Heating a Nexstar controller's LCD screen

When it gets cold the LCD screen on my CGE mount's controller is very sluggish. Some heat would be a great way to keep the screen moving when I'm out in the cold and this is the focus of this project.

The LCD doesn't need a lot of heat, about 1/4W would do according to one such project I've seen for the Nexstar 5i/8i controller (different in those days). That gentleman used a small transistor and made his heat selectable with the LCD backlight. I could do that but it would take much more time than I really wanted to spend on this project. He used two 1200 ohm resistors in parallel to give 240mW while I've selected three 200 ohm parts for 210mW.

Parts and Tools

Fine philips screwdriver
Fine soldering iron
Solder
3 x 200 ohm .25W resistors
Some fine wire

Dissasembly.

1) Remove all 6 screws and put them aside safe.
2) Reach in and carefully unplug the large white connector.



Now you're ready. Looking at the top of the board find the black chip labeled 78D05 just below and to the left of the connector you unplugged. Solder a wire to the left hand 'leg' (12V power), a dab of solder on both the leg and the wire helps. Now put some solder on the exposed part of the tab of the 78D05 regulator and on another wire, then solder the wire to the tab.

Now build your resistor strip. Take three 200 ohm/.25W resistors and solder them together end-to-end. Keep them close so that they fit along the bottom of the LCD with room on each side to spare. Leave the end leads of the resistors long. The resistors don't have to be in contact but if you can arrange it do so. Place them snuggly in the 'corner' between the backlight and the LCD and put a dab of hot-glue on each end of the edge of the backlight to hold them in place. Let the glue harden.

Lastly connect one of your wires from the 78D05 to one end of the resistor strip and the other wire from the 78D0500 to the other. If you wish to be able to turn the heat on/off put a small toggle switch in the bottom of the hand controller, there's just enough room and run one of the wires through the switch. Tuck the wires out of the way, reconnect the 6-pin connector (fiddly but not too hard) and then screw it all back together.