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RE: [beam] phototransistors as substitute for photodiodes in my photovore?

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  • Neil
    ~ hello, is it possible to use phototransistors instead of photodiodes for the 1381j photopopper circuit? ~ ~ thanks, ~ aswin Aswin, The 1381 photopopper
    Message 1 of 2 , Sep 5, 2003
      ~>hello, is it possible to use phototransistors instead of photodiodes for the 1381j photopopper
      circuit?
      ~>
      ~>thanks,
      ~>aswin

      Aswin,

      The 1381 photopopper circuit uses the photovoltaic properties of the photodiodes. Meaning, under
      light, they produce voltage. This voltage is added to the voltage in the storage capacitor
      (47000uF). Therefore the brighter PD will add more voltage to the level across pins 2 & 3 of it's
      1381. The brighter side will reach the 1381's trigger level sooner and turn that side ON. I have
      tried getting voltage out of a phototransistor with a multimeter and it read ZERO. So I'm afraid PTs
      will not work. However a red LED will give 1.2 to 1.3 volts, so you can use LEDs instead. This will
      mean the 1381j will trigger when the storage cap voltage reaches about 1.2 to 1.3 volts, which is a
      bit low. With red LEDs as eyes you may have to use a 1381 with a higher trigger level. A 1 meg of 5
      meg potentiometer may help a little but they can be hard to find.
      Also the potentiometer is mostly for balancing there sides and since there is little current flowing
      theough it, it's effect on the trigger level is minimal. You could also shade the LED's so their
      effect of lowering the overall trigger level of the circuit is less.

      Neil

      1381 popper circuit ref:
      http://www.solarbotics.net/library/circuits/bot_popper_1381.html
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