Here's a little something that a fellow on our BBS found on Internet.
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Newsgroups: rec.radio.shortwave
From: copeland@metronet.com (Charles Copeland)
Subject: new HAMCOM interface with optical isolation
Date: Thu, 23 Jun 1994 03:28:42 GMT
Organization: Texas Metronet, Internet for the Individual
214-705-2901 (info) Lines: 145
Here are two opto circuit I've been playing with for HAMCOM/JVFAX. They
virtually eliminate all computer induced rfi. I've been
using a five foot serial cable and twenty foot audio cable
with this circuit and the results are truly remarkable with
JVFAX.
I've used the first circuit successfully, but recommend
using the second. The first can be built with common Radio
Shack parts. The second requires a little more uncommon
LM358 and TIL181.  The Radio Shack opto RECEIVER is only
rated at 20v but used at 24v and could eventually burn out.
It also may introduced problems since the +9v is not
regulated and induces error into the reference op amp
output ... even though it seems to work well. Perhaps some
engineers out there may have some advice.
============================================================
==================
                         +9v
                          |--------              +9V              ________ 
TR
         9 volt batt. ---------    |  1uf         |              |
                        -----     ===             |              |
                          |        |             [ ] 1K         [ ] 2.2K
                          |--------               |              |________DSR
                         GND                      |C             |
                                         +9v     ___         |/ - C
                                         |       \ /  /\/    |
        .1 uF                         |\ |      -----        |\-> A
  >------| |--*-----------------------|- \        |A             |
              |                       |741 \______|              |
              |                       |    /                      --------RTS
              |               --------|+ /
              |              |        |/ |
             [ ] 100K       [ ] 100K     |
              |              |          GND
              *--------------*
              |              |
       +9V    |     |\       |
        |      -----|- \     |
        |           |741 \___| +4.5 REFERENCE
  100K [ ]          |    /
        |-----------|+ /
  100K [ ]          |/
        |
        |
       GND
============================================================
==================
            INFARED EMITTER             INFARED DETECTOR
                (clear)                    (dark)
               ----------                 ----------
              |          |               |          |
              | |__    | |               | |__    | |
              |    |   | |               |    |   | |
              |  --    | |               |  --    | |
              | |      | |               | |      | |
               ----------                 ----------
                |      |                   |      |
                |      |
These are available at Radio Shack as a pair for $1.99
(#276-142). This shows which is the Anode and which is the
Cathode, not clear on the package.
The LED and DETECTOR must be positioned right up against
each other and exactly aligned to get maximum light
transfer. The sides with the little lense like dots must be
placed together.
=============================================================================
I'd recommend this second circuit for more stability and possibly accuracy.
                                     RS232 connector size ---->  25   9
          +9v                                                    --  --
           |
           o                          +9V              ----o DTR 20
            /   SPST SWITCH            |              |
           o    SW1                    |              |
           |                       R1 [ ] 1K      R2 [ ] 2.2K
  9V       *--------                   |              |
BATTERY    |        |                  |              |----o DSR 6
       ---------   --- C1              |1    TIL181   |5
         -----     --- 1UF         .......................
           |        |              .   |C    U2       |C .
           * -------               .  ___         |/ -   .
           |                       .  \ /  /\/    |      .
          GND                      . -----        |\->   .
                             +9v   .   |A             |A .
                              |    .......................
AUDIO   100 nF            6|\ |8       |2             |4
  o------| |--*------------|- \        |              |
              |            | U1 \_7____|               ----o RTS 4
              |           5|    /
              |        ----|+ /   LM358        2|\
              |       |    |/  |4            ---|- \
         R3  [ ]  R4 [ ]       |            |   | U1 \_1___
(not used) GND      100K |   100K|        |            |
3|    /
  o-----------*-------*--------*------------*---|+ /  LM358
                               |                |/
                              GND
============================================================
==================
|                                   ** NOTE**
| | After constructing, measure LM358 pin 7 with no audio
on the input.         | | It should be high (~8v), if it is
low (~0v) it will drain the battery       | | in only 20
hours or less.
     | | To correct this:  Disconnect R1 from +9v and
reconnect to U1 pin 7.         | |
Disconnect U1 pin 2 and reconnect to GND.                 |
============================================================
Continued on 2/2
 * SLMR 2.1a * Pornography is in the groin of the beholder.
--- Squish/386 v1.10
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