An unusual design which uses an LED both as the light sensor and indicator

In last month’s issue, in the final part of the Light Emitting Diodes series, we showed how the photodiode can be used as a precision light sensor. By generating a photocurrent proportional to light intensity, the photodiode produces an electrical analogue of the measured light source that remains linear over some six or seven decades of light intensity. Furthermore, by combining the basic photodiode function with additional circuitry, semiconductor manufacturers have produced a variety of light-sensitive devices that generate either a direct voltage output or, for example, in the case of light-to-frequency converters, a squarewave signal whose frequency varies according to the strength of the detected light source.

However, in many light sensing applications, there is often a need to detect simply a change in light level, rather than a linear response to light intensity. Typical examples are products like digital alarm clocks that use a light sensor to dim the 7-segment l.e.d. display when the ambient light level is relatively low, or automobile light sensors which prompt the driver to switch on the headlamps when it is getting dark. This project makes use of an l.e.d. as a simple light sensor that generates a photovoltage that varies with light intensity. By comparing the photovoltage with a variable reference voltage, this Light Detector provides a digital output signal that changes state when the monitored light level crosses a preset threshold.

The detector also provides visual indication by illuminating the sensor l.e.d. when the light exceeds the preset level. In this way, the l.e.d. provides a dual function, acting as both the sensor and the indicator. An optional, optically-isolated output is also available.