[1000mp] PTT Interface

Thu, 26 Dec 2002 09:30:09 -0000

Joe:

In your suggested Darlington connection, there is the caveat of the output
voltage not being lower than 1 diode drop plus the Vsat of the
phototransistor. That is often, but not always, OK.

Garry

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Joe Subich, K4IK
> Sent: Wednesday, December 25, 2002 19:32
> To: [email protected]
> Subject: RE: [1000mp] PTT Interface
>
>
>
>
> > 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."
>
> An alternative is to effectively convert the 4N33 to a darlington
> output by wrapping another transistor around it.  The darlington
> configuration yields current gain equal to the product of the
> the 4N33 beta and the beta of the external transistor.  I'd start
> with a 2N2222 or MPS3904 (2N3904) as the external transistor.
>
> The configuration (if I remember correctly) it to tie both of the
> collector leads together (run them to the PTT line), tie the
> "emitter" of the 4N33 to the base of the external transistor, and
> tie the PTT common to the emitter of the external transistor.
>
> Using an external transistor in a darlington configuration avoids
> the need for an external power supply and maintains the galvanic
> isolation between the computer and radio since that's probably
> why you used opto-isolators and transformers in the first place.
>
> 73,
>
>    ... Joe, K4IK
>
>
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