Ozzzy's Place
Dew Zapper control box
and 12V power distribution panel

The need
I have an SCT that seems to collect moisture from about a 100km circle which all ends up on my corrector plate. Therefore the need for heat was paramount. Also, I have some things that use 12V that would be nice to have running (like the scope's drive) without having multiple cables running to my power source (a 1998 Neon).

Therefore I built a power bus/heat control unit, actually a couple of them. The 'control' in these was just a matter of switching in/out resistors in series. Dave McCarter's comment was 'that's a waste of heat'. This may be the case, but what the heck; they all worked, but none of them were what you could call either elegant or efficient.

I've built a new box centred on a custom circuit board that has both my heat/power and the focus motor support with ST-4 and RS232 breakout on the board. See it here.

My last dew controller was designed by Dave McCarter and given to me by Dave Rubenhagen to play with. It was built in the distant past around a 555 timer and an ECG234 bi-polar transistor. While it worked as it was built to work Dave has since re-worked his circuit for better efficiency; this was my starting point. Read all about Dave's Dew Zapper at http://www.astro.uwo.ca/~rasc/HEATER.pdf.

As I only want ONE power connection running to the scope and need to supply juice to things like the drive and a map LED I wanted to be able to pass straight 12V on some of the connectors. I modified the wiring in the case I built for the ECG controller unit to handle the new board and elected not to use a poly-switch (RUE400), mainly because I couldn't find one, using instead a fused cigarette lighter adapter to keep things in line.

Note: Having something switching a resistive load on/off rapidly has been reported to cause problems with computerized scopes plugged into the same power source. Caveat...

Dave's circuit
Exerpted from his PDF
The circuit diagram.

The layout with components shown


The artwork side of Dave's circuit board



How it all works
The circuit board itself has no 'ground' as it is connected in serial with the outputs. Ground is supplied by whatever is plugged in completing the circuit. The duty cycle is adjusted by changing the resistance on the 50K pot. The heart of this is a 555 timer running as an astable multi-vibrator and an IRF530A power FET that turns on/off the juice to the plugs in time with the signal from the 555. This design provides a symmetrical timing curve with the duty cycle adjustable from 5% to 95% with a stable frequency of 265Hz.

The controlled power is sent through the power FET to the RCA plugs that the heater(s) will be connected to. I decided on only two heater outputs as my Telrad can plug into a straight 12V plug. I'll use the two controlled outputs for the corrector and eyepiece.

When the heater power is on a red LED illuminates giving me an indication of the power being provided. There is a second LED above the on/off switch that just tells me that power is available.

The circuits (both sides) have their ground turned on or off with an DPDT switch with center off. One switchable circuit has only the un-controlled output being grounded while the other side will ground both. This way if I don't need heat, I don't have to energize the timer circuit. I've been told that this is a waste of $1.50 but what the heck. It looks cool and gives me the ability to turn the whole thing off without pulling the power cord.

As you can see I built the board on pad-per-hole perf-board. A printed circuit board would be better but I have no facilities to build one. For those places where I needed to carry current I soldered in bare 24AWG wire and ran a big bead of solder along it.

The parts for the board.
        1 x IRF530N       N CHANNEL POWER MOSFET WITH TO220 HEAT SINK
        1 x 1N4148        DIODE 
        1 x 1N4007        DIODE
        1 x LN555         TIMER
        2 x .1uF          CERAMIC CAP
        2 x 2.2k 1/4W     RESISTOR
        1 x 50k           LINEAR POT

        SUITABLE WIRE
        SOLDER
        PERF BOARD
        OINTMENT FOR BURNT FINGERS
        SUITABLE ANALGESIC FOR THE HEADACHES
The parts for the enclosure.
I like the Hammond boxes as they're easy to work with and are built well. You can get cheapo boxes that will do the job for about 1/2 price but we're only talking $8 here. Also, the Hammond boxes come with the 'slots' in the sides that you can use to mount the board. 
        SUITABLE PROJECT BOX (HAMMOND)
        A NICE KNOB FOR THE POT
        MONO PHONE PLUG FOR POWER IN
        RCA PLUGS AS REQUIRED (OR OTHER IF YOU PREFER)
        DPDT CENTRE-OFF SWITCH
        LED'S AND 6.8K RESISTORS AS REQUIRED
        BEER (POST-BUILD ONLY)
The cigarette lighter adapter
Originally I had used an RCA plug to bring power in (hence the strain relief thingy). This trailed 25 feet of speaker wire which wasn't optimal. Now the power is brought to the unit with an 18AWG stranded pair and comes into the unit through a large mono phone plug. 
        FUSED LIGHTER ADAPTER FROM PARTS STORE
        18AWG WIRE (TWO CONDUCTOR)
        MONO PHONE JACK

Note: If you can't find the exact parts, check the data-sheets and get the closest equivalent.

The total cost of the finished controller (minus the beer) came to about $30.00 (Canadian).

The execution
The controller board.
On the board that is in the finished product images below the leads are running out of the rear of the board. To do this I drilled two 5/64" holes in the board and soldered in suitable wire. If you build one that will sit flat in the bottom of the case then you'll want your leads coming out of the top of the board. Don't forget to try to keep static away from the MOSFET. Use normal anti-static procedures. The components could be closer together if you have less space. The circuit is fairly simple. I had room so I placed them further apart so my I could 1) see what I was doing and 2) have room to work (I'm a lousy solderer). I laid out the components as close as I could to Dave's artwork.

The finished product.
Layout your board so that it fits in the supplied slots in the Hammond case; this one fits the case slots nicely.

Notice the strap attached to the top of the case. This is so I can hang the thing from my mount and keep it out of the way. If it was just a heater controller I'd use Velcro (tm) to attach it somewhere on the scope (there's not much room on my scope) so that the leads could be shorter and I wouldn't have to watch for tangled wires. Also, I bent a screw-in hook and put it in the side for strain relief on the 12V input line. Using an RCA plug means that it can pull out at the most inopportune time therefore I changed the RCA plug for a mono phone plug. This gives good positive connection and needs a good tug to pull out. The strain relief I think is still a must.




Here's the whole enchilada laid open along side a diagram of the internal wiring.

As you can see the controller circuit and pot sit nicely down at one end with the rest mostly mounted on the lid. The two LEDs (one for power on the other for heat intensity, the switch and the six RCA plugs fit nicely. The phone plug where power comes in is mounted in the 'bottom' and the eye for the strap and strain relief bar are just screwed into the plastic.

I've used a few drops of hot glue to keep some wires from 'flopping' about and tried to cut the rest so that there wasn't much to have to stuff into the box pushing on things.

Other than that the thing wasn't that hard to put together and it functions very well. With my home-made heaters which you can read about here I've had no problems with dew.

Mounting the unit
This was how I mounted it on the Golden Pyramid tripod. Now that I have the HEQ5 the box is velcro'ed to the 'flat' on the side of the mount. It makes things much simpler and means less wire get's twisted around.

Originally I just put an eye in the top of the unit and just hung it from the mount using a Velcro strap. This worked fine but the unit swung around a bit. Now I have a home-made 'clip on' bracket that I made from an old wall-bracket for a network hub. I bent the thing a bit and now it just clips on to a leg of the tripod. This is a stop-gap until I figure out a way of making a real 'pretty' one out of plastic.

Powering on the unit
When powering on the unit connect ALL of the heater/power cables to the scope, turn the power/heat switch to OFF (center position) and plug in the 12v input (large phone plug) BEFORE connecting the power input cable to the power source. This will save you on fuses and possibly damaged electronics.

NOTE
After many blown fuses I replaced the large phone plug with two bannana plugs; one red and one black. This completely seperates the + and - poles and I don't worry about things anymore.


Clear skies.