RF Column 24 - October 1993
Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED

TOPICS:  Cheap Remote II (Son of Cheap Remote?)
         Using the sun to align satellite dish mounts

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A year ago I described an inexpensive remote control system useful for
monitoring microwave sites and low power transmitters and useful as a
backup for full power stations. That article was based on the first unit
I built. Since then, I've built two more for LPTV monitoring and may
have to put together three more before the end of the year. That
prompted me to come up with a standard design that was easy to
construct. This month I'll show you at full schematic of a complete
interface / isolation circuit for use with the Blue Earth BE-440e micro
controller. I also shot a few pictures of the last unit I built, for use
at our K27EI low power station in Santa Maria, California. 

I shot a new color photo of the remote control for this Internet edition 
of the column. Download file rf24fg1.gif from my FTP site. The schematic 
is available as a GIF file, rf24fg2.gif or as a DXF file (easier to read 
and plot) as rf24fg2.dxf.

The previous articles described the hardware design of the Blue Earth
Micro, some interface circuits I was building and my software program to
run it. I recommend you dig through last year's TV Technology magazine
and read those columns for more details on the circuit. I'll cover some
of the high points here. Since I called the first remote my "Cheap
Remote", I suppose this one will be "Son of Cheap Remote", or perhaps
CR-II to keep it short and save typing.

CR-II takes advantage of most of the Blue Earth Micro's features. I've
configured it for three status inputs and three command outputs - two
open collector and one relay. The 8 bit A/D converter inside the Micro
has eight multiplexed inputs. One is reserved for monitoring the supply
voltage, so seven are available for our use. I like to design "bullet
proof" circuits. You'll notice I've used opto-isolators on both the
command and status inputs. On this board I derived the voltages for the
external side of the opto-isolators from the main supply voltage,
isolated with a 1N4004 type diode and a 150 ohm 1/2 watt resistor. I
figured over-voltages would not get past the diode and shorts to ground
would not pull the supply down, thanks to the series resistor. This
works fine with solid state transmitters. If you are working with
circuits where you really need more isolation, like tube transmitters,
supply the external side of the opto's with a separate, isolated power
supply or use power from the transmitter cabinet itself. The LED's on
the status input side are very sensitive. You should be able to parallel
them across panel LED's or lights if you have to. Watch out for leakage
currents that can keep them turned on. It may be necessary to add a
diode or a zener in series with the input resistor to keep the LED's
from turning on at low voltages. The LED supply and the pull up
resistors on the Micro side of the opto-isolators are regulated by their
own 78L05 100 mA regulator. These 100 ma regulators are cheap, so I used
separate ones for the digital and analog portions of the circuit. Its an
easy way to keep digital noise out of analog circuits.

Blue Earth research says series resistors are sufficient protection, in
most cases, for the A/D converter inputs inside the Micro. I felt the
inputs needed some additional protection - a first line of defense,
outside the Micro box. I used two National Semiconductor LMC660CN quad
op amps for this purpose. They are an improvement over the first circuit
design, which used op amps that needed both positive and negative
supplies to swing from ground to +5 volts. The LMC660CN's solved two
problems. First, I could eliminate the negative supply. Second, by
limiting the op amp supply voltage to +5 volts, I could eliminate the
nasty tendency of A/D inputs on the Micro to "bleed" over into each
other if an input exceeds 5 volts. The LMC660CN allows both input and
output voltage swings from "rail to rail", so I can use a single 5 volt
op amp supply to work with any permissible input to the A/D converter.

Let me quickly describe the analog input circuitry. A 100K ohm resistor
on the input works with the 0.1 uf. capacitor to filter out noise on the
incoming line. Don't worry about it slowing down the readings - the
effect isn't noticeable. The DIP switch and second 100K resistor are
useful if the incoming voltage is too high. Switching the resistor in
drops the voltage into the op amp by at least half. Note that the source
impedance will affect the drop. I used a DIP resistor network for these
resistors and mounted it on a socket so I could easily modify the
divider. Finally, the 1 megohm resistor to ground on the non-inverting
input prevents the op amp from drifting to the +5 volt rail with no
input. The feedback network is simple. Offset currents on the LMC660CN
are low enough that it really isn't necessary to match inverting and
non-inverting input resistance's. This design can be adjusted to work
with full scale inputs from under 1 volt up to 10 volts. I put a 4.7K
resistor in series with the output of each op amp, mainly to provide
another level of protection for the Blue Earth Micro A/D inputs. You'll
notice that, unlike earlier designs, there are no filter capacitors on
the output of the op amps. They were eliminated because the input
multiplexer for the A/D in the Micro can generate little spikes when
switched. These could charge the input filter capacitors and add
unwanted errors to the readings. The op amps are powered by a separate
78L05 regulator, as noted above.

Each of the three versions of the Cheap Remote were built differently,
using lessons learned from the previous one. I've found a couple things
make construction much easier. Use SIP (single in line) or DIP (dual in
line) resistor arrays where possible. They can often be aligned with IC.
pins to make connections simple. Also, use IDC's (insulation
displacement connectors) for cables to the serial port and Blue Earth
Micro I/O port. If you match up the IDC connections so that the flat
cable can be crimped directly onto a DB-9 or DB-25 connector it will
save a lot of soldering. The schematic shows a top view of both the IDC
headers. Wire you board this way and a simple crimp connection is all
that's needed to get the signals off the board. I also found that two
piece PC board/screw terminal connections work great for the inputs and
command outputs. Simply unplug the whole screw terminal portion if you
need to pull out the circuit board or remote control box. No wiring is
needed from a board to a terminal strip. Several connector/terminal
manufacturers make them. The final tip is to use a circuit board with
pads on one side and an isolated ground plane on the other. This keeps
things clean and there's always a ground handy. I used the Vector
"Circboard" for mine, although I hope to get a printed board made up
before building many more units. The cast aluminum box was a bit more
expensive than the ones made out of aluminum sheet, but it gives the
CR-II a nice sturdy look and provides better RF shielding.

The current CR-II monitors an Acrodyne TLU-500KSCE transmitter and a
Uniden CAT-100R satellite receiver. I put the visual and aural forward
powers plus total reflected power readings on the remote, as well as the
two power supply voltages for the final amplifiers. Acrodyne makes these
readings available on their remote control connector. I also plan to
monitor the satellite receiver AGC voltage, taken from the hot side of
the AGC meter level potentiometer, isolated with a 100K resistor and an
additional 0.1 uf. capacitor in side the receiver. I'll let you know how
this works. The last available input is a temperature monitor using the
LM34DZ temperature sensor chip I wrote about a while back. I've also
connected the RF On and Video Present status indications to the status
inputs on the remote. I normally control the transmitter by interrupting
the input video with a relay but also have the capability of turning it
on or off using the two open collector outputs controlling the momentary
On and Off remote commands..

I described the software last year. Little has changed in it with CR-II.
You'll notice I did add two transistors to control the voltage to the
command LED's. This provides a way to disable the command outputs. A low
on port 3, bit 5 turns off Q-1, which also turns off Q- 2, a PNP
transistor supplying the opto-isolators. As I write this I'm not using
that line, however, I'm looking for a way to keep all the commands from
pulsing when power is first applied to the Blue Earth Micro and
interface box. I thought this circuit might do it automatically, if all
output went low at the same time. There is still a minor glitch. My next
experiment is to add a capacitor from the base to emitter of Q-2, so it
takes longer for the voltage to drop low enough to turn on the unit.

Look for the latest version of the software on CompuServe's Broadcast
Professionals Forum (type GO BPFORUM at any system prompt). ((8/95 >
UPDATE! Check for the files on this Internet site - they are NOT on
BPFORUM!)) If you're not yet a member of CompuServe, free intro-packs
are still available to TV Technology by calling 1-800-848-8199 and
asking for operator 175. If outside the U.S. or Canada, call
614-457-8650. The files are in the BPFORUM library as RMTCTL.BAS (the
text file listing the Basic program loaded into the Blue Earth Micro)
and RMTACC.ARC (an archive containing short Basic programs for setting
the time, baud- rate and such on the Blue Earth Micro). I'm also working
on a simple program that will run on IBM compatible P.C.'s and handle
the communications with the CR-II. For now, any terminal program on any
computer will talk to the CR-II, provided it can send control codes
(ASCII codes below 32). Refer to remarks in the RMTCTL.BAS program for
more details. I use Control-G, Control-F, Control-O, Control-N and
Control-R in the program. You can easily modify it to use other standard
ASCII letters if you want. If you build this project, you should take a
look at my program. If you can't get if off of CompuServe, send me a
postcard and I'll mail you a text listing or, if you want it on an
IBM-PC DOS format disk, send me a formatted disk (preferably 3.5", but I
can handle 5.25" if you are patient) in an mailer, with a return address
label and sufficient return postage. Please do not use postage meter
stamps for the return postage. The post office is hassling me about
mailing stuff out with all these different postage meter stamps and have
threatened to stop taking the mail! (UPDATE! IMPORTANT NOTICE -- I AM NO
LONGER ABLE TO MAIL SOFTWARE DISKS. ALL SOFTWARE IS AVAILABLE AT MY
INTERNET SITE)

I was able to find all the parts for my CR-II at Digikey, although I
ended up buying some last minute change items at the local Radio Shack.
Digikey can be reached at 1-800- DIGIKEY in the U.S. or at 218-681-6674.
Their fax number is 218-681-3380. Digikey does not ship directly to
international accounts, but can ship to your freight forwarder. All the
items used here are common, so any major parts distributor should be
able to provide them. If you need to track down other distributors for
the Micro-440 microcomputer, the direct number for Blue Earth Research
is 507-387- 4001.

Please see my earlier columns for information on modem setups for use
with the CR-II. The Computer Peripherals Viva error correcting modem has
worked fine for me. ((UPDATE! This Viva modem locks up after heavy
lightning storms. Use the Hayes instead!)) I'm now testing the Hayes
Smartmodem Optima 24B+FAX96, and it seems to be working. While the CR-II
can issue DTR commands to hang up the modem and use the carrier detect
line from the modem, it does not do hardware handshaking. I can send you
the settings for either the Viva or the Hayes modem if you need them.

We're running out of space this month. Here's a tip I found while
installing the K27EI transmitter. I wanted to double check the alignment
of the transmit antenna. I don't trust compasses, since magnetic
declination varies from spot to spot. A GPS receiver would be nice, but
I can't afford one yet. Here's my solution. It cost less than a hundred
dollars and should be accurate within a degree. Use an astronomy program
(there are many out there - I used EZ-Cosmos for Windows) to find either
the exact location of the sun in the sky at the time you need to check
the alignment, or, if you can wait, find the time the sun will be at the
location you want to hit. A bit of playing around showed me that on
August 31, 1993, the sun would be at 244 degrees and about 38 minutes
from true north at 3:45 PM PDST. I used a spare piece of conduit to
shoot the sun - I aligned it so the sun was pointing down through the
tube to the ground and the shadow was a single circle. The direction the
conduit was aimed was now about 244.5 degrees, pretty close to my peak
of beam of 245 degrees.

Now for the warnings and disclosures. DO NOT LOOK AT THE SUN! You'll
damage your eyes! You can easily see when the conduit is pointed at the
sun by looking at the shadow. As far as CompuServe, the Broadcast
Professional's Forum, or Digikey or Blue Earth Research are concerned I
have no financial interest in these companies (other than it seems like
I keep sending them money) and receive no compensation, credit or
discount from them as a result of these articles. This design is offered
freely to other engineers for their own use in stations they are
employed by (either directly or as a consultant), without charge, but I
retain all rights to the design and software. Perhaps someday, when I've
had a chance to tweak the design and expand it, I'll license the design
for a commercial product.

Next month - more tips, some promised by me months ago, and some nifty
new products you might find interesting. Also, I'm getting close to
finishing the WWV receiver / calibrator. Preliminary results next month.
My work schedule lately has been busy and it looks like I'm going to be
traveling more. CompuServe E-mail is the best way to reach me, my number
is 70255,460. You can also write me at 2265 Westwood Blvd., Suite 553,
Los Angeles, CA 90064 or phone me at 305-884-9664. Please do not be
discouraged if I answer and put you on hold quickly. If you can't hold,
hang up call back later. Around 10 AM or after 6 PM Eastern is best,
when things quiet down a bit.
((8/95 > UPDATE! - Use dlung@gate.net for e-mail!))

Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED