EDN Access -- 09.01.94 Two LEDs perform rudimentary spectral analysi

-September 01, 1994

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Design Ideas:September 1, 1994

Two LEDs perform rudimentary spectral analysis

Peter Bojilov,
Isomatik Sortex Ltd, Pazardjik, Bulgaria

A pair of LEDs having different wavelengths can supply the stimulus for a rudimentary spectral analysis during quality-control sorting operations. By measuring the reflectance of each LED, the circuit in Fig 1 can provide information about the spectral response of the object under test. The circuit's operation is akin to the human eye's using three receptors to deduce information about the entire visible spectrum.

The circuit in Fig 1 has two emitters, D1 and D2, but only one sensor, a TI TSL250 integrated photo receiver, IC1. The clock circuit, IC2, IC3, and associated components multiplex the single sensor between the two LEDs' outputs, as well as sampling the ambient light.

IC3's Q1 and Q6 outputs turn on an LED and its corresponding S/H amplifier via the NAND-gate chains. The delay networks at the inputs of the NAND-gate chains going to the sensor's S/H ICs compensate for the sensor's output-response delay.

IC3's Q4 and Q0 outputs, diode-ORed together, take a sample of the ambient light when the LEDs are off. The circuit subtracts this reading from the LEDs' readings. The MAX 720 programmable filter, IC4, cleans up the outputs of two LEDs' reading.

Potentiometers R1 and R2 allow you to adjust the LEDs' current between 25 and 400 mA. (DI#1580)

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