[1000mp] PTT Interface

[Joe Subich, K4IK]

> 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