Homebrew Portable Power Pack

Posted on by December 13, 2013
Completed Power Box

Completed Power Box

I recently constructed a portable power pack that would serve a dual role: operating our telescopes during observing sessions and running my ham equipment on camping trips.  By coincidence, ham radio transceivers and telescopes have similar power requirements (12V @ 1-10 A).  Most amateur astronomers use either car or motorcycle batteries, auto JumpStart devices or expensive power packs sold by telescope manufacturers.  CelestronPowerPack

The power packs sold by telescopes manufacturers are basically automotive jump starters having one or more female cigarette lighter plugs that are used to power the telescope and accessories.  Not only are these power packs expensive they sometimes  do not have enough capacity to run a telescope and dew heaters for a whole night of observing in the humid midwest.

Motor cycle and automotive batteries have sufficient capacity, but pose a significant safety risk when stored and charged indoors.  A safer alternative is the sealed AGM lead acid battery often used to power security systems and uninterrupted power supplies (UPS).   After a bit of research I determined that a 28 amp-hr AGM (absorbed glass mat) battery would provide enough energy to power our telescope for a weekend of observing and that the same battery would easily power my Elecraft KX3 for several days of QRP operation.

RigRunner4005

I made the decision to utilize Anderson Power Poles for connections because of the reliability and flexibility they provide. I purchased a RigRunner Power Pole panel to provide a connection point for the telescope and its accessories.

For a case, I considered the Pelican cases, but was turned off by their high price and weight.  I didn’t need a case that was 100% waterproof or one that could survive the airport baggage handler gorillas.  I considered using an RV battery box, but they are awkward to handle.  While walking through the Walmart fishing section the other day I stumbled across the perfect box or my application.  It is made by Flambeau Outdoors.  It is  ( 15″W x 9″H  x 8″D  ),  water tight, has a strong handle, a hinged top and a large lockable latch.  The box was large enough to hold the 28 Ah battery, charger and the charger power cord.  There was some room left over to hold other accessories: female cigarette lighter plug to power pole adapter, a power pole Ah meter, spare fuses and a power pole extension cable.

BatteryTender

I used a Battery Tender Plus 1.25 A charger to charge the battery.   These chargers are the ultimate trickle chargers for lead acid and AGM batteries.  They have a number of features:

  • Temperature compensated to ensure optimum charge voltage according to ambient temperature.
  • Automatically switches from full charge to float charging mode.
  • Battery Tender® at 1.25 amps will charge as fast or faster than any 3 amp charger available.
  • Reverse Polarity Protection to ensure user safety. Red & Green Lights Alternately Flash in this condition.
  • Complete 4-step charging program (Initialization, Bulk Charge, Absorption Mode, Float Mode).
Completed Power Box

Completed Power Box

Here are some photos of the finished power pack.  The first photo is of the exterior of the power pack.  Some of the features are the integral voltmeter which shows the current state of the battery charge.  The case has a very strong handle which is important because the finished power pack weighs in at nearly 25 lbs.  I incorporated a three position switch which selects the Operate, Off and Charge positions.  During Charging I the switch disconnects the Power Pole panel from the battery to eliminate any chance that  a failure of the charger could result in a over voltage condition on the Power Pole panel.

IMG_0435The next photo shows the interior of the Power Pack.  The battery is a 28 Ah AGM battery.  The         Battery Tender charge is attached to a Aluminum “L” bracket with a heavy duty wire tie.  I used the “L” bracket to support the Battery Tender for two reasons: so that it was flush with the top of the case so that it is easier to see the charger LEDs and to provide a space below the charger where I could wire tie the extra Battery Tender wiring.  The AGM battery is held in place by 4 pieces of Aluminum “L” channel (3 around the base of the battery and one which captures the batter at the top.  The brackets hold the battery snuggly against the back of the case.  All of the fasteners are stainless.  I also placed 4 rubber feet on the bottom of the case so that the fasteners inserted into the bottom of the box would not scratch any surface the box is placed on during storage.

IMG_0437

 

This photo shows the small compartment in the top of the power pack case which can be used to store cables, spare fuses and the Volt/Amp/Watt-Hr meter I use to check each device I connect to the power pack.

 

 

 

Power Pack with Amp-hr Meter attached

Power Pack with Amp-hr Meter attached

 

The Amp-hr meter can also be used to monitor the total Amp-hrs consumed from the battery by placing it inline between the input to the RigRunner and the Power Pole connection coming from the battery (shown at right).

After building the first power pack I have since built two additional power packs–one for a friend and another to run my wife’s Celestron telescope.  I think my Power Pack will also see a lot of use running my new KX3 on summer camping trips.

Best 73,

Fred

An Arduino Project for the Link Telescope

Posted on by April 19, 2013
IMG_0181

Top view of the Hour Angle Calculator in Operation — note the rotary encoder has an internal RED led.

I have been continuing to work with the Arduino single board computers.  One of the tasks required at the Link Observatory is to align the Argo Navis (digital encoders) using two well spaced stars.  My wife and I have found that the easiest way to pinpoint the alignment stars is to use the telescopes German equatorial mounts setting circles. One of the challenges we had using the setting circles was having to continually recalculate the Right Ascension Hour Angle as we are moving the telescope into position–it takes a bit a time to accurately position a 36″ telescope that weighs over a ton.  Therefore we would have to keep recalculating the hour angle which is calculated by subtracting the stars Right Ascension from the current local Sidereal time ( which is obviously changing).

IMG_0178I decided to use an Arduino to continuously calculate the RA Hour Angle using equations to calculate the sidereal time and the Hour Angle given the selected alignment stars Right Ascension.   This project involved building a circuit which included a real time clock, a rotary encoder ( to select the star from a lookup table) and an LCD display.  I assembled the circuit in a plastic 3″ x 5′ card box I found at Office Depot.

One of the more challenging aspects of the design for me was getting the rotary encoder to accurately count.  I used a 24 pulse per rotation encoder which I purchased from Sparkfun (http://sparkfun.com).  In the initial circuit the software would count extra pulses and sometimes miss pulses.  After I tried several different software schemes to resolve the problem I finally used an  oscilloscope to examine the waveforms coming out of the rotary encoder.  Once I attached the scope I immediately saw the problem.  When  the encoder is being rotated It would frequently have high frequency wiping noise on the encoder A and B pins.  After I attached a couple of .01 uf capacitors between the pins and ground the encoder began working flawlessly.

ChronoDot Realtime Clock Module

ChronoDot Realtime Clock Module

The next challenge was to interface a realtime clock to the Arduino.  This turned out to be very easy using a ChronoDot clock module.  This little gem interfaces to the Arduino using the I2C serial interface.  It will run over 8 years on a single battery and will maintain the time with an accuracy 1 minute a year.  The module requires four connections (5V, Gnd, SCL and SDA).  SCL and SDA are the serial interface connections.  The neat thing about the I2C interface is that many devices can share the same interface.  In this project I  also used the I2C interface to interface a 4 line by 20 character display.

 

IMG_0184I spent quite of bit of time developing the software–mainly because I was also learning how to program in ‘C’ at the same time I was developing the code to for the project.

The software to interface to the realtime clock, rotary encoder and display were developed by modifying examples readily available on the web.

I also had to develop the software to calculate the Sidereal time from GMT using one of several algorithms I found on the web.  One problem I experienced was an error of about 5 to 30 seconds in the Sidereal time because of floating point round off errors in the Arduino software.  This occurs because the  Arduino software does not support double precision floating point calculations.  This did not present a major  problem in my application since the resolution of the mechanical setting circles on the Link Telescope only have a precision of around 1 minute.

The followingIMG_0187 photo shows the internal circuitry of the Hour Angle Calculator.  The circuit board at the top of the photo is the backside of the 4 line by 20 character display.  The four wires connect the display to power and the I2C interface.  The circuit board shown in the 3 x 5 file card box is the prototyping shield attached to the top of the Arduino.  This board holds the realtime clock, the pull-up resistors and bypass capacitors for the rotary encoder and other wiring to needed to interface to the Arduino.  The circuit can be powered for several hours using a 9V battery.

73,

Fred

Late Winter Storm

Posted on by March 24, 2013

Just when I thought Spring was finally arriving, the National Weather Service issued a winter storm warning for the next two days.   We are expecting up to 10″ of snow this afternoon and tonight.  I guess I will just have to stay inside chasing DX and working on the Arduino project I have been putting together for the Link Observatory.  Shucks!!!

I’ll try to post a few photos when the snow arrives–assuming our HughesNet satellite internet does not go out.

73,

Fred