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Can anyone give me a basic rundown of this circuit?

Discussion in 'General Electronics Discussion' started by Archie Moore, May 20, 2016.

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  1. Archie Moore

    Archie Moore

    May 20, 2016
    I found this circuit online, it is not my own design, and I thought it was pretty cool! Can anyone just explain what some of the more complicated components are in this circuit and how the whole thing works, e.g. I think the first component on the right is a phototransistor but I am not sure! I am only in GCSE electronics so it would be great if you could keep it reasonably simple ;)!
    Last edited: May 20, 2016
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    This circut isn't cool at all. It will output a distorted audio signal.

    The idea of th is circuit is:
    • Audio input (left) drives Laser diode via transistor Q1 - Light intensitiy is modulated by the audio signal.
    • Phototransistor (right circuit, lower left) receives light from the laser diode and modulates collector current accordingly.
    • Modulated collector current creates AC voltage drop across 10k collector resistor.
    • AC voltaeg (audio) is amplified by Q7 and Q6 to drive the speaker.
    Issue 1: The sender does not control the LED current very well. There is no current limiting for the laser diode. There is a Dc current for an input signal of 0V which will cause heating of the laser diode and correspondingly a change in operating point.
    Issue 2: The amplifier (Q7, Q6) is very simple. It will work but it will not produce quality sound.

    Résumé: The circuit will work, but quality can be substantially improved.
  3. 73's de Edd

    73's de Edd

    Aug 21, 2015

    Sir Archie Moore . . . . . . .

    Didn't you have the rest of the accompanying explanatory text with the schematic ?

    NOT much there, so don't expect much.

    Yes that is a photocel which is the receptor of the audio modulated laser beam from the left.
    Its audio is then amplified by the two transiostors to the right, to then activate the systems speaker.
    Of which, we should not expect a wall shattering sound level from.

    Specs and supplier for that photo cell is here :

    If I were building that circuit I would start off with a much lower powered transmitter, in the order of initially using a common RED led connected to an audio sources output and then
    tune in to get a constant banter of speech such as a talk show to drive it.
    That would then let you "see" the speech peaks and lulls to set up to . . . . . akin to using a radio announcers VU meter.
    Then place the LED near the photocell to experience the first reception of sound and play around with backing away to evaluate its range capability.
    Back away until you can just barely hear the sound.
    THEN . . . .you toy with the overbiased base connections of both transistors.
    Start with placing a 10 K linear variable trimpot on the base of the Q7 . . . . . . HEY ! . . . . Q7 . . . . .Q7 . . . . . .maybe the reason for its deficiency, is that he forgot to show us how he used
    his Q2 thru Q6 transistors ? Ha Ha Ha.
    Rotate the pot fully Counter clockwise and note the side terminal that would leave the pot wiper being against.
    That terminal gets connected to ground. The center wiper connection goes to the base of Q7 transistor.
    Power up again and initially you will have no sound, then as you rotate the wiper clockwise your sound will reappear and you adjust the pot for the best sound.
    Next you power down and hook up another 10 K linear variable trimpot on the base of the Q8 output transistor .
    Same procedure on its hook up as with the other.
    Adjust it also for the best sound level and quality with the use of the LED transmitter being positioned to that distance of just intelligible sound reception.
    Then you go back to the 1st trim pot and see if audio can be improved upon.Then adjust the 2nd trim pot the same way.
    By this time you will have optimized for "middle of the road" biasing on both transistors, to the best as can be expected.
    They are now responding to a weak input signal and then have biasing latitude to increase the gain progressively as input increases.
    As it was, the audio amplifier circuitry was either needing quite a hefty signal input to get audio output or else going into clipping and saturation in a hurry from the incoming signal.

    If that receive portion is working good then, with the ncorporation of a collimating lens lens in front of, and matching its focal point, to the photocell apex, I could see muuuuucccccch greater range possibility than that party of the first party ever expected . . . . with the final use of the audio modulated laser

    BUT on the laser transmitter, incorporate those other suggested drive procedures until you can see the production of good voice modulation swings on a white sheet of paper.

    73's de Edd

    Last edited: May 20, 2016
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The transmitter part of the circuit can be improved by assuming that you're using a laser diode with a constant current driver.

    The next step is to bias the transistor so the laser diode is operating at about 1/2 its normal current.

    The audio should be coupled to the base via a capacitor so the signal causes the laser current to rise and fall.

    The receiver circuit should be similarly modified.

    An even better approach would be to use a 1 bit A to D on the transmitter and a 1 bit D to A on the receiver end. This would be far more immune to noise and would have better range.
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