photodiode trigger due to continuous DC

Discussion in 'General Electronics Chat' started by unlv007, Jan 12, 2014.

  1. unlv007

    Thread Starter Active Member

    Apr 5, 2008
    I want to generate a trigger circuit to detect that a WHEEL is stationary. The wheel has slots on its periphery.
    There is a LED transmitter in front and a photodiode behind the wheel.
    As the wheel rotates and the slots come in front of the LED and then pass by, there are pulses recorded by the photodiode.
    When the photodiode comes to rest with the slot in front of LED, there is continuous DC signal output.
    The difference between motion and stationary wheel appears as a pulse train and continuous DC respectively.
    Based on this, I would appreciate your assistance to design a trigger mechanism that discriminates between pulse train and Continuous Dc and triggers only due to the continuous signal?
    The pulses are about 10-20ms wide and occur every 50ms, as shown in attachment.
  2. pwdixon


    Oct 11, 2012
    You could lookup retriggerable monstables but your question is wide open to a wide range of solutions especially if you have a processor involved.
    unlv007 likes this.
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    A simple 555 timer circuit would do it. Download the NE555 data sheet and read the applications section, or read this
    unlv007 likes this.
  4. ScottWang


    Aug 23, 2012
    What kinds of the WHEEL?

    What's the detector and controller of power come from ?

    If the power is provided from battery, then you can choosing the cmos type 555 as ICM7555, and make a oscillator for the LED transmitter, and make a 555 monostable as a receiver.

    The output of 555 monostable → diode rectifier and cap filter → DC output.

    It means that when the output side of the receiver is Low then there is no signal from the LED transmitter, if the output side of the receiver is High then it has received the signal from the LED transmitter.
  5. unlv007

    Thread Starter Active Member

    Apr 5, 2008

    Thanks everybody.
    The wheel is similar to the optical encoder wheel.
    It rotates on a shaft and has slots in it.

    The LED is powered by constant current driver such as LM555.
  6. ScottWang


    Aug 23, 2012
    The optical encoder usually use it to counting the length of wires, types, clothes or something needs to know the length.

    Could you take a picture for the optical encoder and shaft and slots?
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
    why not just simply use a capacitor to couple the pulses?