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.
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:
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.
- 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
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