Electronic Design

Economical Circuit Captures Light Pulses

For those needing a method to capture a light pulse and even color, the circuit in the figure may offer a solution—without investing hundreds of dollars. Although typically employed as a display, an LED can be used in reverse to capture light and emit a small voltage. With a bright white light, some LEDs can output over 1.0 V.

Depending on the color, the LED can be used to discriminate against other colors. A green LED will produce a voltage output with a green light source, but almost no voltage with a red light source. This comes in handy if you need to ensure against detecting the wrong-color LED. Note that the LED doesn't discriminate against white light, as it inherently has all colors in the source.

The voltage of LED1 is fed into comparator U1 (LM2902), which controls the set point where the circuit determines a valid light pulse. The set point is controlled through potentiometer R4, a simple voltage divider. The output of U1 is then sent to U2, a 74HC04 logic inverter, and subsequently fed into U3 to trigger a monostable circuit.

Taken from the family of the 555 timer circuits, U3 (an LM556) is a dual-timer IC that can hold the valid trigger up to seconds, as determined by the time constant of resistor R1 and capacitor C1. The output on pin 5 can source or sink 200 mA and easily drive an LED or relay.

The monostable offers the benefit of holding the pulse on, providing adequate time to verify if an LED circuit has its output on. The ability to capture such an event improves the ability of testing some LED circuits that may have no control over duration.

See the figure

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