Heating of the hand-controller of the NexStar N5i/N8i ... - DD1US

Heating of the hand-controller of theNexStar N5i/N8i/N8GPS/N11GPSMatthias Bopp, Langenbrettach Dec. 18 th 2002It is entirely possible that Celestron will changecircuitry inside the hand control at any time. Ifyour hand control appears different in anyway fromthe pictures in these instructions, do not proceedunless you have the proper background inelectronics.All NexStar models suffer from problems using thehand-controller at low temperatures. Differentsolutions have been discussed in the NexStarYahoo Group, one of them is heating of the handcontrollerdisplay internally. The following picturesshow how to modify the hand-controller to heat itwith approx. 240mW. The same power wasformerly also used in the hand-controller of aNexStar N5 and has proved to be sufficient.Hand-controller front side still closedWe use 3 resistors (3.9kOhm, 1.2kOhm, 1.2kOhm)and a BC327 or similar Silicon-NPN transistor. Thetransistor is used in a common emitter circuit withthe 3.9kOhm resistor in front of its base and the two1.2kOhm resistors connected in parallel forming theload between the collector of the NPN transistorand the 12 Volt supply. The dissipated power inthose 2 resistors provides the heat for the display.The transistor is switched on and off by the samesignal used to switch the backlight of the display onand off. This signal can be tapped at the existing1kOhm resistor R5. Thus the heating can beswitched on and off using the software of the handcontroller (together with the light of the display).Hand controller rear side, the 6 screws like the onein the upper left corner have to be removed to openthe hand-controller1/4

The display is still in the hand-controller, the flexcableis pulled out of its connector and the clampfor the backlight optical fibre cable is pulled off theLED (marked by the 2 arrows). Next the 2 screwsfixing the black plastic holder for the display haveto be removed.Here the black plastic holder with the backlightoptical fibre cable is removed. The small blackrectangle marked with the arrow is a chip-on-glassdisplay controller.The 3.9kOhm SMD-resistor (marked with thearrow) is soldered to the lower pad of the 1kOhmresistor R5. The other side of the 3.9kOhm resistoris soldered to the base pin of the NPN transistor.The emitter of the NPN is soldered to ground,which is the lower right pin of transistor U10.Finally a wire (here yellow) is soldered to thecollector of the NPN transistor. This wire goes tothe resistors on the backside of the display. Anotherwire is soldered to the pads J4 which is the +12Vsupply of the hand-controller. Also this wire goes tothe resistors at the back of the display.This is the front side of the plastic holder showingthe backlight optical fibre cable guiding the lightfrom the LED on the PCB to the LCD-displayNow the modification to the electronic of the PCBcan be done and the 2 resistors can be glued on thebackside of the LCD-display (close to the displaycontroller).The wires of the 2 resistors are isolated (purpleinsulation) to avoid a short circuit with the displaycontroller.3/4

It is entirely possible that Celestron will changecircuitry inside the hand control at any time. Ifyour hand control appears different in any wayfrom the pictures in these instructions, do notproceed unless you have the proper background inelectronics. I cannot give any guarantee for themodifications nor held responsible for any damage.Further modifications including a simpler designwith permanent heating using only two 2 resistorscan be found on my webpage http://www.dd1us.deMany thanks to Mike Swanson, Dan Hupp andFrank Dilatush for reviewing these instructions andgiving me helpful hints for improvements.Questions and comments are always welcome.Please send them to dd1us@amsat.org.Kind regardsMatthiasThe resistors have been glued to the back of thedisplay using “superglue” (Cyanacrylat) and thenthe display and its holder have been reassembled.Also the connections of the flex-board and theoptical fibre cable are reassembled.Modified hand-controller before back part of thecasing is reattached4/4