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Celestron Nextstar GT Mount encoder sensor for motor SLT-F00-1A

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#1 clearabskies

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Posted 29 November 2021 - 12:26 AM

Hello, I have bought Celestron Nextstar GT Mount from second hand shop few days back, after aligning it using two star alignment method i found that its ALT motor is rotating randomly, so I opened the mount and found that encoder sensor for this motor is not giving output as required. I have checked motor control board outputs by plugging AZM motor there and everything is working fine. Can anybody let me know how this encoder sensor works? which led and transistor is used ? what is the exact output is expected at two encoder wires other than 5V wires? can anybody let me knoe the replacement circuit for the same? Hereby attached all the images for reference. Thanks in Advance

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#2 photonicsguy

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Posted 12 May 2022 - 11:18 AM

Hello,

Your board is slightly different than the one (I worked on the NXW430, yours is the NXW435)
There are only four chips on your board, perhaps the op-amp chip is on the bottom of your board, or the internal op-amp/comparator of the PIC16F87 is being used directly (pins RA0 & RA3)
Top left: L293 motor controller (instead of the L298 on the one I worked on)
Top right: 74HC24? buffer/line driver (for communication with another board/controller)

And the bottom two PIC16F87 Microcontrollers (the brains of the board)
 

 

Pin 1 - Motor: goes to L298 Motor controller on board (You've identified as 12V, will go to 0V when motor is running in the other direction))

Pin 2 - Motor: goes to L298 Motor controller on board (You've identified as GND, will go to 12V when motor is running in the other direction)

Pin 3 - 5V

Pin 4 - Ground

Pin 5 - Goes to LM324 Op-amp Pin 5 (+input)

Pin 6 - Goes to LM324 Op-amp Pin 5 (+input)

 

The clear package is an IR LED, the 909Ohm resistor limits the current. (Resistor is on the encoder (tiny) board)

The Black package with the blue paint is a quadrature photodiode/phototransistor/receiver. IIRC, center pin goes to ground, 

Pins 5 & 6 have a very small signal, I was just seeing 0v & 0.3v, but they each had a 1K pull-up resistor. (The 1K resistor was on the controller board, not the encoder (tiny) board on the motor)

 

The quadrature photodiode/receiver is what's probably bad, but if you're lucky, it's just a solder joint.
The same component is used in mice for the scroll wheel and the old fashioned roller balls as well. I haven't found a replacement, nor have I been successful in locating the components themselves. I almost wonder if the components are even still made in favour of optical/hall sensors.
I tried one from an old mouse, but it didn't work correctly, as the spacing between the two photodetectors also matters to ensure proper quadrature output. 

 

IIRC, the center pin of the receiver is tied to ground and the two outer pins are the outputs which pull down the line to 0v when it detects a signal.

 

It's hard to pick up the IR LED with a camera, but an IR remote will trigger the photodetector.

 

TLDR: If it's not a bad solder joint, buy a new motor assembly. Good luck!




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