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BPX81 photoTransistor circuit

  1. Oct 1, 2009 #1
    Hello forum i would like to ask about this circuit ( in attachment ) if it's possible to make it.I need to have on IC1.1 a 3.3V , or you have another good idea.
    Thank you in advance,

    Attached Files:

  2. jcsd
  3. Oct 1, 2009 #2


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    Science Advisor

    Yes it's possible to get close to 3.3 volts at the input of IC1.1, but only just. You'll need to get the bpx81 almost to saturation which means you'll need at least 1mW/cm^2 illumination at 850nm and more if the wavelength is other than this optimum value.

    You haven't exactly said but I assume that the problem you are having is that you're not getting enough voltage right. Well since the dark current is less than 1uA for T<50C you could probably increase the value of R3 up to several tens of K-ohm without any problems. Don't increase R2, leave it at 500 or less, but perhaps place a catch diode from IC1 pin 1 to the 3.3 volt supply to prevent getting too larger input voltage.

    BTW. Exactly how high you should go with R3 largely depends on how fast the circuit needs to operate, a larger resistance will give a slower switch-off transient which may or may not be a problem.
  4. Oct 1, 2009 #3
    Thank you alot , till now i didn't try it , just i took your opinion about this , i will try it and i will let you know
    Thank you,
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