lm 555?

Discussion in 'General Electronics Chat' started by mhester, Sep 17, 2008.

  1. mhester

    Thread Starter New Member

    Sep 17, 2008
    New to electronics, Trying to count light pulses from photodiode and drive an output high. Need to be able to vary (idealy through a pot) the count to drive high. time in micro or at least milliseconds.
  2. bertus


    Apr 5, 2008
  3. RiJoRI

    Well-Known Member

    Aug 15, 2007
    It sounds to me like mhester wants to count light pulses, and after a certain count drive an output high. The pulses will come in at Mhz or KHz frequencies. (Will a photodiode react that quickly???)

    If my interpretation is correct, a signal conditioner may be needed, feeding into a counter chip (or chips). If a pot is definitely required, perhaps the output of the counter(s) could feed an R2R ladder and go into one side of a comparator. The other side would be fed from a multi-turn pot.

    OR, replace the entire thing with a microcontroller with built-in A-to-D (to read the pot).

  4. mhester

    Thread Starter New Member

    Sep 17, 2008
    Thanks for your reply, the photodiode is a light to voltage conv. into a 272, first input for op amp, then second as a comparator, the rise of the light sensor is around 8μ. yes you are correct, try to be able to drive output high through transistor, would like to be able to adjust to go high on first pulse up to several pulses lasting a max time of about 2sec. would a 555 with a variable capacitor work?
  5. mik3

    Senior Member

    Feb 4, 2008
    If you want to count pulses then you can use a counter to count them. Also, use an ALU (arithmetic logic unit) to subtract the value of this counter from a presetable value (the number of counts you want to measure) and when the subtraction equals zero then the ALU will give signal to other parts of the circuit to control whatever you want.
    Another method is to use a counter again and then a DAC to convert the digital value of the counter to an analog voltage. Then you will use a comparator to compare this value with a presetable value from a pot and when the value of the counter is greater than the pot's value then the comparator will give you an output signal. The value of the pot's voltage will be proportional to the number of pulses. Another option is to use a microcontroller to make your life easier.