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= Light-Sensing Circuit Yields Human-Like Dynamic Range =

Design Idea URL: []

The human eye can detect very small differences in light changes among many different settings. From a dimly lit room to a brightly shining light source, the eye can still detect these small changes because the eye has a dynamic, logarithmic range. The light-sensing circuit design intends to mimic this human function by making use of the logarithmic relationship between voltage and current of the bipolar junction photo transistor. This relationship yields light-sensing applications that can operate in any ambient light condition as long as the test object produces a quick change in the light level [1]. This proximity detection saves on the amount of energy usage caused by unnecessary outdoor lighting, which ultimately cuts the cost of generating electricity from the coal burned. According to a carbon reduction plan by Peter Miller, the most savings from carbon emission comes from efficient lighting of buildings [2]. Efficient lighting comes from using bulbs that are energy efficient in their emission as well as lighting areas of a building when necessary.

**Four E's of Sustainability:**
The amount of energy used by this design application varies depending on the load circuit that the output terminal is driving. For example, the output circuit could be a controller that turns on a lighting system automatically if it detects a moving object in a dark setting. If the light-sensing circuit is configured to sense objects in the dark, then a 1V rail is used on the output as the steady-state voltage. Anything that disrupts this steady-state voltage would drive the output and the controller high. This circuit in an idle state causes a consumption of power P = 1^2 / 101k = 9.9uW at the output while the input biasing resistors cause a consumption of power P = 1^2 / 10M = 1uW. Having this system run during night hours would result in a minimum power consumption of about 10.9uW * 12 hours = 130.9mW*hr of energy or 471.21 joules. Power supplies or battery sources would power this circuit. If a battery source is used, then maximum efficiency of power usage is necessary to reduce the amount of batteries consumed. However, battery consumption is a rising issue since it adds more toxic wastes to the landfills. Use of power supplies is not sustainable as well since coal power plants are a major source of power generation and they already generate 72% of the world's emission of CO2 [3]. Overall, this circuit has a net savings of energy when the cost to operate the circuit is compared to the cost of unnecessary lighting, and the nonhazardous effects of light inputs and light outputs ensures that this design is inherently safe.
 * Energy**

The circuit and its applications ultimately derives its source of power from coal power plants, water power plants, and wind generators. It also takes an amount of carbon footprint and chemical to create the circuitry for this design. The emissions generated causes an excessive discharge of acids or akaline solution into water systems that affects microscopic, plant, and animal life [5]. However, the circuit attempts to cut costs in operation of current lighting systems and therefore, reduces the amount of CO2 emission from coal plants. Because it is not completely eliminating dependence on burning natural resources for energy, it is not considered completely sustainable [4]. It still impacts the Earth's atmosphere by decreasing the CO2 emission by a little and slowing any effects caused by global warming. However, it will take more effort on all
 * Environment**

In terms of human capital, it provides people with jobs to install these kinds of circuitry in their lighting and it financially provides a source of income for them. It also increases savings monetarily for businesses and offices that rely on providing lighting to their workspace too much. However, it does increase the manufactured capital of electronic circuitry and the cost to generate it. The design initially costs the environment in its production stage. Once it is implemented over a long period of time, the environment, businesses, and people benefit from the savings it generates. The lifespan of the product should last a good amount time, but constant maintenance checks on the sensitivity and performance of the photo transistor diode within the circuit are required. Since this design senses the movement of people, the circuitry undergoes constant wear and tear and eventually needs replacing. This total process generates financial cost.
 * Economics**

This circuit and its applications impact most of the stakeholders in a positive way. Owners save lighting costs and the environment benefits from reducing greenhouse gases, but power companies lose in financial profit. However, energy not used in lighting costs can be diverted to other important causes, such as the powering of medical equipment. So in effect, power companies don't lose much in the process.
 * Social or Political Equity**

Embodiment of the green engineering design principle:
From a systems standpoint, this design improves proximity detection of every system and would improve the entire infrastructure of lighting if it was put to that application. Although there are wastes associated with the generation of this circuit, it also generates savings in the environment and in the financial state of individual businesses and companies. To a small extent, this design affects and improves natural ecosystems indirectly (through reducing power consumption and reducing CO2 emissions) while providing human comfort and health associated with good lighting. The applications to this circuit are limitless and it generates a number of options for the life expectancy of the design to be maximized. Most if not all of the inputs and outputs are considered to be safe. The inputs and outputs to the system do not require any hazardous material or energy. It only involves light energy. Production of this circuit requires more natural and chemical resources. Therefore, it does not minimize depletion of natural resources, but increases it instead. Although there is carbon footprint waste associated with each unit, the simplicity of the design optimizes its function onto the smallest components of electronics as possible. This design is cognizant of people's comfort and ease of accessibility. It was not designed with the intention of changing cultures, aspirations, or local geography, but it gave opportunities to apply engineering in improving people's standards of living. This circuit goes back to a simplistic design and uses innovation of simpler design methods to solve the problems associated with current light sensing techniques. The logarithmic relationship between voltage and current established a simple connection between the realm of electrical engineering to the realm of human anatomy. Designs achieve maximum potential when it attempts to mimic the processes of life that nature has evolved over the millions of years. This design challenges communities to improve the design for better controller circuits that can yield the same or better perception of moving objects. Better designs would yield efficient circuits, both in cost of production, operation, and savings.
 * Engineer processes and products holistically, use systems analysis, and integrate environmental impact assessment tools.
 * Conserve and improve natural ecosystems while protecting human health and well-being.
 * Use life-cycle thinking in all engineering activities.
 * Ensure that all material and energy inputs and outputs are as inherently safe and benign as possible.
 * Minimize depletion of natural resources.
 * Strive to prevent waste.
 * Develop and apply engineering solutions, while being cognizant of local geography, aspirations, and cultures.
 * Create engineering solutions beyond current or dominant technologies; improve, innovate, and invent (technologies) to achieve sustainability.
 * Actively engage communities and stakeholders in development of engineering solutions.

REFERENCES
 * 1)  S. Hendrix, "Light-Sensing Circuit Yields Human-Like Dynamic Range," //Electronic Design//, Available: []. [Accessed April 30, 2011].
 * 2)  P. Miller, "Saving Energy Starts at Home," National Geographic, Vol 215, No. 3, March 2009, p. 60-81, Available: [] . [Accessed April 30, 2011].
 * 3) P. Epstein, J. Buonocore, K. Eckerle, M. Hendryx, B. Stout, R. Heinberg, R. Clapp, B. May, N. Reinhart, M. Ahern, S. Doshi, and L. Glustrom, "Full cost accounting for the life cycle of coal," //Annals of the New York Academy of Sciences//, Vol. 1219 (1), Feb. 2011, pages 73-98. [Accessed April 30, 2011].
 * 4) D. Braun, "4E Sustainability Analysis," //HOME - Electrical Engineering Department - Cal Poly//, California Polytechnic State University, Available: [|http://courseware.ee.calpoly.edu/~dbraun/courses/4E-SustainabilityAnalysis.html]. [Accessed April 30, 2011]
 * 5)  E. Williams, Environmental impacts in the production of personal computers, in //Computers and the Environment: Understanding and Managing Their Impacts//, R. Kuehr and E. Williams, Eds. Dordrecht: Kluwer, 2003, pp. 41-72 . [Accessed April 30, 2011].