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Posted by Sam Goldwasser on May 22, 2006, 11:16 am
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> Rahgu Seghar wrote:
> > How to modulate laser pointer
> > =====================
> > I want to modulate a Red and Green laser pointer (laser diode) with
> > audio sweep signal 1-10KHz square wave with adjustable duty cycle.
> > For the modulation I'm planning to use a simple transistor circuit
> > where the base of the transistor will be conneced to a signal
> > generator and the laser pointer will be connected to the collector
> > of the transistor. The transistor will be used to switch on/off the
> > laser diode.
> > How to sweep the modulation signal and rotate the laser pointer
> > ===========================================
> > Signal generator will sweep the frequency from 1KHz to 10KHz in
> > 1second. During this 1 second period laser pointer will be
> > physically rotated from 0 degree to 180 degree (From Left to
> > Right). Then signal generator will sweep the signal from 10KHz down
> > to 1KHz and laser pointer will be rotated from 180 degree to 0
> > degree (From Right to Left).
> > Laser receiver sensor
> > ===============
> > There will be a receiver/sensor on an arbitrary position at a
> > distance of 100meters (or more). When the laser light hits the
> > sensor, I want to detect the laser signal and demodulate the
> > received signal. The frequency of the demodulated signal will
> > indicate the angle of the receiver/sensor with repect to the laser
> > pointer.
> > Questions
> > =======
> > 1.) Can you advise a good way of modulating the laser pointer?
> Direct pulse modulation is easiest. A mosfet or bipolar transistor
> will do.
> > 2.) What can I use as receiver sensor which can operate even in daylight?
> A silicon pin photodiode would be sensible. You don't state the range
> you need. A narrow band interference filter at the laser wavelength
> in front of the diode will greatly reduce the effect of sunlight.
>
> > 3.) Can you advise a good way of demodulating/decoding the received signal?
>
> I'd use a carrier in the 100khz to 1 mhz range (fixed frequency) so it
> would be easy to easy to filter out low frequency noise. THen you can
> audio modulate the signal. Rather than using tone you could send the
> angular positon as pulse coded data whhich is likley to yield better
> angular resolution vs time than an angule vs frequency scheme. I'm
> assuming you have a miroprocessor on both ends
>
> > 4.) Would it make any difference if I use either Red or Green laser
> > pointers?
>
> Red will have a little less solar interference, better sensitivity for
> silicon detectors, and much lower cost for a given power level laser
> and have less absorbtion by the atmosphere. Infrared in the 800 to
> 950 nm range would do better still in all respects.
In fact, if you get the absolutely dirt cheapest Far East import red
laser pointers, they have no regulator and no filter caps (just a current
limiting resistor) so directly controlling power will be able to modulate
from DC up to many MHz.
Better ones will have some sort of filter capacitors on the power and
modulating at more than a few Hz may not be possible without going inside.
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> =====================
> I want to modulate a Red and Green laser pointer (laser diode) with audio
> sweep signal 1-10KHz square wave with adjustable duty cycle.
> For the modulation I'm planning to use a simple transistor circuit where
> the base of the transistor will be conneced to a signal generator and the
> laser pointer will be connected to the collector of the transistor. The
> transistor will be used to switch on/off the laser diode.
>
> How to sweep the modulation signal and rotate the laser pointer
> ===========================================
> Signal generator will sweep the frequency from 1KHz to 10KHz in 1second.
> During this 1 second period laser pointer will be physically rotated from
> 0 degree to 180 degree (From Left to Right). Then signal generator will
> sweep the signal from 10KHz down to 1KHz and laser pointer will be rotated
> from 180 degree to 0 degree (From Right to Left).
>
> Laser receiver sensor
> ===============
> There will be a receiver/sensor on an arbitrary position at a distance of
> 100meters (or more). When the laser light hits the sensor, I want to
> detect the laser signal and demodulate the received signal. The frequency
> of the demodulated signal will indicate the angle of the receiver/sensor
> with repect to the laser pointer.
>
> Questions
> =======
> 1.) Can you advise a good way of modulating the laser pointer?
> 2.) What can I use as receiver sensor which can operate even in daylight?
> 3.) Can you advise a good way of demodulating/decoding the received
> signal?
> 4.) Would it make any difference if I use either Red or Green laser
> pointers?
>
> Thx.
>
> Rahgu
>
>
>
>
>