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Path: utzoo!linus!philabs!cmcl2!seismo!umcp-cs!cvl!umd5!zben
From: zben@umd5.UUCP
Newsgroups: net.analog
Subject: Any 555 mavens or PUT weenies out there?
Message-ID: <469@umd5.UUCP>
Date: Tue, 9-Apr-85 14:15:17 EST
Article-I.D.: umd5.469
Posted: Tue Apr  9 14:15:17 1985
Date-Received: Fri, 12-Apr-85 04:46:31 EST
Reply-To: zben@umd5.UUCP (Ben Cranston)
Organization: U of Md, CSC, College Park, Md
Lines: 48

The situation is that I want to build a bitbanger serial interface for my
kid brother's Apple ][+ that plugs into the game port, which has several
TTL inputs and outputs.  The problem is that the only power supply available
on the game paddle connector is +5 - and I'd rather not kluge in the Apple's
two 12 volt supplies on the unused pins or run more wires.  I need a nominal
amount of + and - 12 for the TTL-RS232 converters (MC1488 or MC1489) and for
some strapping for DTR type stuff.  Rather than build a separate plug into 
the wall supply or run on (ick) batteries, I thought I could design a small
DC-DC converter to run off the available +5 and produce these voltages.

I had thought about something like this circuit:

       R1
   +-/\/\/\-+---+---o +5
   |        |   |
   |    +---+---+---+            5-1        o +5
   +----+7  8   4   |        TRANSFORMER    |
   |    |          3+---------~~~~~~~~~-----+
   /    |           |         =========               +----+--/\/\/\--+--o +12
   \    |    555    |      +--~~~~+~~~~--+            |    |          |
R2 /    |           |      |      |      |   +-----+  | + ---       \---\
   \    |           |      |     ===     |   |     |  |   ---        / \ ZENER
   |    |           |      |      =      +---+ ~ + +--+    |         ---
   +----+2,6    1   |      |                 |     |       +----+-----+
   |    +-------+---+      +-----------------+ ~ - +--+    |   ===    |
  ---           |                            |     |  | + ---   =   \---\
  --- C1       ===                           +-----+  |   ---        / \ ZENER
   |            =                            BRIDGE   |    |         ---
  ===                                      RECTIFIER  +----+--/\/\/\--+--o -12
   =


with the 555 set up to run somewhere in the audio range, hopefully in the
efficient range of the transformer, which is an audio transformer.  My
questions are:

Can a 555 drive a transformer like this?  Is it better to run it to +5 like
in this diagram, or to ground?  Would it be better to use a tapped primary
and/or a drive to "halfway" provided by a voltage divider?  Would it be
better to set the 555 at 60 cycles and use a power transformer?  Would it be
better to use a Programmable Unijunction Transistor (PUT) in this application?
Would it be better for me to wind a torroidal transformer, and if so, about
how many turns do I need on the primary (I can compute secondary given this)?

Ad(thanks)vance for your thoughts on this matter.

-- 
Ben Cranston  ...{seismo!umcp-cs,ihnp4!rlgvax}!cvl!umd5!zben  zben@umd2.ARPA