Electrical – Calculating range of between LED and phototransistor


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I would like to tell briefly about circuit. When LED is ligth(on),it will trigger ptohotransistor for work. I wonder that between these two component of range is important. Is there any calculate metod for unknown range ? How can I design productive this circuit ? How many length should be between LED and Phototransistor ?

Best Answer

The LED emits light power. That light power is projected across a spherical arc (maybe 60 degrees) and the power density (watts per square metre) reduces with the square of distance from the LED.

The photo transistor has an active surface area and a conversion factor of incident watts to collector current. So, work backwards: -

  • What collector current do you need?
  • What incident light power is required for this current?
  • How many watts per square metre is that equivalent to?
  • What distance does the LED have to be from the photo-transistor to produce this power density.

If your data sheets don't tell you these figures find devices that do. Also make sure that the peak sensitivity of the transistor matches the colour of light produced or you will have less distance.