ROOFLOP

Well I missed ROOPHLOCH this year. No time really, with other 
weekend jobs needing doing and no energy to spare during the weeks 
(away from huddling under the heater in cold evenings anyway). 
Nevertheless I'm not actually disappointed because by 10:30PM last 
night I did finally get all the electronics working for my optical 
communications test proposed in 2023-09-30Last_Radio_Roophloc.txt, 
but wiring it up to RS232 adapters and setting everything up 
outside in an hour and a half before midnight was more than I could 
take, on a weekday at least.

I thought I was fairly set when I started on things Sunday 
afternoon, having just finished my more important weekend job of 
fixing the instrument cluster in the farm ute which kept blowing 
the fuse (after pulling everything apart I still couldn't see what 
was, or even could have been, blowing a 10A fuse, but it's not 
blowing anymore so I guess that's a win?). I also rebuilt the 
transmitter circuit, even though it seems I did already build one, 
because I couldn't make sense of it, though this was probably 
because I kept getting the power transistor pin-out wrong since the 
datasheet list the pins in the opposite order to their left-right 
position in the numbered diagram and this sent my brain completely 
nuts. Unfortunately while my receiver circuit connected to an LED 
looked to be toggling on/off nicely with light pointed at the 
sensor, an oscilloscope revealed that the amplifier circuit was 
actually oscillating and therefore useless for receiving data 
(short of filtering, and that way lies madness).

Already a bit mentally exhausted from the ute (after I got the 
instrument cluster working, the water-in-fuel-filter light was on 
and I managed to stuff up something else draining the water), I 
gave in at that point. One thing that didn't help is that I got my 
notes for the project out at the start of the month and then, 
having not had time to study anything properly, they all got 
carelessly pushed aside during the week as unprofitable nonsense, 
and now I can't find them. I don't even know the model of the PIN 
photodiode that I built into my cardboard-tube adjustable lens 
assembly now.

Yesterday I took a quick look in the morning which only served to 
completely distract me for the rest of the day thinking about it 
while trying to make myself work on other stuff. Finally in the 
evening I printed out a different application note to whatever I 
was trying to use before and had more success copying the figure 2 
circuit from that, though with a common TL-082 FET Op-Amp instead 
of the OPA128:
https://www.ti.com/lit/an/sboa061/sboa061.pdf

Once I figured out which lead to the photodiode was which, that 
worked great - no nasty oscillations on the 'scope, just nice 
straight lines. Albeit not going Low enough out of the comparator 
stage to tuck under the 0.8v LOW threshold of TTL logic, so I'll 
have to use some silicon diodes to connect it with the FTDI RS232 
adapter, but that should work. Bandwidth might also be an issue, I 
can't get one of my USB RS232 adapters to use a baud rate lower 
than 200 baud, and the application note says to expect max. 30Hz 
from this circuit, so I might need to try one of the lower 
sensitivity circuits - ideally I'd prefer longer distance 
communications over high speed though.

Extremely rushed ACSII-art circuit diagrams:

OPTICAL DATA TRANSMITTER
                                         12v
                                          o
             ______ ______________________|
            |      |                      |
            |     |~|                     |
           |~|    | |680R   ->     ->     |
       100K| |    |_|  ____|\|__|\|_ ... _|
           |_|  ___|_|/    |/|  |/| ->
  100K    __|_|/     |  RED ULTRABRIGHT LED ARRAY
   ___  |/    |      |\>
o-|___|-|     |\>      | 2N4922
^       |\>     |      |
| BC549B  ------------
|              |
|            -----
 \            ---
  TO RS232     -
TTL USB CABLE
     TX

OPTICAL DATA RECEIVER

       ____||___
      | 5pF||   |
      |  ___    |
      |-|___|---|
      |10M  4.5v|
   ___|__|\ o   |
  |      |-\|   |   |\ TL082
 --- <-  |  \___|___|+\
 /_\ <- _|  /       |  \__|\|_|\|____   TO RS232
  |____| |+/|       |  /  |/| |/|  |  TTL USB CABLE
 ^     | |/ |     __|-/    1N4148  |       RX
 |    |~|   |    |  |/            |~|
 | 10M| |   |    |   4.5v         | |10K
 |    |_|   |   \/    o           |_|
 |     |    |  ____   |            |
 |      ----*-|____|--           -----
 \ PIN      |  250KB              ---
PHOTODIODE  |                      -
          -----
           ---
            -

Besides finally putting this to use (perhaps once it's more solidly 
built than the current mess of breadboards and clip leads), I also 
want to try it for detecting radiation since these PIN photodiodes 
are supposed to do that in a similar way. In fact that's what I 
originally bought them for!

 - The Free Thinker