NOTES ABOUT THIS SCHEMATIC
;* Author 		: Brady Mayes
;* File Name 		: e5_3_schematic.sch
;* Date			: May 26, 2005
;* Last Update		: June 20, 2005
;* Version		: 3.0.0
;* Support Telephone	: withheld
;* Support Fax		: withheld
;* Support E-mail	: brm016000@utdallas.edu
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The attached schematic is intended for use with the Digital Code Lock.asm program (versions 1.0.0, 2.0.0, 3.0.0).  Here are some general recomendations about the parts featured here.

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BYPASS CAPACITOR
================

The capacitor C3, should be placed as close to the microcontroller in the circuit board as possible.  This will aid in noise reduction 

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ISOLATOR
========

Optical isolator IC4 is a generic isolator.  Virtually any type may be used so long as the output can drive the electro-locking relay (not shown).  If the isolator cannot drive the relay directly, it must drive a BJT or MOSFET transistor to conduct the large current needed.

This schematic assumes the isolator's LED has an internal 100-Ohm current-limiting resistor.  If your isolator has no current-limiter, resistor R4 must be increased to at least 220 Ohms.

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KEYPAD
======

The keypad is not shown.  This schematic assumes a 4x4 matrix keypad.  If you are working with a 3x4 matrix keypad, use a 74C921 chip in place of the 74C922 chip shown (IC2).

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PINHEADERS
==========

JP2 is a set of eight pinheaders to the keypad.  Take care to read the 74C9CC chip data sheets to make sure you have the proper row and column connections to this keypad.

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CLOCK FREQUENCY
===============

The operating frequency of the AVR microcontroller is not critical, however, speeds in excess of 6.6MHz may be excessive due to the propagation delay times of the 74C922 encoder.

Check http://www.geocities.com/digitan000  for more details.

12:20 AM 6/20/2005
