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  • Automatic Light/Dark Indicator

  • Created: Mar 21, 2016

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This project uses NXP NCX2220GU that compares voltages or currents and outputs a signal indicating, which is larger. This circuit also has a sensor component that works in response to the ambient light condition. The output of the sensor is compared to a set reference value. Whenever the light condition changes either from dark to light, or light to dark, the output of the comparator goes high. As the light level decreases, and the LDR meets the maximum threshold resistance, the circuit automatically switches ON the LED L1 acting as a dark indicator. On the other hand, as the light level increases, and the LDR meets the lowest threshold resistance, the circuit automatically turns ON the LED L2 working as a light indicator.

The surrounding illumination is observed by the light dependent resistor (LDR) that will act as a sensor for the circuit. The voltage drop across this LDR corresponds to a particular intensity of light. This value can be taken at the inverting input of NCX2220GU comparator (pin IN2P). The resistor R3 (1kΩ), along with LDR, forms a potential divider at its input. The non-inverting input (pin IN2N) is connected to a variable resistor (preset), which is used to set the reference voltage of the comparator. NCX2220GU works on the principle that its output remains HIGH as long as the non-inverting input is at a higher level than at its inverting input. Here, the output (pin OUT2) is connected to a LED. This concept can be utilized in two configurations (night activation and day activation, respectively) by changing the positions of the R3, and the LDR as shown in the circuit diagrams. The reference voltage is set to correspond to a light intensity between day and night. A preset R1 (100kΩ) is used which is set to a value of 10kΩ. The resistance of the LDR varies between 1kΩ and 1MΩ when it is bright and dark, respectively. In the first case, the LED will glow when the LDR is completely dark. In such condition, the resistance of the LDR is higher, which is offered by the preset. This causes a lower potential at pin IN2P than that at pin IN2N, and so, the output goes HIGH. Thus, lighting the LED. The second case is exact opposite to this. This time pin IN2P goes lower than pin IN2N when the LDR receives bright light.

Automatic light/dark indicator systems have very common utilities in day-to-day life. Such applications include street lighting at night, morning alarm, automatic emergency light and dark activated buzzers.



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