Help with this switch design

Discussion in 'General Electronics Chat' started by Firethelaser, Sep 29, 2011.

  1. Firethelaser

    Thread Starter New Member

    May 11, 2010
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    Hello,

    I have a laser diode driver that has a modulation input. How this works is that if this input is connected to the power supply, the laser will operate in continuous wave mode (always on). If it is connected to a 0-5v TTL signal, it will modulate the laser (on-off) at the given frequency.

    I want to implement a SPDT switch that will select between the TTL input and the power supply. I have this design (shown below) that incorporates an indicator LED that turns on when the modulation input switched on:

    [​IMG]

    Only problem with this setup is that the LED will drop the voltage of the TTL input (5 volts) before it is fed to the driver. This may cause issues with getting the modulation to work.

    Is there a simple way to get the modulation voltage I need and have the indicator LED do its thing. Do I have to implement some sort of buffer?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    What is the laser supply current? About how many volts does it take to get that current?
     
  3. Firethelaser

    Thread Starter New Member

    May 11, 2010
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    The driver has a separate input for the supply power. The modulation input simply serves as way to apply modulation. The driver supplies constant current to the laser so I can apply anywhere from 5-12 volts. I am running the laser at 100mA. I don't really think this will affect the problem Im describing.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, it does.

    Seems to me that the TTL output should be shorting the laser constant current supply to modulate the signal, and that your switch should simply disable the capability of the TTL level circuit to short the laser supply.

    If it were a constant voltage supply, I'd say to interrupt the voltage supply.

    [eta]
    Here's what I'm talking about:

    [​IMG]

    V1 is the output from your TTL device.

    An ideal current source has infinite impedance.
    An ideal voltage source has zero impedance.
     
    Last edited: Sep 29, 2011
  5. SgtWookie

    Expert

    Jul 17, 2007
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    OK, let me re-think that; I had a rather long interruption between first reading your post, and replying to it.
     
  6. Firethelaser

    Thread Starter New Member

    May 11, 2010
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    Im not quite sure what you mean by the TTL shorting the current supply.

    The driver has a V+ and GND input where the power supply connects to (I did not include it in the drawing). The third input of the driver is the Modulation input which can either be connected to the supply ("always on" mode) or to a TTL signal (modulating mode).

    The selector switch will allow me to choose which mode I want, with the LED indicating whenever I am in modulation mode. The LED shouldn't affect the TTL voltage I need. I hope this makes things more clearer. Thanks
     
  7. Firethelaser

    Thread Starter New Member

    May 11, 2010
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    Ok, I understand what you mean from your diagram.

    However, I don't think it is necessary to interrupt the main supply. That is why the driver has a separate input for modulation signals.

    If it helps I have attached the manual for the driver.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Here, try it like this:

    [​IMG]

    The LED will only be lit when the TTL input goes low, and modulate is selected.
     
    Firethelaser likes this.
  9. Firethelaser

    Thread Starter New Member

    May 11, 2010
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    Looks good! When I get my parts in I will give this a try. Thanks!
     
  10. circuits148

    New Member

    Sep 21, 2011
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    THIS IS CRAZY TALK TO ME! Hahah Someone explain
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    The original poster wanted an LED to flash on and off only when they were flashing a laser beam (modulating the amount of light it is putting out) using a stream of data from a TTL level circuit, likely some kind of microcontroller.

    The easiest way to do that was to only turn on the LED when the TTL input went low, if the TTL input was driving the laser modulator. I accomplished that by wiring the anode of the LED to Vcc (+5v) using a resistor, and connecting the cathode of the LED to the modulator input. If the switch is connecting the modulator input to +5v, the LED will never turn on.
     
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