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Question about mosfets after the SMT730D/850D

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Shayan Ghaffari
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Joined: 2 months ago


I was looking over the schematics for the source optode in the NinjaNIRS 2021 and saw the circuit that comes after the SMT730D/850D LEDs I was wondering what the signals going to the MOSFET are for is other than to control which LED is turned on. From what I gathered it seems that it is used to control the intensity of the LED. Any help on this would be appreciated.


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Bernhard Zimmermann
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Yes, correct, these are for selecting the power level.

Pin 3 on connector J1 selects whether the output power is low or high.

E.g. when pin 3 is high and the modulation signal for the 850nm LED (J1 pin 4) is also high, both transistors Q2A and Q2B will turn on, resulting in a large current in the LED. If J1 pin 3 is low and J1 pin 4 is high, only Q2B will turn on, resulting in a much lower current in the LED.