Message-Id: <9311221952.AA14178@tortuga.media.mit.edu>
To: beyer@erim.org (James Beyer)
Cc: fredm@media.mit.edu
Subject: Re: another try at explaining a 2.11 issue 
In-Reply-To: Your message of "Mon, 22 Nov 93 12:03:26 EST."
             <9311221703.AA11705@ipf690a.erim.org> 
Date: Mon, 22 Nov 93 14:52:51 -0500
From: "Fred G. Martin" <fredm@media.mit.edu>

> 	The 2.11 board had a LM386 (dual lo power op-amp) chip on it.
> One half of this chip is you to amplify the signal to the IR transmitting
> board.  I don't know if the other half is used or not.

No, the LM386 is a single, -high power- op-amp.

> 	The 2.21 board has no LM386 on it.  The manuals (appendix B) has
> not been updated to reflect this change, so I don't know what the new IR
> transmitting circuit is.   Was the amplifier simply removed?  

The TIP120 power darlington performs a similar power-driver function
as did the LM386.
 
> 	Some of the 2.11 board builders have been querying me.  I'm thinking
> now the best approach would be for them to simply assemble the circuit in
> the original fashion.  But for the 2.21 crowd, it would be nice if that
> circuit was available.  I presume that someone figured out the amplifier 
> was not needed, and left it out.  I'm still not sure what role the TIP120
> (the larger transistor) plays in all this, but its nearby, so it might have
> something to do with all of this.

----
>From the file
cherupakha.media.mit.edu:pub/6270/debugging/v211-vs-221-differences.txt:

4.  The rev 2.21 uses a simplified infrared transmission circuit on
the main board, replacing an amplitude-controllable LM386 power op-amp
circuit with an on/off TIP120 darlington transistor.
----

There's the difference.  The LM386 allowed modulating the -strength-
of the IR output.  The TIP120 does not.

The two circuits require different software drivers.  Other than this,
either is functional and you should install the parts for whichever
one your board was made for.

	-Fred