[RTTY] optoisolators

[Kevin Normoyle]

before the arrl roundup, I updated an Icom 751 to FSK keying. (actually 
kinda during, HI )

This is well trod ground, and I'm a newbie, but:

I've had problems with an older icom 730 not keying (from the 
computer serial port) with a  simple 4N33 opto-isolator on PTT, 
so looked around on the web.

I've noticed it seems to be quite common, with people doing various
things with relays or an extra transistor to get more current 
into the opto isolator, so more load current.

But I was intriqued by a comment in a design from KK7UQ, that said
he changed to using a solid state relay, with lower Ron, to 
cover keying issues with rigs like the icom 706, or older rigs.

basically just another opto-isolator, with different specs. Got
me wondering why people use the 4N33 so much if it has issues.

Clinton used a ps7141-1a, which has a Ron of only 20-30 ohms, (with a small If)
would seem to cover a lot of cases that have had issues.
(He mentions Icom 706, but another example might be
Yaesu FT-101, reputedly PTT current ~110-180ma at 13.5v?)

http://kk7uq.hfradio.org/model_ii.htm
schematic at
http://kk7uq.hfradio.org/download/Schematic.pdf

I had ordered a couple of the ps7141-1a from mouser. $1.90 each.
Used one for my FSK keying on a icom 751.

Just added a diode and resistor in series to the serial port's
TXD, and 4 connections to the ps7141-1a and was done. Works great
(ok, I did have the polarity reversed on the output at first)

the data sheet is available here for example
http://www.cel.com/pdf/datasheets/ps7141a.pdf

The "Connection C" setup seems to allow 300mA Iload, 
although I only used the pin 6-5 connection (150mA Iload)

a nice comparison of different types of NEC solid state relays at
http://www.cel.com/pdf/datasheets/ssrsel.pdf
I'm not sure why kk7uq decided on ps7141-1a :)

Quite an amazing variety of "solid state relays" with various specs.
Made me wonder why so many homebrew (and other) circuits still use the old
warhorse 4N33 (or similar). 

Yeah I know there's  a lot of ways to skin the cat, but I'm guessing most
people don't have opto isolators laying around, so if they get one, they 
can probably get something better than a 4N33, nowadays?

(yes I'm ignoring all the real old rigs with higher voltages on PTT)

also was thinking about: 

I have an old Yaesu FL-2100b that I was thinking of keying
with my new icom 756 pro II. Wanted to isolate it to avoid 
the documented reed relay failures, ... although there's a negative keying
voltage on the FL-2100b, the ps7141-1a can handle that with 
reversed output connections. 

And doubling up the outputs ala
"Connection C" should give me 300mA, more than enough for the FL-2100b 
keying circuit. (-16v/125mA)

-Kevin
KE6RAD