Help, Laser Communicator

A

amdx

Guest
I'm helping my son make a laser communicator.

It uses a cheap laser pointer and a Radio Shack 1000ohm to 8ohm audio
transformer. The 1000 ohm side is in series

with a battery and the laser diode, the 8 ohm side is driven by a radio.

The receiver is a Cadmium Sulfide sensor (CdS) in series with a 9v battery
in series with the input of an amplifier. This all worked fine. But I looked
a little further a see the modulation transformer is causing very little
modulation, maybe 20mvpp on a 4vdc laser drive voltage.

So, I'm thinking about winding a better transformer.

Some data:

The laser impedance is about 160 ohms (100 to 200 ohms depending on
input voltage)

I'd like to see about .5v of modulation.

The radio will output about 4vpp into 8 ohms.

If I point the laser with 2.5Vdc input at the CdS, the R of the CdS is 115
kohms.

If I point the laser with 3.0Vdc input at the CdS the R of the CdS is 52
kohms.



Any hints on improving the modulation and the receiver?

Thanks, Mike
 
On Thu, 22 Nov 2007 11:29:40 -0600, "amdx" <amdx@knology.net> wrote:

I'm helping my son make a laser communicator.

It uses a cheap laser pointer and a Radio Shack 1000ohm to 8ohm audio
transformer. The 1000 ohm side is in series

with a battery and the laser diode, the 8 ohm side is driven by a radio.

The receiver is a Cadmium Sulfide sensor (CdS) in series with a 9v battery
in series with the input of an amplifier. This all worked fine. But I looked
a little further a see the modulation transformer is causing very little
modulation, maybe 20mvpp on a 4vdc laser drive voltage.

So, I'm thinking about winding a better transformer.

Some data:

The laser impedance is about 160 ohms (100 to 200 ohms depending on
input voltage)

I'd like to see about .5v of modulation.

The radio will output about 4vpp into 8 ohms.

If I point the laser with 2.5Vdc input at the CdS, the R of the CdS is 115
kohms.

If I point the laser with 3.0Vdc input at the CdS the R of the CdS is 52
kohms.



Any hints on improving the modulation and the receiver?

Thanks, Mike
Click to expand...
Suggestions:

Reverse the transformer (to step down) and make sure the laser supply
is heavily bypassed, big electrolytic.

Use a better detector, like a photodarlington. CdS cells are
incredibly slow.

Use a lens on the receiver.

That 160 ohm laser impedance sounds fishy to me. I'd have guessed a
lot lower. You're not including the transformer resistance there, are
you?

John
 
amdx wrote:

I'm helping my son make a laser communicator.

It uses a cheap laser pointer and a Radio Shack 1000ohm to 8ohm audio
transformer. The 1000 ohm side is in series

with a battery and the laser diode, the 8 ohm side is driven by a radio.

The receiver is a Cadmium Sulfide sensor (CdS) in series with a 9v battery
in series with the input of an amplifier. This all worked fine. But I looked
a little further a see the modulation transformer is causing very little
modulation, maybe 20mvpp on a 4vdc laser drive voltage.

So, I'm thinking about winding a better transformer.

Some data:

The laser impedance is about 160 ohms (100 to 200 ohms depending on
input voltage)

I'd like to see about .5v of modulation.

The radio will output about 4vpp into 8 ohms.

If I point the laser with 2.5Vdc input at the CdS, the R of the CdS is 115
kohms.

If I point the laser with 3.0Vdc input at the CdS the R of the CdS is 52
kohms.



Any hints on improving the modulation and the receiver?

Thanks, Mike

That's kind of tricky to get a laser diode transmitter to follow
Click to expand...
in a linear track..
If the diode was a 3 legged unit with an internal PD detector in it,
you could use that to bias an op-amp to insure the laser output is
following linearity.
normally, these PD are used for the laser driving circuit to ensure
that it only supplies enough to archive output. If you have one of
these and haven't redirected this circuit, it maybe working against you.

----------------------------- ---------------------------
On a second approach....

If you really want it to work ..
I would suggest a PWM transmitter for the laser and
high gain comparator on the receiving end..
Of course, if you take this path, you'll need to sit
down and do some engineering :)
it's not really that bad when its all done.


--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:g2gbk3tnki8l41p6orbuej2fkmeiv1duqb@4ax.com...
On Thu, 22 Nov 2007 11:29:40 -0600, "amdx" <amdx@knology.net> wrote:

I'm helping my son make a laser communicator.

It uses a cheap laser pointer and a Radio Shack 1000ohm to 8ohm audio
transformer. The 1000 ohm side is in series

with a battery and the laser diode, the 8 ohm side is driven by a radio.

The receiver is a Cadmium Sulfide sensor (CdS) in series with a 9v
battery
in series with the input of an amplifier. This all worked fine. But I
looked
a little further a see the modulation transformer is causing very little
modulation, maybe 20mvpp on a 4vdc laser drive voltage.

So, I'm thinking about winding a better transformer.

Some data:

The laser impedance is about 160 ohms (100 to 200 ohms depending on
input voltage)

I'd like to see about .5v of modulation.

The radio will output about 4vpp into 8 ohms.

If I point the laser with 2.5Vdc input at the CdS, the R of the CdS is
115
kohms.

If I point the laser with 3.0Vdc input at the CdS the R of the CdS is 52
kohms.



Any hints on improving the modulation and the receiver?

Thanks, Mike




Suggestions:

Reverse the transformer (to step down) and make sure the laser supply
is heavily bypassed, big electrolytic.

Use a better detector, like a photodarlington. CdS cells are
incredibly slow.

Use a lens on the receiver.

That 160 ohm laser impedance sounds fishy to me. I'd have guessed a
lot lower. You're not including the transformer resistance there, are
you?

John

Thanks John,
Click to expand...
The lens on the receiver, is that to spread out the light? As it is,
the laser dot only fills a small portion of the CdS area so I don't want to
focus it any more.
Re; laser impedance, I rechecked just to make sure,
The test circuit is a variable P.S.in series with a 4 ohm resistor
and laser diode. Measured voltage across P.S. and 4 ohm resistor.
At 2.25v current is 9.3ma equaling 243 ohms.
At 4v current is 33.8 ma equaling 119 ohms.
So 160 ohms is correct for this laser diode.
See laser at ;
http://www.scitoyscatalog.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=LASER&Category_Code=L
Although I paid $1.00 at the dollar store.
Mike
 
"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:rBj1j.28$Mc.7@newsfe07.lga...
amdx wrote:

I'm helping my son make a laser communicator.

It uses a cheap laser pointer and a Radio Shack 1000ohm to 8ohm audio
transformer. The 1000 ohm side is in series

with a battery and the laser diode, the 8 ohm side is driven by a radio.

The receiver is a Cadmium Sulfide sensor (CdS) in series with a 9v
battery in series with the input of an amplifier. This all worked fine.
But I looked a little further a see the modulation transformer is causing
very little modulation, maybe 20mvpp on a 4vdc laser drive voltage.

So, I'm thinking about winding a better transformer.

Some data:

The laser impedance is about 160 ohms (100 to 200 ohms depending on
input voltage)

I'd like to see about .5v of modulation.

The radio will output about 4vpp into 8 ohms.

If I point the laser with 2.5Vdc input at the CdS, the R of the CdS is
115 kohms.

If I point the laser with 3.0Vdc input at the CdS the R of the CdS is 52
kohms.



Any hints on improving the modulation and the receiver?

Thanks, Mike

That's kind of tricky to get a laser diode transmitter to follow
in a linear track..
If the diode was a 3 legged unit with an internal PD detector in it,
you could use that to bias an op-amp to insure the laser output is
following linearity.
normally, these PD are used for the laser driving circuit to ensure that
it only supplies enough to archive output. If you have one of
these and haven't redirected this circuit, it maybe working against you.

Yea I don't really know where the nonlinearity is worse the laser or the
Click to expand...
CdS. I suspect the laser is ok between 2.5v and 3v and the CdS is the major
source of nonlinearity. But.....
 
On Thu, 22 Nov 2007 11:29:40 -0600, "amdx" <amdx@knology.net> wrote:

Any hints on improving the modulation and the receiver?
Click to expand...
I helped a student with exactly this project a year ago - while the
modulation seems tiny, it's actually quite effective, but the receiver
circuit he had found was a faulty design. Given crap on the web, I
wonder if you've got the same one. Once that was sorted, the thing
worked stunningly well (if you could line up the laser beam) despite the
appearance of "not much happening". Stray light did not bother it, you
could do some interesting modulation (of the already modulated signal)
by running your fingers or a comb through the beam. With the circuit he
had found, there was a great deal of interference and noise pickup, with
very little signal getting through. I also thought about increasing
modulation on the laser end of things as a solution, but stumbled across
a way to improve the receiver while trying to figure out what was not
working.

Toss ether a link to or an ASCII sketch of the receiver circuit, and
I'll try to remember what it was. It was bog-simple, just adding a 1200
(value not critical) ohm resistor, but I'd either need to see and
recall, or go find the student's report, if it's still here somewhere.
Best I can manage at the moment, after cogitating while tending to the
T-day cooking, is that it was across the output - as far as I recall,
the receiver _was_ just


-----------
| |
_ \
- Batt / CDS
| \
| |
| ------------
| Out to amp
|
----------------------

Which meant there was not much of a current path, as the inputs of an
amp are typically high impedance. My "stunning" modification (as best I
recall) was simply to provide a path for some current to flow in a
relatively low impedance path, which made the thing work easily 10 times
better.

-----------
| |
_ \
- Batt / CDS
| \
| |
| ------------
| \ Out to amp
| / 1200 R
| \
----------------------

--
Cats, coffee, chocolate...vices to live by
 
"amdx" <amdx@knology.net> wrote in message
news:4590d$4745bc82$18d6b40c$1016@KNOLOGY.NET...
I'm helping my son make a laser communicator.

It uses a cheap laser pointer and a Radio Shack 1000ohm to 8ohm audio
transformer. The 1000 ohm side is in series

with a battery and the laser diode, the 8 ohm side is driven by a radio.

The receiver is a Cadmium Sulfide sensor (CdS) in series with a 9v
battery in series with the input of an amplifier. This all worked fine.
But I looked a little further a see the modulation transformer is causing
very little modulation, maybe 20mvpp on a 4vdc laser drive voltage.

So, I'm thinking about winding a better transformer.

Some data:

The laser impedance is about 160 ohms (100 to 200 ohms depending on
input voltage)

I'd like to see about .5v of modulation.

The radio will output about 4vpp into 8 ohms.

If I point the laser with 2.5Vdc input at the CdS, the R of the CdS is 115
kohms.

If I point the laser with 3.0Vdc input at the CdS the R of the CdS is 52
kohms.



Any hints on improving the modulation and the receiver?

Thanks, Mike

I found a much better receiver. I pulled a phototransistor from the
Click to expand...
junkbox (detector from Radio Shack 276-142) and a set of high impedance
magnetic headphones (3000 dc resistance), I wired these in series with a 9v
battery. With as little as 5mvpp out of the radio I can here the signal on
the headphones. If I crank the radio to 3vpp it's to loud to listen to
comfortably.
Mike
 
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

amdx wrote:
I'm helping my son make a laser communicator.

It uses a cheap laser pointer and a Radio Shack 1000ohm to 8ohm audio
transformer. The 1000 ohm side is in series

with a battery and the laser diode, the 8 ohm side is driven by a radio.

The receiver is a Cadmium Sulfide sensor (CdS) in series with a 9v battery
in series with the input of an amplifier. This all worked fine. But I looked
a little further a see the modulation transformer is causing very little
modulation, maybe 20mvpp on a 4vdc laser drive voltage.

So, I'm thinking about winding a better transformer.

Some data:

The laser impedance is about 160 ohms (100 to 200 ohms depending on
input voltage)

I'd like to see about .5v of modulation.

The radio will output about 4vpp into 8 ohms.

If I point the laser with 2.5Vdc input at the CdS, the R of the CdS is 115
kohms.

If I point the laser with 3.0Vdc input at the CdS the R of the CdS is 52
kohms.



Any hints on improving the modulation and the receiver?

Thanks, Mike
Click to expand...
ChibiChn created a digital transmitter for audio a while back. Although
his instructions are no longer available on the internet, he offered to
send schematics and details to interested people via e-mail ( ChibiChn
__AT__ ou.edu )
It's based on a 555 pwm modulator, if I remember correctly.

- --
Brendan Gillatt
brendan {at} brendangillatt {dot} co {dot} uk
http://www.brendangillatt.co.uk
PGP Key: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0xBACD7433
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.3 (MingW32)

iD8DBQFHRvt1kA9dCbrNdDMRAqDKAJ4wSsHJIOm7NpMW2FikCn15oU/lygCfZYgu
BPZ1uU1+4y+40aFE7893THE=
=MkSF
-----END PGP SIGNATURE-----
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top