Electronic Design

Laser Barrier System Uses Tone Detection

In a laser barrier system, the receiver circuit generally uses signal amplitude for detection purposes. This configuration yields a complex circuit with a number of components, including an amplifier, a filter, and a threshold detector. But you can build a simpler circuit that detects the signal's frequency through a tone detector.

An emitter and a receiver make up the laser barrier. The emitter is based on the LM555 timer operating in variable mode. It drives a laser diode with a square wave of frequency f0. The receiver circuit in Figure 1 captures the optical signal through a BPW77 phototransistor (Q1). Q2 then converts the Q1 collector voltage into a square wave, a necessary step due to Q1's frequency response.

Detection is amplitude-independent in this circuit. The LM567 tone detector incorporates a type-I phase comparator and a voltage-controlled oscillator. When the frequency of the signal at the input (pin 3) is in the range of f0 ±(BW/2), the tone detector produces a logic zero state at its output (pin 8). The central frequency, f0, and the detection bandwidth, BW, function as the circuit parameters. The central frequency is:

In the circuit, f0 is set at 10 kHz and fine tuned by potentiometer R7. BW is set at 1.2 kHz. The central frequency value is limited by the phototransistor's speed, and BW is defined by a system response-time requirement. C2 and C3 set bandwidth detection to 12% of f0. NP0 capacitors, such as a polyester type, are used because of their stable behavior over a wide temperature range. The tone detector lock-up time varies between one and 10 cycles of central frequency, so the maximum response time is 1 ms. Q3 drives LED D1, which indicates the barrier state.

The optical path is critical for correct operation. As shown in Figure 2, lenses focus the beam emitted (A) and facilitate the alignment process (B). The lenses also maximize the phototransistor active area (C).

This barrier can be used for precise detection of a target interrupting the optical path. Applications include the detection of a small object in an industrial environment, or the high-speed crossing of a vehicle in competition.

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