[1000mp] PTT Interface

[Pete Smith]

At 07:47 AM 12/25/02 -0500, you wrote:
>The circuit used in the NOMIC is a 4N33
>optoisolator driven by RTS or DTR line from PC serial port through diode and
>680 ohm resistor.  The output side of the 4N33 has a floating base lead, with
>the emitter and collector leads used to key the radio PTT.
>
>Any suggestion for a simple mod to the circuit to allow compatibility with the
>rear PTT/packet jacks?  Thanks in advance.
>

I would measure the voltage when the control line is active.  I have had an 
oisolator keying interface that would key the radio but not trip the VOX on 
my TS-930, and that was eventually traced to the control line not getting 
close enough to zero.

Here's what AE0Q, the keying interface designer, had to say:

"our problem is that the Opto's effective Beta multiplied by the available
input current is less than the radio's pull down current.  So, you need
more pull down current.   Simply build two inverters and cascade them.
Such a circuit is called a buffer, or more generally, an non-inverting
amplifier.  These two invertors would have to be powered by the
transciever's power supply to maintain the isolation from the computer you
apparently have put the opto-isolator in the circuit to obtain."


73, Pete N4ZR
Happy Holidays