[Laser] Ramsey Kit range

Tue, 11 Nov 2003 20:37:31 EST

Thank you all for the many comments.

Allow me to give some more details on the LBC6.=A0 The receiver consists of=20=
a=20
phototransistor with a 1M collector resistor capacitively coupled to an=20
inverter amp with a gain of 2 and a tensy bit of capacitive feedback for sta=
bility,=20
then to a sharp cut off 6kHz low pass filter (the identical LPF they use on=20=
the=20
transmitter) to a pot for gain control and an audio amp (LM386).=A0 This is=20=
a=20
simple light sensor with DC blocking and filtering above the desired audio=20
band.=A0 The receiver has no trouble "hearing" amplitude modulated light, su=
ch as my=20
700-800Hz pulse laser circuit or the hum from a farm light.

The transmitter starts with an electret mike and an amp with a gain of 100,=20
the afore mentioned LPF, to a microcontroler chip.=A0 I can't say from the=20
circuit diagram, but the manual says that the chip provides automatic gain c=
ontrol,=20
pulse width modulation(The advertisements say 16kHz, the cover of the manual=
=20
says 20kHz, and the text of the manual says 18kHz.=A0 I have not checked.=
=A0 It=20
works.), and a test tone function, as well as driving a BS170 FET, which tur=
ns=20
the laser pointer on and off.

There are several places you can break into the circuits.=A0 For the receive=
r,=20
I would think that you would want to build an entirely separate light sensor=
=20
circuit, mounted to the optics, an use the LBC6 receiver for what it is, a l=
ow=20
pass filter and audio amp.=A0 The transmitter is probably limited in drive=20
current, but it works, for that laser pointer.=A0 I have a circuit using two=
 555=20
timers that I think is a lot cheaper and easier to modify, but I haven't tes=
ted it=20
yet.=A0 The LBC6 has what I think is a good mike, amp, and LPF.

In short, the LBC6 does what it claims, and might be souped up.=A0 I don't=20
think I would recommend it for someone that planned to mod the transmitter.=
=A0=20

When I built the units, made a few "improvements" like changing the TX power=
=20
switch from push-on-pull-off to pull-on-push-off to reduce the hazard of=20
draining the battery if the switch is accidentally bumped.=A0 If I were goin=
g to=20
build another one, I would consider using the power switch to change from to=
ne=20
mode to mike input, and=A0 use a power cord to external batteries.=A0 I was=20=
really=20
annoyed when the RX=A0 combination audio gain pot and power switch did not f=
it in=20
the mounting holes=A0 -=A0 I have the tools and experience to modify the cir=
cuit=20
board or the switch but others may not.=A0=20

Both the TX and RX units are mounted in 2" plastic pipe (which the manual=20
says they are sized for, but does not give even a hint of how to do it.=A0 I=
t took=20
longer to work out the first installation with cut and try than it took me t=
o=20
solder all 4 units.

I have 50mm and 100mm lenses mounted repectively in 2" and 4" plastic pipe,=20
with black paper to reduce stray light.=A0 The RX unit will fit to the pipes=
, but=20
I have not attempted focusing.=A0 Work on the optics and electronics were pu=
t=20
on hold while I work on two table mounts.=A0 They will stand about 4 feet hi=
gh on=20
three 2X4 legs with an 18" diameter "table" made of kitchen counter top=20
pressed board with a hole in the center for 1/4" bolt for an azimuth axle.=
=A0 The=20
optics and electronics will mount on a board 18" X 48" which is made from vi=
nyl=20
clad pressed board.=A0 The "board" rests on the "table" with a disk of felt=20=
as a=20
separator for smooth rotation 360 degrees.=A0 The three legs use 3/4" bolts=20=
to=20
adjust the height individually.=A0 Once the "board" is pointed close to the=20
target, it is clamped.=A0 The optics then are mounted to the "board" with a=20=
fine=20
adjustment mechanism which I have not worked out yet.=A0 I plan to use an 8-=
32=20
threaded rod and a pivot distance of at least 31.25" for on milliradian angl=
e=20
change per revolution of the rod.=A0=20

I think I better close for the night.

James

John,

You are right, but the Ramsey RX runs into some special IC before it=20
goes on to the LP filtering, which makes
me suspicious of what is really going on.=A0=A0 Like most folks, I just put=20
the parts in the board and soldered it
together, not questioning the magic of the black box.=A0 There is a lack=20
of buzz with the lights, which also makes
me think there is more too it, but that might be because the thing is=20
absolutely deaf relative to the
PD-based receivers I've built.=A0 3KHz B/W will be a much better S/N than=20
100Khz for sure....

Andy K0SM/2

John Matz wrote:

>I just thought I'd mention ... if it really is true PWM, not on a subcarrie=
r
>... then the bandwidth need only be 3 kHz on the receive end.=A0 Recovery i=
s
>simply lowpassing the PWM transmission.
>John Matz KB9II
>----- Original Message -----
>From: "Andrew T. Flowers, K0SM" <[email protected]>
>To: <[email protected]>
>Sent: Sunday, November 09, 2003 7:01 PM
>Subject: Re: [Laser] Ramsey Kit range
>
>
>>This reminds me,
>>
>>It seems there have been several folks who have put together these
>>ramsey kits.=A0 In fact, Dave and I have had the pleasure of using these
>>kits in a radio contest or two.=A0 Does someone want to find a way to "hot
>>rod" these things to get a little better performance out of them?=A0 It
>>seems to me that someone could come up with a "front end" that beats the
>>heck out of the phototrasistor that comes with it, keping in mind that
>>it needs to have a respose up to over 100KHz or so (it's 18KHz PWM, and
>>you need to resolve the waveform fairly accurately I think).=A0 It's
>>pretty clear to me that most of the light is getting thrown away due to
>>the teeny-tiny active area of those devices.=A0 If someone wants to look
>>at this I can show you a schematic what the RX looks like.=A0 It might be
>>a worth doing since many folks seem to be getting interested in laser
>>comm stuff through these kits...and it's also in the a "ham spirit" to
>>figure out how to make a good thing better....
>>
>>Andy
>>K0SM/2, Rochester NY
>>
>>David D. Rea wrote:
>>
>>>On Sun, 2003-11-09 at 15:20, [email protected] wrote:
>>>
>>>>On the other hand, the area of beam, or the power in it, may not be
>>>>
>descibed
>
>>>>by a mathematical relation to the square of the distance.=A0 For "short
>>>>disatances" the intensity of the received beam may be linearly
>>>>
>proportional to these
>
>>>>"short" distance numbers.=A0 If that is the case then the power on the
>>>>
>detector
>
>>>>would be proportional to the area of the lens, so that the useful range
>>>>
>is
>
>>>>factored by the square of the diameter ratio of the lens to the
>>>>
>detector.
>
>>>>Both ways of thinking about range seem to be oversimplification of the
>>>>
>"real"
>
>>>>world, but is either a practical estimating tool?
>>>>
>>>Hi James-
>>>
>>>You're fairly close to on-target with your theory. The received power
>>>will fall off with the square of the distance, but this only occurs in
>>>freespace. Keep in mind that you've got atmospheric nasties to deal with
>>>as well - after you get above 1 Km or so, you'll start noticing the
>>>effects of humidity and purturbations in the air; i.e. you'll lose power
>>>due to absorption and dispersion of water mollecules, and you'll see a
>>>"shimmering" effect as the beam traverses different thermal planes
>>>between the transmitter and the receiver.
>>>
>>>There has been quite a bit of work done in this subject, as I found out
>>>when writing a paper on laser communication during an undergrad
>>>independent study. You can get as deep in the math as you want; there's
>>>no shortage of folks out there who have written PhD theses on this
>>>stuff...
>>>
>>>To add a layer of complexity, remember that no lens is perfect, either.
>>>If you're dealing with a nice AR multi-coated glass lens, maybe you'll
>>>throw away 5% of your signal (if you're lucky). But if you're using a
>>>less expensive (but MUCH larger area) fresnel lens, plan on tossing
>>>about 50% of the inbound light back toward the receiver. This is where
>>>you compensate for reflection with sheer lens size...
>>>
>>>Anyway - hope this helps a bit...
>>>
>>>73 de Dave K2THZ

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