DIY controller for ZWO AEF. Question about the resistor value.

[ofranzen]

I want to build a controller for ZWO EAF based on this https://www.williamliphotos.com/diy-eaf-handcontroller and I was wondering if it is as simple as the resistor value determines how much slower the "slow" speed is? Or, does anyone know if it has to be fixed ad 4,7k for the electronics in the EAF to switch in to slow mode?

Thanks in advance

/Ola

[symmetal]

Hmmmm. Hard to say. Some background...

The controller plugs in the temp probe socket on the EAF. The temp probe uses a 3 pin 3.5mm jack while the hand controller uses a 4 pin 3.5mm jack so there must be a 4 pin socket as the EAF temp probe input and therefore ring-2 and sleeve (R2 and S in the diagram) are shorted together in the EAF when the temp probe is plugged in.

The temp probe is very likely the DS18B20 which usually works with +5V, Gnd and data pins. Looking at the diagram for the diy hand controller having +5V on one of the wires would mean it gets shorted to ground at some stage which isn't good so there can't be raw 5V on any of the jack pins.

It's therefore likely that the temp probe is being used in parasite mode, where only a Gnd and data pin is used. The temp probe gets power from the data pin via a 4.7k pull up resistor to 5V inside the EAF.

The EAF likely has a microcontroller similar to the arduino inside. As the EAF has a 4 pin socket the unused pins when used with the temp probe may go to spare input pins on the microcontroller. I don't think the 4.7k in the diy is that critical , but it's likely that changing it's value wouldn't alter the focuser speed, unless possibly one of the analog input pins was being used in the microcontroller and the 4.7k in the diy controller is part of a potential divider to determine the focuser speed.

Changing the value in the diy controller won't cause any harm, and it may or may not alter the focuser speed depending on how it's connected in the EAF.

Long story short, I don't know, but the above may give you some insight. 😁

The person who published the diy controller likely just took one apart, and copied what was inside.

Alan

[Tomatobro]

I would say that the circuit diagram replicates what is found in the ZWO hand controller as suggested by Alan. The EAF is an expensive bit of kit to go experimenting with so would stick with the suggested circuit and only experiment with the resistor value if for some reason its performance is not acceptable.

 

[bosun21]

Personally I would just buy the designated hand controller. They are nice and light and provide good fine focus adjustments. They are not expensive in relation to the EAF itself. Good luck if you decide to go the DIY route.

[ofranzen]

Thanks for good info. 

My guess is that S is ground and T, R1 and R2 are data pins. R1 and R2 probably have pull up resistors internally and T gets two different values depending on whether the pin is grounded or if the pull-up resistor in the controller is engaged. That, I think, would explain how it also functions with the temperature probe and the controller circuit without anything shorting. 

I'll just build it as the diagram shows as I agree that it is probably a boolean value for fast/slow and not an analogue input.

As for why not just buy a controller. Apart from the controller costing over a quarter of what the entire EAF costs and has components for only a few dollars (all which I already have lying around), I'm simply a tinkerer. I get so few clear nights so to be honest, I spend more time upgrading and tinkering with the rig than doing actual astrophotography.

[Carbon Brush]

I have no idea what goes on inside the the EAF.
However, if we believe the circuit works and does no harm....
You could fit a 5K variable resistor in place of the 4K7 fixed and see what happens.
I suspect at some point the focus will switch between fast and slow, rather than produce a variable speed. But am happy to be proved wrong!
HTH, David.

[malc-c]

18 hours ago, Carbon Brush said:

I have no idea what goes on inside the the EAF.
However, if we believe the circuit works and does no harm....
You could fit a 5K variable resistor in place of the 4K7 fixed and see what happens.
I suspect at some point the focus will switch between fast and slow, rather than produce a variable speed. But am happy to be proved wrong!
HTH, David.

I guess it depends on how it uses the reduced voltage to govern the speed.  If it's simply a direct connection to a motor.  If it gets fed to an analogue to digital pin of a micro controller then the results might be a very variable speed.  The problem is unless the EAF is dismantled and the PCB examined we are all second guessing. 

[ofranzen]

R1 and R2 have resistors on the PCB and I got the resistance to 10 k on them, but that can be off as I need to change batteries in my multimeter  That's how far I have inspected the PCB and I'm quite satisfied with just building the controller as article shows. That's built and verified by at least a couple of people and it'll be used so rarely that I don't need to be more fancy than that.