!!! You need to be very careful with laser. Never look into the laser beam !!!
Optics
From my friend I get an old optical head, which were used as wirelles
internet poitn. There were several nice things but the best one was
laser optics.
I do have several laser diodes from DVD-RW. One which I use I tested up
to 100mW optical power. For sure I am using it at 60mW. Now it should
be bullet-proof. With this optics it has 15mm diameter beam.
Laser bias
I used laser modulator from K3PGP.
It is source of constant voltage with shunt modulator. It was fine, but
I decided to build one with source of constant current instead. It
should provide better protection to laser diode.
12Ohm resistor provide soft start. With 1N4148 diode it is reverse
polarity protection. It is followed with constant current source with
LM317. Resistor R setup the flowing current.
R = 1.25 / Iop [Ohms, Amps]
In my case I wanted 150mA current therefore the R is 8.3 Ohms.
Transistor BS170 is shunt modulator. For CW modulation you can use 555
timer with adequately frequency for example 700Hz conected to Mod.
pins.. If you want to use AM phone than you will need laser modulator.
Laser modulator
Laser bias module is able to provide phone, but we can not connect the
microphone directly to the Mod. input. We do need a modulator.
This is microphone amplifier. The heart of it is LM358. Its gain
is biased with relation of resistors R1 to R2. You can use computer
headset with this modulator. Computer hedset usualy has electret mike
which needs some voltage (R3, R4). Working condition is setuped
with trimer R5. It is quite critical, therefore you need to use
multi-turn trimer. Working conditions is changing with voltage supply,
therefore there is 7809 voltage stabilisator.
At the end we need to setup linearity. The best is use signal generator
with 1kHz to MIC input. Laser beam is pointed to the wall. Its
reflection we receive with optical receiver with osciloscope. We need
to setup nice sine wave with trimers R5 and R6. You can use your
computer with sound card instead the signal generator and osciloscope.
Good software is Visual analyser.
