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Review: LCD Keypad Guide

by David Ross on 1 June 2003, 00:00

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Lesson in LCD Basics...

What is a GPO?

This three letter acronym (yes another one!) stands for General Purpose Output.  These are basically outputs which can be switched from 5v (max 20mA) to off, via the software controlling the LCD, and some models even have 'high power GPO's', which can handle 12v and 750mA.

What if I want to get more power out of my GPO's

Your in luck!  Cheese @ Bit-Tech has come up with a neat 'piggy-back' device that will allow each of your GPO's to support 32W @ 12V.  Click here to check it out.

Why did you go for a PCB based approach?

Simple.  To keep the wiring minimal and to create a tidy package, which could be applied to many LCD modules (even VFD's!) and module locations.

(left) This is where the LCD module is mounted in the case window and (right) the LCD is in a bay direct below the keypad

The entire design of the keypad is based on the LK204-25-WB-V manual, and may be applied to other models with slight alterations, such as the LK204-25PC  I.E. if you have a model that supplies 12v (and a current over 30/50mA) you will need to connect a current limiting resistor in line with the Led's

For example:

12v source, for a 5.0V, 25mA LED (Ifmax = 30mA): R = (12-5)/.025 = 280Ω, possible resistor: 330Ω, resulting in a 21mA current
12v source, for a 4.1V, 25mA LED (Ifmax = 30mA): R = (12-4.1)/.025 = 395Ω, possible resistor: 330Ω, resulting in a 23.9mA current
12v source, for a 1.8V, 25mA LED (Ifmax = 30mA): R = (12-1.8)/.025 = 408Ω, possible resistor: 470Ω, resulting in a 21.7mA current

The inline resistance is simply calculated as follows: (Vs-Vf)/Ir.  Vs is the supply voltage, and Vf is the voltage drop across the diode (you can get this from the LED's tech specs) and Ir is the current you choose to run it at, and in this case it is close to the MAX rating of If.

At the start of this project, it was designed to work with an LCD, but I will be finally applying the keypad to my VFD,
showing how versatile the 'custom PCB' is, due to Matrix Orbital units having the same keypad controller on the bulk of their units!


I recently purchased a Lian Li PC71-USB case and decided that I wanted to install an LCD module.  After some researching I came across the LK204-25-WB-V by Matrix Orbital and based upon it's online manual I set about creating a custom keypad that will allow me to control a) Winamp, b) the LCD it's self, C) other miscellaneous functions and D) LEDs to notify me about temperature warnings detected by MotherBoard Monitor.

The LCD Module - MatrixOrbital LK204-25-WB-V

For this task I decided to employ LCDC to control the LCD.  LCDC supports most of the LCD modules by MatrixOrbital, and has keypad and GPO support and many more features such as complete plug-in support.

When starting off, I wanted the wiring of the keypad to be minimal, and the components replaceable.  The final location of the LCD wasn't a main factor as well, and this left me with one logical I wanted to avoid this:

My initial *want* of a keypad started when I saw this little baby by CoolHand, but I wanted to take it one step further...

The Printed Circuit Board and the Keypad Connector

[Click the second image for a 4x size image.  When printing the image needs to be reflected on the y-axis

The above PCB was designed in Cadsoft's EAGLE PCB design software, and sent off to based in Malaysia for manufacturing.  The completed PCB:

This PCB cost £25 as I requested soldermasking, which cost an additional USD10.00!  I decided to use header connectors in the design, as these plugs are easy to handle and can be salvaged off old PC's.  Of course if you got an UV box and know how to etch your own PCB, you could make this easily, but the size of the tracks and track to track distance may cause some problems if you use my 'design' directly...

...Understanding the PCB

The simplest way to understand how this circuit works is like this:


Look at the 10 pins above next to the serial port on the right. Pin1 on the left corresponds to R1 and the right most pin is C5.  This is how they are divided:

You should be able to see that the 10pins can support a 25key keypad (5x5).  A 'contact' between one of the pins on the left and one on the right will result in the closing of a 'switch'.  Looking back at the PCB diagram, JPROW will connect to R1-R4 an JPCOL will connect to C2-C5. This leaves R5 and C1 unconnected for our purposes.

This is why I decided to have the PCB interface the LCD to the keypad, because this will require the least amount of soldering of the switch terminals.