lumpy darkness

Observation notes (with equipment details), photos, sketches, reviews, software tips, random thoughts, by an amateur astronomer.

SUNDAY, JUNE 2, 2019

DIY dew heater controller

I had thought about building my own PWM dew heater controller for a long time but it remained low priority. When I thought I had damaged my Kendrick Type IV and that the cheap LED dimmer from eBay wasn't working, I reopened the project. Another driver for proceeding was that when multi-tasking, say imaging with the DSLR while visually observing with the telescope, I'd need more dew busting gear. I knew I could make a custom unit cheaper. None more black.

 

inside of custom dew heater controller

 

technical info:

The Kendrick Type IV is great with 4 outputs but it only has 1 control. My custom unit has the following features:

  • 4 RCA outputs
  • 2 independent controls
  • pulsing LED indicators
  • pulse-width modulation
  • reverse-polarity protection
  • fused
  • slightly longer power cord
  • input power: 8-18V (recommended 12V)
  • current per channel
    • maximum: 3A
    • recommended: 1-2A

build costs:

Commercial dual control units are around CAD $150

  • digi-key parts (including shipping and taxes) CAD $45
  • parts in my bins - free sorta?
  • parts gifted from Dietmar Kupke - priceless

research and build time:

  • start date 9 Jan '19
  • finish date 17 Feb '19
  • approx. 32 hours

references:

  • Michael Vlasov's dual control 556 design
  • Math Heijen's simple 555 schematic
  • RASC Journal, October 2014

lessons learned?

  • In ExpressSCH and PCB, if you make a "new" object for a polarised or multi-pin component, verify the pin layout! Many times I have decided to stand radial electrolytic capacitors upright for space reasons. One must verify the positive and negative terminals.
  • Trust your instincts! Early on I found when I rebuilt a prototype circuit following the schematic, it worked. But my circuit on the PCB didn't. I should have carefully checked my PCB layout at that point...
  • Be careful about assumptions, interpretations of tests, etc. In the end, the cheapo LED dimmer did work. But other factors conspired to throw me off the scent. Better notes would have helped, stepping back, re-testing...
  • Avoid testing high current circuits with precious equipment. I should have done all my heat output tests with a sacrificial (or simple DIY) heater.
  • Use a drill press for increased precision.

 

 

SUNDAY, JUNE 2, 2019

bits and bobs

Parts for the dew controller build. Many accumulated over the years from various sources including A-1, Sayal, The-Store-Formerly-Known-As-RadioShack, and Active Surplus. All items denoted with an asterisk (*) were purchased from digi-key.

 

schematic for dew heater controller

 

  • SW1 - switch SPST
  • U1 - LM556 dual IC timer
  • IC socket (optional)
  • R1, R2 - variable 1M resistors *
    (Bourns Inc. / PDA241-SRT02-105B0)
  • 2 knobs for pots *
    (APEM Inc. / MPKES90B14)
  • R3, R4 - 47k resistors
  • R5, R6, R7, R8 - 2.2k resistors
  • D1, D2, D3, D4 - 1N4148 diodes
  • D5, D6 - diffuse red LEDs 5mm
  • 2 LED snap-in holders
  • D7 - Schottky diode *
    (SMC Diode Solutions (VA) / 95SQ015)
  • C1, C2 - 10nF capacitors
  • C3, C4 - polarised 1µF capacitors
  • Q1, Q2 - 2n2222 NPN transistors TO-18
    (obsolete but I had some in stock)
  • Q3, Q4 - TIP31 NPN transistors *
    (STMicroelectronics / TIP31C)
  • 2 heatsinks TO-220 for Q3 and Q4 *
    (Aavid 507302B00000G)
  • PCB perf board
  • 4 RCA female jacks *
    (CUI Inc. / RCJ-031)
  • 12v male CLA fused
  • fuse 3-5A
  • project box
  • Richlok F-4 cable strain
  • fine gauge solid wire for jumpers
  • light gauge insulated wire for jumpers
  • medium gauge wire for output lines
  • phone cord wire for input line
  • 2 bolts and nuts for heatsinks 4-40

 

Tools, equipment, software, etc. used. You can do your own thing of course but this is what I used.

  • computer resources
    • AutoDesk 123D software
    • Express SCH and PCB software
    • word processor
    • laser printer
    • interwebs
    • Evernote
  • testing
    • digital multi-meter
    • logic probe
    • breadboard
    • 12v SLA battery
    • 12v lithium-ion battery
    • alligator jumper cables, for jumping
  • tools
    • soldering iron, adjustable
    • sponge
    • helping hands
    • alligator jumper cables, for heatsinking
    • Fancii loupe glasses
    • drill (press is best)
    • drill bits, 1/4", 5/8", 1/2", 1/32"
    • side cutters
    • wire strippers, with small bolt cutters
    • heat gun
    • pliers
    • small vice
    • needlenose pliers
    • tiny screwdrivers
    • small spanner
    • small round file
    • hack saw
  • supplies
    • heat shrink tubing
    • solder
    • water
    • electrical tape
    • paper
    • clear tape
    • thermal gel
    • matches
    • contact cement
    • fuses for the multi-meter
    • desoldering braid

 

And I used a digital camera to photo-document things...

 

 

FRIDAY, SEPTEMBER 6, 2019

blog links

Scattered through the blog proper are detailed reports and updates on the build with some posts showing photos. If you dare... Reverse chronological order.

  • first full run in the "field" under clear skies, 31 Aug '19
  • tested LED dimmer again, 18 Feb '19
  • finished dew controller, 17 Feb '19
  • built working circuit board, 8 Feb '19
  • more debugging while soldering, 3 Feb '19
  • exported the 3D rendering, 2 Feb '19
  • considered physical layout, 27 Jan '19
  • prototyped half the circuit, 26 Jan '19
  • made a discovery, 26 Jan '19
  • gathered parts, 25 Jan '19
  • designed schematic, 12 Jan '19
    (this is the final corrected schematic)
  • looked up prices for commercial units, 11 Jan '19
  • considered Arduino solution, 9 Jan '19
  • tested gear, 5 Jan '19
  • noticed dew wrap not hot, 5 Jan '19

 

 

 

 

links

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(*)   builds and hacks
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