741 Op Amp Transistor Circuit Trouble

Discussion in 'Homework Help' started by kim89, Sep 17, 2013.

  1. kim89

    Thread Starter New Member

    Sep 17, 2013
    Hi guys, I'm building a circuit for an assignment and I am having trouble getting it to work correctly. I am new to Electronics so I have a LOOOOT to learn. But basically I have built the circuit but the LED's are not doing what they should be and I am not getting the right voltage swings from my Op Amp. One LED flashes on and off (very slowly) about 3 times and then just goes hard on. The other does not turn on. I have a 0V junction rail a +9V rail and a -9V rail. When I pull out the positive one LED comes on and when I pull out the negative the other LED lights up so the polarity is good. I have checked the continuity of the ciruit it is all good. On the 741 I have the non inverting input (pin 2) hooked up after the Cap, the inverting input (Pin 3) hooked up the the 22K resistor, positive supply to the op amp is on pin 7 and negative on pin 4. Output is pin 6. It appears as though I am not getting an equal swing on the output of the op amp..
    I am not sure where/what to do next. -.- Hiopefully someone on here can help me understand what I am doing wrong or what I am missing!
    I have also attached my redrawn circuit in Yenka - it doesnt work at all on Yenka but I couldn't put in the correct model Transistors so that may be why??
    I have wired the circuit as it is on Yenka, have I intepreted it correctly?

    Details of the circuit

    Using two 9V batteries connected in series, (giving a 0V line at their junction), power up the opamp circuit as shown in the diagram. The LEDs should pulse on and off alternately. By inspection determine the LED current when either of them are on. By timing 10 pulses, determine the period of the pulsing (and frequency). Also measure the peak output voltages (high and low) at the opamp’s output . (40 marks)

    From your observations and any circuit analysis calculations done, explain the operation of the circuit. (20 marks)

    Submit a report covering the circuit. Include reproduced neat circuit diagrams, measurement results (tables), any necessary calculations and observations. Include the circuit operation description.
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    The capacitor value in your simulation is 1mf which is 1 milli-farad or 1000uf.
    Your schematic calls for 1uf. Try that. :)
  3. kim89

    Thread Starter New Member

    Sep 17, 2013
    Thanks but I accidently put that in but I am actually using a 1uF Cap. I have now got my circuit so that one led flashes. When I pull out and change the pins on the transistors the opposite LED turns on but. Am I missing something with the transistors?
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    I do not know your simulation program. But these are the common PSpice prefixes. And all circuit simulators based on Spice use them. The one mega Ohm resistor may have been given a 0.001 Ohm value
    10^12 - "T" or "t" tera
    10^9 - "G" or "g" giga
    10^6 - "MEG" or "meg" mega
    10^3 - "K" or "k" kilo
    10^-3 - "M" or "m" milli
    10^-6 - "U" or "u" micro
    10^-9 - "N" or "n" nano
    10^-12 - "P" or "p" pico
    10^-15 - "F" or "f" femto
  5. kim89

    Thread Starter New Member

    Sep 17, 2013
    Never mind, figured it out. Will post back after the assignment due date as to what was wrong