Moon at night.
       17. Simple electronic devices.        

    17.20 Green LED as light sensor.

  What would we do with electronics, sooner or later we begin to require simple light sensor. You can certainly purchase such sensor on well-known Web stores where there are hundreds of them, but this is not a fun. In addition to spending extra money, delivery takes a time, and we don't want to wait.

  LED's are an alternative for light sensors. As most of the semiconductors they generate voltage when lit. More or less, but produce. This property has been used in the switch that turns white LED off when illumination exceeds a certain level. How it works you can see in the picture.


  As I said, all LED's produce voltage. From widespread, this is best obtained from Green LED's. If you're wondering, the practical comparison you can look here.

  Most LEDs produce voltage in range 0,5v - 1.8V. The only disadvantage - is their high internal resistance which on average from 100 to 1000 kΩ. The latter shortcoming doesn't matter if you use MOSFETs at the input. They do not consume current, and managed only by a voltage.

  A schematic diagram is simple and easy to understand. When the LEDs (D1-D4) are lit, they begin to produce voltage. When this voltage level exceeds 2 volts, transistor Q1 opens and the voltage on his 'drain' electrode drops to 0V. This closes transistor Q2 and LED D5 stops glowing. R3 - current limiter for LED.

  Resistor R1 (50 Mega-Ohms) reduces the sensitivity of the circuit. Without it, the circuit will be too sensitive to light level. In addition, when illumination falls, much time will be spent on switching LED 'ON' because the input of MOSFET transistor represents as capacitor (~1000 pF) and needed a seconds, to discharge it.


  To test this concept all the parts have been mounted in a box from CF card. Self-adhesive copper tape as carrier for SMD components. Because value of resistors have a limit of 20M, resistor R1 consists of three resistors 10, 20 and 20M.

  It works, but the surprise was that it worked better than I expected.

August 28, 2016  SKootS

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