Simple Light Sensor Circuit Features High Dynamic Range

Ambient light is increasingly being used as a source of energy. To help designers of such systems, this circuit accurately measures ambient light intensity over four decades of measurement. The design is highly cost effective and, with the sensor mounted in a waterproof glass enclosure, can be deployed in the field for continuous monitoring in all
Simple Light Sensor Circuit Features High Dynamic Range - schematic

weather conditions. The sensor in this application, a BPW21R photocell (from Vishay), is used to generate photocurrent proportional to the ambient light intensity ( see the figure ). A photovoltage measurement technique would not offer the high dynamic range desired. The LMC555 CMOS timer converts the photocurrent from the BPW21R into a transistor-transistor-level (TTL) frequency source (10 Hz to 100 kHz). Any other equivalent timer IC can also be used, but component quality is very important in order to get high dynamic range with accurate measurements. Capacitor C1 should be an NPO/COG or X7R type. NPO/COG capacitors have the lowest temperature coefficient and minimize frequency drift with temperature. C2 is a filter capacitor for the comparator voltage references. C1 and resistor R1 are timing elements. While charging time depends on the photocurrent from the BPW21R and the value of C1, the discharge time for C1 depends on its capacitance and the resistance of R1. Since both charging time and discharge time contribute to the output`s frequency, it`s important to select R1 to keep the discharge time as low as possible. In this case, R1 is 1 k © and C1 is 1 nF, making the discharge time 1 µs. The photocell`s dark current is about 2 nA and its maximum output is about 60 µA at saturation level. The cell`s linear sensitivity is 9 nA/lx or 9 µA/klx. This high dynamic range allows the photocell to operate from 10 nA to 50 µA...

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