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Help with op amp between 3D projector and transmitter to glasses

Discussion in 'General Electronics Discussion' started by JeffD, Jan 9, 2015.

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  1. JeffD


    Jan 9, 2015
    I have a 3D projector that has an output jack for external transmitter. The external transmitter needs ~+5v signal that pulses from 0v to 5v ( +1v->+3v seems to trigger it) to keep the glasses in sync with the PJ. The PJ is only outputting a 0-1v signal.

    I've built a simple op amp (from someone else's design) that uses a 5v usb power supply and is suppose to take the signal coming from PJ and send ~5v signal to the external transmitter when the out jack is at 1v.

    First I'm a complete noob to electronics. Second the op amp I built works to amplify the signal and trigger the external emitter. It generates a 0.7v->3.5v signal. I tested this on my bread board and it worked with the PJ. I then built a box with a cable coming in from the PJ and out the transmitter. This is where things went wrong. The op amp is too sensitive. If I touch the op amp input cable with my hand static from my hand triggers the op amp.

    How do I fix the below diagram to make it less sensitive but still trigger quickly on the 1v signal form the PJ or is there a better design I should use?:

    thanks, Jeff
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome to electronicspoint.

    Whlie you could reduce the sensitivity by adding a low-pass filter to the input, let me say that the TL082 is not a particularly good choice in this application. It cannot drive the output to Vcc or GND (not a rail-to-rail output in technical terms):
    Also the output stage suffers in that it can never reach +5V. Even with the base of the transistor at 5V (which the TL082 can't, but let's ignore this here), the emitter can reach only 4.3-4.4V due to 0.6-0.7V base-emitter voltage drop.

    May I suggest a simple transistor based level shifter?
    1V at the input will turn on Q1. Q1 in turn will pull down the base of Q2 and the output will go to +5V (minus Vcesat of Q2 which will be a few 10 or 100 mV only).
    0V at the input will turn off Q1. R3 will then pull up the base of Q2 turning Q2 off. R4 will then pull down the output to 0V (ground). If the load current in the output low-state is too high, the current flowing through R4 will develop a voltage drop that rises Vout above 0V. In that case you can lower R4. You can make it as small as the collector current of Q2 allows (consult the datasheet for Q2). Note that in this case you need to adjust R2 to supply enough base drive current to Q2 when on.


    Attached Files:

    hevans1944 and Arouse1973 like this.
  3. JeffD


    Jan 9, 2015
    Thank you very much for the quick and detailed response. I will certainly give this circuit a try. This does make more sense to do it this way. I'll have to hit radio shack as I don't seem to have any PNP transistors in my stock and work on it this weekend. I have a 0-10k pot that I can put in at R2 to adjust the output. The emitter works when it gets a pulse that goes from 0.8v to 3.5v so I have some slop room on the signal

    Right now I'm using a separate 5v PS. However, the PJ has a 12v supply that comes in from the same cable. while I'm at RS I might pick up an LM7805 and build an onboard 12v to 5v converter.
  4. JeffD


    Jan 9, 2015
    Success. I put a 7805 with a couple of 1uf caps on and used the 12v supply from the PJ. For Q1 I used a 2N2222 and for Q2 I used a 2N4403. I had to go down to 10ohms for R2 to get the emitter to trigger. The emitter did lock onto the PJs signal once before and I recall they worked a little better maybe vs. using the logic gate. I ordered an O'Scope and will be checking the signal on both sides to see how different they are. I know that some of the settings on the PJ change the voltage or cycle rate.
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