With the negative input and REF pin grounded, and V IN+ applied to the positive input, the gain is: Where G is the circuit gain, V IN+ and V IN– are the voltages at the positive and negative inputs, respectively, and V REF is the voltage at the REF pin. The circuit shown in Figure 2 provides an output voltage that responds to both the input voltage and the light intensity, with the photoresistor serving as the gain resistor for the AD8226 instrumentation amplifier (in-amp). The sensor can be mounted in a clear, waterproof enclosure and, thus, deployed in any field of operation under all weather conditions. Each application requires an appropriate circuit and physical setup, and some calibration may be required for the exact lighting scenario encountered. Able to detect large or small fluctuations in light levels, it can distinguish between one or two light bulbs, direct sunlight, total darkness, or anything in between. Its resistance decreases from millions of ohms in darkness to a few hundred ohms in bright light. The sensor is a light-dependent resistor (LDR)-photoresistor Model 276-1657 from RadioShack-which has a resistance that varies with ambient light intensity, as shown in Figure 1. This design idea describes a simple, cost-effective circuit that provides a voltage proportional to the intensity of ambient light. At the heart of an energy-harvesting system is the ability to measure ambient light accurately. Ambient light is increasingly considered as a source for harvesting energy to power heartbeat monitors, bathroom fixtures, remote weather sensors, and other low-power devices.

Ambient light sensor