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 NXP Semiconductors


  • Simple Light Detector

  • Created: Sep 30, 2014

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Summary

Simple light detector is a device that uses 74AHCT00D NAND gate chip and photoresistor to effectively detect light. The 74AHCT00D NAND gate chip is a high speed CMOS device compatible with low-power Schottky that also provides quad 2-input NAND function. Concurrently, the photoresistor detects the presence of light with its light-sensing ability. However, its resistance changes in proportion to the amount of light it is expose to. In darkness, its resistance is high while in bright light, its resistance drops dramatically. In addition, the device has an LED as an indicator that turns ON when the presence of light illuminates the circuit, such as from a flashlight or laser.


The circuit has 5V supply voltage that drives the 74ACT00D NAND gate chip, which is connected to pin 14 and pin 7 respectively, connected to the ground. The voltage divider circuit is composed of a photoresistor and a 6.8kΩ fixed resistor where voltage is distributed across the components in direct proportion to the amount of resistance each component offers. The more resistance a component offers in relation to the other, the more voltage that will fall across that component. When a photoresistor is exposed to darkness, it has very high resistance, in the order of a few Megaohms (MΩ). With high resistance, most of the voltage from the power supply feeding the voltage divider circuit falls across the photoresistor with very little voltage falling across the 6.8kΩ fixed resistor. Thus, when connected to the inputs of NAND gate, with voltage so high, the NAND gate will essentially interpret the voltage divider as HIGH or logic 1. When the voltage feeding the inputs of a NAND gate are greater than half of the power supply voltage, it will interpret it as a HIGH value. When a photoresistor is exposed to bright light, its resistance drops dramatically and significantly turns ON the LED.


The light detector circuit is a basic project that is favorable for beginner electronic enthusiasts. This project can be integrated to security or safety devices such as burglar alarm that constantly senses a beam of light where any changes in light causes it to alarm. Moreover, it can be used to motion sensing devices as well as to computers, wireless phones and television to automatically control the brightness of a screen especially in low-light or high light situations.

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