Talking over a laser beam - at least I've got the circuits designed and simulated!

Talking over a laser beam - at least I've got the circuits designed and simulated!

This is a discussion on Talking over a laser beam - at least I've got the circuits designed and simulated! within the Off Topic & Humor Discussion forums, part of the The Back Porch category; Designing the circuits was pretty easy - making myself take the time to do it has been the hard part Sometime back when I was ...

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Thread: Talking over a laser beam - at least I've got the circuits designed and simulated!

  1. #1
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    Talking over a laser beam - at least I've got the circuits designed and simulated!

    Designing the circuits was pretty easy - making myself take the time to do it has been the hard part

    Sometime back when I was posting about the hearing amplifier I was developing for my brother-in-law, I mentioned my next project would be to talk, i.e. send voice, over a laser beam. You speak into a mic, the audio is sent via the laser beam to a photodiode, amplifier and power speaker driver and that's it. I expect the laser will easily reach 100 yards and with some tuning could go much further.

    I want to start by de-mystifying the concept. All this does is change the intensity of the laser beam proportionally to the voice. So our voice, or sound, causes the laser to get brighter on positive going peaks of the voice waveform and dimmer on the negative going portions. At the receiver, the photodiode converts the modulated (brightening and dimming) laser beam into a modulated electrical signal that can be amplified and applied to a speaker.

    This is exactly the principle of an AM radio except of course the radio uses radio waves instead of a laser beam. The radio version is far more complicated but it uses the very same principle of transmitting signals.

    I've been wanting to use what's called pulse width modulation which is more power efficient and should extend the range, But it's quite a bit more complicated requiring amplitude to pulse width conversion, which I've done, it's just a lot of trouble and I really don't have a need for the trouble.

    Anyway here's the laser modulator circuit with the microphone pre-amp. I could have used integrated circuits, but what fun would that be? I'm a bipolar transistor, old-school fan And besides, I have all the components for the bipolar version, well, I will have to order some NPN and PNP power transistors to drive the speaker.

    I have to admit I was strongly tempted to use the LM386 amp and speaker driver IC. It's pretty much all the amp and driver needed. It does have one caveat, it's a small 8 pin DIP (Dual In Line) plastic package and could have problems with handling heat dissipation in the summer heat. Heat sinking then becomes an added requirement and that's not straightforward. Anyway the two power bipolar transistors half the power requirement and have metal tabs to help dissipate heat. Did I mention I like to use bipolar transistors?

    Strong negative feedback (although it sounds negative , that's a good thing) is used in both the mic preamp and the laser modulator circuit. The makes the gains of the circuit very stable and independent of temperature variation effects.

    The only thing not shown, and I will add this, is a laser over-current protection circuit. Laser diodes are quite susceptible to over-currents, even very short over-currents. For example, a loud sound occurs near the mic. That could cause an over current to occur in the laser diode. If it does, it could very well destroy the laser diode. The circuit is simple, just a single bipolar transistor and resistor.



    Here's the photodiode preamp circuit. Remember these circuits are for simulation. I can't simulate light shining on a photodiode so I replaced the photodiode with a signal generator. I show a voltage source, but it really should be a current source signal since that's what a photodiode would be. But the voltage/current versions are compatible and using Norton's theorem one can convert a voltage source into an equivalent current source. JIC you wanted to know that.

    Again, lots of negative feedback for gain stabilization and I used a feedback circuit that increases the input resistance as well - not that that's needed, but if I hadn't used this approach, I would have had to use an inverting type of feedback that loads down the input signal.



    This is the speaker driver circuit. I won't say much about the speaker driver, it's a pretty common configuration. I will say that I used the special "Darlington" output drivers instead of the more common true Darlington pairs. This configuration allows a bit higher peak voltage swings and at a low 4.8 volt supply, large voltage swing is needed. I should be able to get a nice, clean, low distortion, 100 milliwatts of sound from this circuit.



    Now the next step, and I can see it's going to be challenging, is to make myself sit down and build the circuits.
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    VIP Member Array G26Raven's Avatar
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    Tangle, I don't understand the electronics at all, but when I was a kid, I asked for a set of walkie-talkies for my birthday one year. My grandmother ended up buying me a set that didn't look at all like the walkie-talkies I imagined, but were supposed to work the way you describe. I doubt they were laser driven, but they emitted a red light that the two units were supposed to shoot directly at each other- line up the beams- to get them to work. I don't think they ever actually worked.
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    Quote Originally Posted by G26Raven View Post
    Tangle, I don't understand the electronics at all, but when I was a kid, I asked for a set of walkie-talkies for my birthday one year. My grandmother ended up buying me a set that didn't look at all like the walkie-talkies I imagined, but were supposed to work the way you describe. I doubt they were laser driven, but they emitted a red light that the two units were supposed to shoot directly at each other- line up the beams- to get them to work. I don't think they ever actually worked.
    Sounds like the same thing as the laser, but using LEDs instead of the laser. Actually, I could drop an LED right into the circuit and do the same thing I'm doing with the laser. The LEDs are far more range limited than the laser version.

    Another thing I'm tempted to do, in fact almost went in this direction, was to get a cheap but high power LED flashlight and use the LED(s), head, and reflector as the transmitter. The only reason I decided not to is the power drain on the battery would be much higher than a laser diode. I'd have to beef up the driver circuit a bit to handle the higher current of the high power LEDs, and other than that about the only problem is the much higher drain on the batteries.
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    We use some in-classroom sound systems that are IR based. The technology finally became viable on large use scale with the advances made in the late 90s and early 2000s with lighting technology. The microphones have LEDs that transmit the light out. They can now work by bouncing light off of reflective surfaces in the room (to a degree) as well as with direct line of sight.

    We had one building built, that I support, that has windows on the interior of the building. I had to play with the location of the IR receivers so that the teachers would not start talking in the classroom across the hall. I also had that problem occur in another building if the weather conditions were perfect to allow the IR signal to travel across a courtyard. It was such an intermittent problem, it took me a while to figure that one out and make it stop happening.
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    Quote Originally Posted by SFury View Post
    We use some in-classroom sound systems that are IR based. The technology finally became viable on large use scale with the advances made in the late 90s and early 2000s with lighting technology. The microphones have LEDs that transmit the light out. They can now work by bouncing light off of reflective surfaces in the room (to a degree) as well as with direct line of sight.

    We had one building built, that I support, that has windows on the interior of the building. I had to play with the location of the IR receivers so that the teachers would not start talking in the classroom across the hall. I also had that problem occur in another building if the weather conditions were perfect to allow the IR signal to travel across a courtyard. It was such an intermittent problem, it took me a while to figure that one out and make it stop happening.
    Ahhh, the realities of life - it works too good sometimes and not so good in other conditions. Sounds like you resolved the issues - good job.

    I had some IR headphones that worked similarly to what your systems do. These are far more sophisticated electronics than my simple designs and I'm sure the use the radio AGC, and/or FM instead of AM.

    If I wanted to do long range outside, even say 50 yards or so, rain would likely completely block all transmission. You can just imagine a few drops of rain intercepting the beam at any point along the path would pretty much block the beam. One way to deal with that, not that I need to, is to use the flashlight approach.which would have a much wider beam and hence be less susceptible to complete beam blockage.

    Unfortunately that wouldn't really solve the problem. Since the technique I'm using is AM (amplitude modulation) even if the rain didn't block the beam (maybe flood-beam would be more accurate), the rain would amplitude modulate the light and we would truly hear the rain!

    The solution to rain modulation would be to use the pulse width modulation instead of amplitude. PWM is kin to FM but less difficult to modulate and especially to recover. The information would be in the duty cycle of the light rather than the amplitude or intensity. It would have a lot of immunity to rain drop modulation.
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    And I think I've accomplished something when I mow the lawn... You're taking lawn mowing to another level.

    I worked electronics in the Air Force and readily admit I never really understood it. But I'm good at taking test, so I breezed through tech school. Most of the components I worked on were sealed unit and repairs were depot level, and those units we did repair were limited to circuit board replacement. Today I try to limit my electronic knowledge to Off, On, Volume, and Channel.
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    Quote Originally Posted by OldVet View Post
    And I think I've accomplished something when I mow the lawn... You're taking lawn mowing to another level.

    I worked electronics in the Air Force and readily admit I never really understood it. But I'm good at taking test, so I breezed through tech school. Most of the components I worked on were sealed unit and repairs were depot level, and those units we did repair were limited to circuit board replacement. Today I try to limit my electronic knowledge to Off, On, Volume, and Channel.
    LOL - "...I try to limit my electronic knowledge to Off, On, Volume, and Channel." - that's why the hard part is making me do it. There's always another Gunsmoke, Have Gun Will Travel, or Andy Griffith, and more of late, Tales of Wells Fargo!

    I've got to get some reloading done, "Tee" time is 1:30 !
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    We don't have a gun problem in the US, We have a people problem.
    The problem we have is people that want to kill large numbers innocent people
    in Gun Free Zones.

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