Unusual opto fork setup

[Roger HartParticipant @rogerhart88496]

In the process of reworking a telescope Alt/Az drive system.  Uses dc motors with opto forks.  The Alt axis opto emitter is driven by a 4 resistor D-A setup.   In this case 510,1000,2000,3900 ohms to a microprocessor port. Presumably to ‘fine tune’ the sensitivity of the dual photo transistors that enter the software feedback loop.

I had not come across this approach before.  I suppose it avoids having pot twiddlers to set up and can handle variations in opto sensitivity etc.  Not found any examples of this kind of approach (but not looked all that hard).

I had considered using steppers but space is very limited and I don’t want to spend much more money on it and the dc motor drive works OK.

The Azimuth axis opto driver seems a fixed current drive (so far as I have worked out so far).

 

 

[John HaineParticipant @johnhaine32865]

LED brightness degrades over time so it may be to allow the drive current to be adjusted as they age? I recently worked on a 50 year old clock using LED/phototransistor pendulum sensing where the LEDs had basically given up.

[SillyOldDufferModerator @sillyoldduffer]

My guess is the circuit adjusts for ambient light so the mechanism isn’t confused by bright moonlight or changing cloud cover.  Automatic if done by the microcontroller and a sensor, or manual via a control panel, and both could be available.

Might be a LED corrector as John suggests though my experience of electronics is that designers don’t think that far ahead.  Unfortunately!  (And if they do, it’s to part us from our hard-earned with a dose of planned obsolescence!)

Looks like an old-school binary weighted divider to me.  Old-fashioned in that modern designers are more likely use a DAC chip rather than separate resistors and op-amp, cos chips are cheaper and more reliable than components, especially potentiometers!  Shouldn’t be a problem – separate components will do the job and are much easier to work on.

Dave

 

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