Newbie - really need some explanations

Discussion in 'General Electronics Chat' started by VoltAmpere, May 30, 2014.

  1. VoltAmpere

    Thread Starter New Member

    May 30, 2014
    Hi guys !

    I`m glad to join this forum. I`m sure I can find a lot of people from which I can learn.

    My riddle is the following :
    I`m trying to do a project at my college, using a laser pointer, a fiber optic cable and a receiver. The problem is that I have to explain the circuit in the receiver and I don`t know it completly.

    So here are some pictures :

    Ok, so from what I understand the circuit goes like this :

    • First, the light transmitted via the fiber optic is used to power a Photodiode which actually does this process of conversion between light and voltage.
    • Second, the voltage is amplified with an Op-Amp, powered by pins nr. 3-Vcc respectively 2-Gnd, which also have a bypass cap for noise.
    • The output from the Op-ams is then used to control the base current of an npn transistor, wired in the collector-emitter configuration.
    My questions are the following :

    • Which is the role of the RL - the pull up resistor - as it is parallel connected and obviously the current from Vcc won`t be limited by it when it goes to the Op-Amp and why do I connect it together with the output voltage - Vo ?
    • How come that I get 5V when there is no light through the fiber and 0V when the fiber has light through ? How exactly does the transistor work ?

    Thanx !
  2. Lestraveled

    Well-Known Member

    May 19, 2014
    Hi Andrew and welcome to the forum
    The type of output that your receiver has is called "open collector". The output transistor can only pull to ground and requires RL to pull the output high. This open collector output allows the output to be set for different logic (voltage) families. Also Google "wired OR".

    Light = low, dark = high. That is just the transfer conversion that this receiver has. You can get them any way you want.

    Hope this helps

  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    And about the photodiode. It does not generate a voltage. It acts as a strange light-controlled resistor (actually a light-controlled current limiter). It is part of a series circuit, and as the light intensity increases it allows more current to pass through it.

    Keep in mind that the little schematic on the datasheet is not detailed. It shows only the basic elements and their relationships.

    Les is right about the output. Looking at the datasheet, the part is powered by 5V but the output can interface with circuits up to 18V. It is a generic part intended to be adaptable to a wide range of equipment. Some inputs have internal pull up resistors, and only need a switch to GND to function. For those, pin 4 is left floating. OTOH, a true TTL input circuit needs to be driven both high and low. For those, RL pulls up and the output transistor pulls down.

  4. VoltAmpere

    Thread Starter New Member

    May 30, 2014
    Ok guys, thx a million :)

    I guess I will redraw the circuit and I will have some good explanations on it.