on the clock and data lines via J6 and J7.

C-language software routines, running on a Microchip PIC18F452, turn on each emitter-receiver channel one by one via the I2C interface to scan the sensing window. This scanning technique prevents adjacent IR signals from getting around the object in the light path and into an active IR receiver. The C source code to scan a 16-channel IR emitter and IR receiver, transmit the location via RS-232, and provide a simple graphic output is available for download here.

This simple circuit can be applied to a variety of applications, such as robotics, automation, alarms, and monitoring.

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