Modified Handpad For Use With Scope I

One of the major advantage of a system such as Scope I, developed by Mel Bartels is the ability to add additional functionality by modifying existing program modules.  There is a disadvantage  however, namely the need to have an attached computer and the necessity to go back to the computer to enter commands.

There are several modules that reduce the need to go back to the computer. One of them is GrandTour.  With this module, a file containing a list of objects can be loaded and the observer can step from one to the other using the handpad.  If the list is short, the observer knows which object is next in the list.  However, the list can be very large, such as a list of all Messier objects plus nearby bright stars to aid in locating them.  This list can be almost 200 objects long, necessitating trips back to the computer to remind the observer what object he's now at.

To get around this problem, I decided to add a liquid crystal display, (LCD) to the handpad that displays the object name.  As the LCD I choose has two lines the coordinates of the object are also displayed.

Choosing the LCD

Several parameters need to be considered when choosing the LCD:

• Number of columns
• Number of lines
• Color
• Physical size
• Voltage and power
• Computer interface

The object name field in Scope I is 24 characters long so a 24 column LCD would be ideal.  However, a smaller unit would only mean truncating some object names. As some names are already truncated in the program, this is not a major issue.

Displays with 2 and 4 lines are readily available so additional useful information can be written to the handpad display.

The ideal color is reverse red.  This gives red alphanumeric characters on a black background.  A white on black display could be used, but a red overlay would be required.

My current handpad is 2" X 4" by 1" thick.  It fits comfortably in my hand and all functions can be activated by one hand.  The smallest LCD displays I found were about 4" wide and building one into a monolithic handpad would result in a unit that requires two handed operation.  Enclosures designed to hold a display plus keyboard functions are available, but all are quite large, being able to contain a full function computer keypad.  I chose to use a non-monolithic design, attaching a 2.5" X 5" by  1.5" thick enclosure to the existing handpad.

There is already +5V to the handpad, so a display powered by a +5V source is preferable.  Some displays use an electo-luminescent backlight rather than a light emitting diode.  These displays draw less power, but an inverter is required, taking up even more space.  As the backlight needs to be either reduced in intensity or only on momentarily, using an LED should not be an issue.

There is a standard interface for displays, consisting of a 14-pin or 2x7-pin IDC header.  It is best driven by a dedicated processor, not by the same parallel port used by Scope I.  Serial and USB interfaces are also available, but there's no standard for the commands sent over the interface.  However, the interface only requires that two commands be sent, plus the alphanumeric characters.  These commands are:

• Clear screen
• Set cursor to a row and column

I found only two suppliers of serial interfaces and chose one from Matrix Orbital as it provided the most options, including commands to adjust backlight brightness, baud rate, etc.

Matrix Orbital LK202-25-R

This is a 2 X 20 reverse red display coupled with a serial interface module.  The display and interface are joined together via a 2X8 IDC header, but the modules could be mounted separately via a cable.  The backlight is an LED and can be adjusted in both brightness and contrast via serial commands.  The unit is rather long, at 4.56" but fits snuggly inside a Radio Shack enclosure.  I did lower the height of the 4-pin power and data connector and cut down the enclosure to 1.5" resulting in a reasonably compact handpad.  The two enclosures are bolted together through a block of black nylon to orient the display at 45 degrees for easy viewing.

Programming the Display

The main objective in developing the handpad display was to show the object currently in view.  However, with two lines available, I chose to also show the Ra and Dec of the object.  Both strings are written to the screen in Scope I in the same subroutine so it was simple to modify that subroutine to also write to the handpad display.

The LK202-25 display supports a large number of commands, including the ability to save many settings in memory so the display powers up in that mode.  The default baud rate is 19200 and as my computer supports that, I left it alone.  I implemented a menu routine to adjust the backlight intensity and contrast.  As the display is used at night, the backlight intensity is set to a very low value.  This also greatly reduces the current requirements for the display.

Other Handpad Tasks

As the goal for the handpad display was to reduce the need to return to the computer, I looked at other tasks that could be accomplished at the handpad.  The first was the need to acknowledge a slew greater than a configuration preset value. The subroutine that displays that message on the computer screen was modified to display it on the second line of the handpad and wait for an acknowledgement from the Left Key.  Any other key would be considered a "No."

Alternative Displays

Matrix Orbital also makes a 4 line display, the LK204-25-R.  It is actually shorter than the LK202-25-R, but of course is wider and just slightly thicker. However, RS232 input is restricted to the DB-9 connector, so the enclosure would need to be at least 1/2" thicker.  It does not allow control of backlight brightness, only turning it on or off.

Commands for clearing the screen and moving the cursor are identical, so programming to support either display would not be a problem.  Making use of all four lines might involve object name and Ra/Dec on the first two lines, slew status on the third line and dedicating the fourth line to slew distance confirmation.

Matrix Orbital also makes a series of lower cost MOS displays as well as displays identical to the LK20x series having a wider temperature range.  The latter might be important if the installation is to be permanently mounted in a cold temperature region as LCD displays can be damaged by very cold conditions.

Other Considerations

Support for a handpad display requires a Comm port.  Most computers come standard with at most two ports, so if the installation already supports LX200 and encoders, a third port would have to be installed.  Matrix Orbital also makes displays with USB interfaces, but I have not looked into them.