Astronomy.CJDawson.com

Power box
Intro and planning

Contents
As this is quite a large project, I've decided to split this project into several pages. Each page dealing with a different aspect of the project.

Planning

Overview
This is perhaps my most ambitious project to date. Having been using my scope, it's amazing how I've still got the same problems that I had at day 1. Namely these problems are powering the scope, transporting the equipment and keeping everything simple to setup! I've changed my scopes equipment boxes several times, but always end up cramming stuff in. Toolboxes just don't do the job. I was constantly worried that my eyepiece might get damaged, and I even managed to scratch my Autostar (I call it a battle scar).
Also, there are problems with setting up the scope, as the effort required to plug in my dew heater mean's that most nights it's just sitting in the box, and not helping to stop the inevitable onset of Dew.

That's enough of the problems, now for the solution. I've just bought a new 85Ah Marine/Leisure battery; it's about the size of a 44Ah Car battery. And it weighs in at 23Kg's. Not light, but it's more than enough to supply power for my scope and all camping type needs for an entire weekend. I know it's kind overkill, but it's cheaper than a 24Ah Sealed Lead Acid battery. (About 1/2 the price, give or take). Besides, I'd rather have too much power than not enough.

I figured that this is OK for lugg-ability, as the finished unit will probably weight in at about the same my OTA or tripod. But I don't want it to be much more.

At the moment, I'm still working on the design of the box. Currently I'm in the process of getting the components together for what will be built into the unit. The idea is that I'll be able to pull one lead from the box, attach to the scope, and everything will be powered up in one foul swoop.

Another consideration that I've got is to try and help preserve the scope and increase its life. To that end, I'm going to make a power distribution box, that will attach to the scope, this will be used to supply power for the scope and dew heater, as well as take the strain from the weight of the wiring loom. (Currently the bulk of the weight is taken buy the scope's power connector, which is not a good thing)


Basic Design
This project will be sub divided into two parts, firstly there's the power box itself, then there will be the distribution box on the scope.

One thing that is not a concern for this project is saving power, there's 85Ah! To play with! 85Ah!!!!! Seriously though, There's still not excuse for wasting power needlessly, it's still worth considering the environment. And already I'm 1/2 thinking about a solar charger project which will power my scope forever, for free! I'm only half thinking about it as I want to be able to add this, and not design my box in a way that rules it out.

Power box contents
What's going into the power box?

This one of the most important questions for the project, if I get this right, it'll cut down the need for toolbox's alot. Currently I'm working on the devices that will built into the box, here's a list of the items that I want to build into the box.

  • Dew Heater Controller
  • Battery level meter
  • Lighting
  • Fuse Board

Each of these devices is a seperate circuit in it's own right. Lets spend a little time talking about each of these. I'll skip the battery as you already know about that. (85Ah! Still can't get over it)

Dew Heater Controller
This is the controller from the Dew Heater project. If you like, you could call this the Mk2 Heater. Again it's based on the same design, I'm just implementing it in a slightly different way. The controller will be run directly from the battery, then a single pair of wires running out to the scope. (See the section on the distribution box for what' happens next) Also, I'm going currently researching a circuit that I can used to give a visual in the dark feedback on what setting the heater is currently on. (It'll be a variation on the Battery level meter)

Battery Level Meter
This is a simple circuit that will display a bar graph of the current battery condition, I'll know in plenty of time for when the battery will need recharging.

Fuse Board
I think a really nice touch would be to have a proper fuse board, this is a really simple circuit, and will help to finish the box properly.


Lighting
This is another important thing to think about, Instead of using torches for most of the work I'm thinking about making a Red light lighting circuit. As we all know, it's a real pain in to work with a torch all night, Afterall I'd need to hold the thing all night and it just plain gets in the way. I've tried a strap to attach it to my head, but it's just irritating as the thing never points where I want. My idea here is to make a few low power lights that can be turned on and off as desired. There will be one in the Power unit, lighting the control panel, and box itself. A little desk light, that can be used for looking at maps/books and stuff like that (probably won't get used much). Also another light on the scope itself, to illuminate eyepieces, and other stuff on the scope.

Others
I did think about putting in a little FM Radio, that would be a fun item, as it wouldn't pull alot of juice, and during a long lonley observing session would give me something to listen to. Also an illuminated clock, would be nice. These items I've decided are a little to much for the moment, but there will be room on the box to do this later.

After alot of thought, and an attempt at constructing a purpose built box, I found that it would weight too much. There would also be practical problems of fitting the electronics into the box. So for the time being, I've had to drop the idea of a purpose build box. Instead, I've opted to leave the battery loose, and fit a cable and plug, which connected into the project box.

Scope power distribution box
This box will be used for a couple of jobs. First, it'll provide a single point of contact from the power box, One plug and I'll be up and running. Second, take the physical strain from the weight of the cable. (better that than the scope) Third split the power on the dew heater so that I can plug in several heaters at a time. And finally, provide a place to plug in the Red Lights.

It's important that this box can do all of the above, whist still being as small as possible, if it's too big, it'll get in the way of other devices that attach to the scope. This is perhaps the simplest part of the project, and provided that I design it properly, it can be build and uted before the rest of the project is completed.
I'll need to build a custom lead, which accept power for the scope. That'll be enough to start with.

Scope Power Distribution Box

Overview
This box is attached to the scope, it is used to as a connection point for the power box, and the electrical elements on the scope.

Design
Whilst in my local Maplin store, I decided to pick up an Ice Blue plastic Enclosure, outer dimensions are 100x50x21(mm). It's semi transparent, so you'd better be good a soldering and wiring - it's on display!

A quick look at the box will show that it's got groves in the side walls, which are just the right width to slot in a PCB, however, it means that the PCB needs to be less than 21mm wide! So that it can fit into the box. For my purposes this is ideal.

The plan is to place two PCB's into the box, one to provide power to heater elements, and the other, will provide power to the scope and also to the lighting circuit.

Finally, in middle of the box, I will place a standard 8 Pin Din socket. This will be where the cable that travels to the power box will plug in.

PCB creation
For this part of the project, you'll need two small PCB's. I used a single sided PCB, to create both boards at the same time. Then seperated the boards with a hacksaw. This part of the project wastes alots of PCB. As there isn't a standard size PCB available.
The two PCB's are 10cm long, and 20mm wide.

To use this image, print the board layout into transparency, the use the photo-etching techinque to produce the two boards. Once etched, you should then seperated the two boards.

The top board is used to distrubute power to the heater elements. To complete this, you'll need to solder 6 PCB mount 3.5mm Mono jack sockets onto the board.


The bottom board has one mono jack socket, this is for the lighting loom. the other pair of pads and tracks are for the main telescope power. You might want to omit these from your box.

The pads are will be used to connect to the DIN socket. The order for connection is entirely up to you, but it's important that you know which circuit is which. The last thing you want to do is damage your expensive scope, with incorrect wiring. (I've got a test box, that lights an LED when the connected to the scopes power supply.)


Box Modification
Next you'll need to modify the box, by cutting out the holes needed for the jack sockets, DIN socket and wire that connects to the scope.

I placed the PCB's into the box and pressed them hard into the two outer most slots. Then I marked where the holes needed to be for the jack sockets. And drilled them out using a bench drill. Take your time with this, and make sure that you're accurate, as it will ruin the box if you make a mistake.

Next I drilled the large hole for the DIN socket. As there's a plate in front of the socket, accuract isn't as vital here. It's important that the hole remains covered by the plate though. Also drill the big hole first, then the two smaller screw holes, that wat the socket will act as a template and mark the holes for you

Once you've completed drilling all the holes, you'll need to knock off the top corners of the PCB, so that it will fix firmly against the box lid. Then attach the sockets through the holes and tighen up the nuts on the sockets. This will hold the PCB's in position.

Add the wires that connect the PCB's to the socket, and solder everything together. Also don't forget to make the lead that attaches to the scope. You'll also need a hold in the box to allow that lead to escape. This is part of a power lead, so uses a DC 2.5mm jack plug at the free end. Make sure that you cut this cable long enough so that it will run from the place you intend putting the box, into the DC jack on the LX90.

Finally, use sticky backed velco to attach the box to the scope.

Dew Heater enhancement control enhancement

Overview
From experience using my dew heater, I found that it wasn't easy to tell what setting the heater is on in the dark. To make this extremely easy, I designed (with a little help) a circuit that will visually display the setting of the heater. This circuit will attach in between K8004 and the 1K pot from the original dew heater. It will display the setting in the same style as a VU sound level meter.


Circuit diagram
Here's the circuit diagram for the bar graph.

Here's how the circuit works... The POT VR1 is the 1k Pot taken from the original dew heater circuit. The three connections vRef, DCIn and Gnd, are the three connections that the Pot was originally connected to. This is the voltage that we'll be measuring.

SW2 us used to switch the chip from DOT mode, to BAR mode. In DOT mode only a single LED is lit, this means that the enire ciruit will used about 10mA of current. In Bar mode, with the heater at 100%, the circuit will pull about 120mA of current. When building the final circuit, this switch will be a pair of jump pins, like the ones used on computer mother boards, and hard disk drive.

As I'm want DOT mode to be available, I needed to add the resister R5. The reason for this resister is that there's a quirk on the chip. When operating in DOT mode, of any of the LED's are lit, pin 1 of the chip will pull about 500uA, this is enough for the diode to light about about 1/4 of the normal strength. Placing this resister across D1 will mean that some of the 500uA is drawn away from the LED, stopping it from lighting. When the LED is supposed to come on, it will be slightly dimmer than the others, but it's not noticable, even in bar mode.

I then made this into a PCB, and added the circuits. Here's a link to the board layout image PCB Layout. When you print this it make sure that the long edge of the boarder is exactly 100mm long and the short edge is 75mm. This is the same size and the size 1 boards from Maplins. For my boards I used the SRBP board, as it's cheap.

Although I've finished making up the PCB and tested it. I've not quite finished. This circuit won't be complete until I've worked out how, and fitted it into the box.

Note: If building this part of the project again. I would spend some time re-arranging the positions of the components and tracks so that I can drill a large hole. This hole will allow me to insert a screwdriver, through to the battery monitor circuit, so that I can trim the potentiomiter easily.

Battery Level Monitor

Overview
The Autostar unit provides a battery level meter that's accessable via the mode button (press and hold). This is all very well, but I've found that it's not sensetive to detect the levels on a 12v battery, until it's almost completely discharged. To that end I wanted to add a circuit to my control box, that allows me to instantly see what the battery level is.


Circuit diagram
Here's the circuit diagram for the battery monitor.

I can't take credit for the basic design of the circuit. As I got the design from another website. I did make a small modification to the original circuit to correct the dot more quirk, that everyone seems to miss.

Final adjustsments are simple and the only thing needed is a digital voltmeter for the necessary accuracy.
Connect an input voltage of 12.65 volt between the positive and negative poles and adjust the 10K trimmer potentiometer until Led 10 lights up. Lower the voltage and in sequence all other Led's will light up. Check that Led 1 lights up at approximately 11.89 volts.

At 12.65 volt and higher the battery is fully charged, and at 11.89 is considered 'empty'.
The green Led's indicate that the battery capacity is more than 50%, the yellow Led's indicate a capacity of 30% - 50% and the red Led's less that 30%. This circuit, with the components shown, uses less than 10mA.

Again I laid out the components to fit nicely onto a Size 1 PCB (100mm*75mm), This was so that I could stack it with the dew heaters bar graph. Here's the layout.
Note: Since creating this project, I've found that there's a couple of issues.
First, deep cycle batteries can be run down to about 10.5V, Since I use an 85Ah battery, I re-calibrated my circuit to show fully discharged at this voltage. As this is used for an indication more than a mark to recharge the battery at, I'm not worried if this voltage is close to the absolute minimum that the battery should be allowed to read.
If I were to remake this circuit, I'd spend a little more time altering the track layouts on this and the Dew heater monitor circuit, so that I could drill a hole in the Dew heater circuit to allow a screwdriver to gain direct access to the trimming preset. This would help to make the calibration a little easier, especially after I upgraded the box with the extra sockets. The sockets also have an LED to indicate that they are connected.

Fitting the electronics

This is the part of the project where we start pulling the other parts of the electronics together. I wanted a little fuse board inside the project box, to help provide a little protection to the scope, heater and aux circuits. For this I etched a board and fitted 20mm fuse holders. I've made provision to hold 4 fuses, but I'm not sure if/when the fourth will be used. Also this provides a neat way of providing power to all the circuits, and it will cut down on the number of wires. You wouldn't believe it from the spaghetti in the pictures.

I didn't take any contruction photos for this project, as I wanted to crack on and get it finished. The method for construction was to cut all the holes in the box, then fit the circuit boards. The only thing holding the two graph circuits in place is the pressure on the LED's you need to be careful not to break the tracks holding the LED's in place. Once fitted, I then added the wires to connect all the switchs and boards together. I've used wire that is overrated for the amount of current that I'm going to use for each part of the project. On the right hand side of the box, you'll see the 8 pin DIN that is used to for the cable that connects to the scope's distribution box. It's important that this is wired corrected and tested before plugging into the scope.

The fuses that I used in this project are all the quick blow type, with ratings of 2amp for the scopes power line, 3.1amp for the aux circuit, and 6.3amp for the dew heater circuit. The Vellman board provides the upper limit for the dew heater circuit, as it has been rated as 6.5amp Max, 6.3 should mean that the board is sufficiently protected, whilst allowing maximum performance from the heaters.

It would have been nice for the switchs to be illuminated when they are turned on, however, the type of which that I used needs a much high voltage in order for that feature to work. If I had to re-make this project, I'd use different switches that will illuminate on the 12v supply. Maybe I'll do this modification one day. It'll mean a new box lid though.

New heating Element

Having used my old dew heater for a couple of years, I've found that it works quite well. However, I've also started thinking about the effect that all the insulation is having. With a layer of heat strink, and some inulation tape there's quite a bit of stuff for the heat to get through before getting to the scope. Even then, one is gets to the scope, it's still got to penetrate through the outside of the OTA to get to the corrector plate. All in all, that's along way.


I've seen several other websites showing ideas for heating elements, two of them seem to strike more of a cord that the others. The first site suggested using copper tube, then threading several high wattage resisters through the tube. The second suggests using resistance wire, wrapped around the outside of the OTA and held tight by a spring.

I was thinking about this and like the idea of using copper tube to conduct the heat evently around the corrector plate. Also being placed inside the lip of the scope, it's practically invisible and delivers the heat right on target. But the website for talking about a 10" LX200. So I needed to adapt this for the LX90. The answer came from the second site. Combine the copper tube with the resistance wire and you'll have an element that's easier to contruct and works efficiently.

Now I wanted to think small, so I got some 4.7mm copper tube. Which I bent to shape, this was accomplish by using the lip on my focusing aid as a guide. This left a ring that was oversized. Then with a little persuasion, I forced this into the lip of the OTA to pull the ring in a little tighter.

Next job was to place the resistance wire into the copper tube. But I didn't want direct contact, as this could short out. So I cut the resistance wire to length leaving some extra so that the wire will hook over the edge of the OTa, allow connection to the supply wire.

Next I threaded the wire into some heat shrink. Then connected the ends of the wire to a 12v battery. This will give you some idea of the power that this heating element will have. The wire got hot enough to cause the heat shrink to er, well, shrink! And that was with not extra help.

I then threaded this into the copper tube and attached some speaker wire to the ends of the resistance wire. Also I added a little more heat shrink to cover the bare wires. This time I used a hair drier to do the shrinking. As the other end of the speaker wire is a 3.5mm jack plug which connected into the power box.

Finally I sprayed the copper pipe matt black, and took the picture above. You can see the heating element around the outside of the corrector place housing. It's almost invisible in black. I think this has a really nice finish.

Finishing off

To finish off this project, I figured that it would be nice to show the whole thing together. The powerbox, the distribution box and that 85Ah battery :-)

The idea is that I can put the battery anywhere, Ideally I'd have liked a box to place that and the control unit into, but the wooden box that I was thinking off would have been way to heavy. The battery weighs 23Kg's! But it'll be worth it.

And to bring this to a close, I wanted to show leave you with an image that will instantly show both the power status, and the setting on the heater.