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Light sensors - two part question...

Discussion in 'Electronic Basics' started by The Grim Reaper, Dec 6, 2006.

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  1. Part 1 - I need to count/log the number of times a 3mm red LED flashes. The
    flash (on) period is very short - looks like half a second-ish, and the
    period between flashes varies between 5 seconds and 2 minutes. I can't
    interact with this circuit, so I've got to read it optically.
    I've got a simple 741 voltage comparator setup that works OK with an LDR
    (using a green, not red, LED as test input). However, I believe the
    response time can be improved. Google as I might, I can't seem to find a
    lot of information on phototransistors or other high speed, but cheap
    optical sensors.
    Any ideas appreciated.

    Which brings me to Part 2; I've found what I thought was a phototransistor
    in my pile of spare bits - photos here: but I can't for the life of
    me find any specs on this part, and I can't even remember where I got it
    The numbering on the side is R8305462T102. For an idea of scale, it's the
    same size as a BC107 (T 192 case?!).
    There is no base connector... I tried wiring it up as a simple input (in the
    same circuit, replacing the LDR) but get no change in voltage, resistance or
    current from any legs!! Think it's bust?!?
    Again, any ideas appreciated!!

    The Grim Reaper
  2. Guest

    Try searching on keywords:
    photodiode amplifier schematic

    Here's a simple one:

    The usual way to do this is to use a photodiode (or phototransistor)
    feeding an op-amp that's wired as a current/voltage converter.
    Connect your PD or PT between V+ and the op-amp's neg input pin. (If
    using a PD, connect it to V+ backwards so it's turned off.) Ground
    the op-amp pos input pin, and use a feedback resistor between neg input
    and op-amp output. This converts tiny PD currents into large output
    voltages. Resistor value determines sensitivity, and would be between
    2.2K and 2meg.

    Try putting 100K in series to limit current, hooking it to 9V supply,
    then measure the voltage across the 100K resistor while shining an
    incandescent flashlight at it. If the voltage changes a little, or
    stays at 0v, then try connecting the power supply backwards. When
    polarity is correct, sensitivity will be maximum. If it never budges
    from 0v, then it's shorted out.

    ((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
    William J. Beaty Research Engineer
    UW Chem Dept, Bagley Hall RM74
    Box 351700, Seattle, WA 98195-1700
  3. Chris Jones

    Chris Jones Guest

    I guess this is an electricity meter that has the flashing light - am I

    I found this:

    Probably a phototransistor or a photodiode should work, but getting it close
    to the LED will help a lot.

    I would suggest trying the photodiode or phototransistor in series with a
    very high value resistor (try 1M as a starting value, or a 1M variable
    resistor in series with a 10k safety resistor would allow adjustment),
    across a 9V battery. The photodiode would need to be connected in the
    direction so that its anode is more negative than its cathode, otherwise it
    will always conduct regardless of light level. A phototransistor would be
    more sensitive, but the sensitivity would be less stable with temperature
    and when substituting different phototransistors of the same type, As the
    other poster suggested, you can measure the voltage across the resistor
    with a digital multimeter, and this will indicate when current is flowing.
    By using a high resistor value, the circuit will be more sensitive. You
    then need to use some kind of comparator with a high input impedance, you
    could use a 741, but the standby power consumption would be lower if you
    use a CMOS inverter chip line CD4069, CD40106, 74C14 etc.

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