Following on from my post about DIY Nichrome Dew Heater Bands, I’m going to show you how to build a cheap and effective dew heater controller. Whilst preventing dew formation is essential to avoid interruptions to the night’s imaging, we do not want to generate excessive heat as this can cause air currents in front of the telescope’s optics and degrade the quality of the image.
A dew controller enables you to manage the power output of the heater bands, and also helps to conserve battery life if you are away from the mains supply. Commercial dew heater controllers retail for £70 to £100+, but I’ll show you how to make a simple one for as little as £5. Please note that this device is suitable for heater bands at 12V to 24V only.
|DIY Dew Heater Controller|
The basic problem we want to solve is to vary the power output of our heater bands according to the current environmental conditions. As I noted in my previous post, there is no simple way to calculate the correct power output to keep the scope’s optics just above the dew point. What we have to do is use a Feedback Loop to monitor the current conditions and increase or decrease the power output accordingly.
There are two ways in which this can be achieved:
- We could use temperature probes to monitor ambient temperature and the temperature of the optics, plus a micro-controller to adjust the power output of the heater band to keep the optics at a set temperature above ambient. Some commercial dew heaters offer this function (at a premium price), and a number of DIYers have built similar devices using Arduinos and other programmable micro-controllers.
- Adjusting the power output by hand as required. This is the approach I have taken as it is a lot simpler. As an imager I already have too many complicated things to manage and I don’t wish to add to the list. I don’t image in extreme conditions so the temperature tends to stabilise fairly soon after dark. This allows me to determine whether I need to set the heater on low, medium or full power and I rarely have to change the setting thereafter.
If your local conditions are different you may need to be more proactive in monitoring for the signs of dew. Using a dew shield is a good idea as dew will form on the exposed parts of the scope before it starts affecting the optics, thus giving you sufficient warning to crank up the power.
If you followed the calculations in my previous post, you’ll probably realise that you could vary the power output of your band in one of two ways:
- By Changing the Input Voltage: Reducing the input voltage results in lower power output from the heater band. The various means by which we might do this are generally less efficient that the method used below.
- By Changing the Circuit’s Resistance: Increasing the total resistance of the circuit also results in lower power output. It would be very simple to insert a Potentiometer or Rheostat in series between the power supply and the heater band, allowing us to adjust the total resistance of the circuit. This is a less than ideal solution as the potentiometer gets hot and also wastes valuable power, a significant issue if you are using a battery.
|12V LED Dimmer|
- LED Dimmer, 12-24V, 8A: Search on eBay for these devices. There are lots of sellers and they currently retail for about £2.00 (including postage) from sellers in Hong Kong and China, or about £5.00 from UK sellers. Bear in mind that delivery from HK by surface mail takes three to four weeks. Buy one dimmer for each heater band that you wish to control. Given the long lead time, I bought a few spares in case of failure; others have reported that these cheap devices giving out on them after a while, but mine have been reliable thus far. I also wanted an additional power channel for my LED light box (to be covered in a future post).
- Connection Wire: You will need insulated hook-up wire or speaker wire that can carry the full load that you calculated in the previous article. Generally 1mm – 1.5mm wire should be fine.
- Plastic Project Box: I bought a project box from my favourite electronics retailer; you just need to make sure you have enough space to hold all of the controller innards (which are much smaller than their own plastic enclosures), plus the connecting wires, etc.
- Red LED and LED Holder: If you are using a 12V power supply, you will need one 12V LED and holder per dimmer. If you are using 5V LEDs or a different supply voltage, use this LED Calculator to identify the correct resistor to put between the power supply and the LED.
- Chassis Fuse Holder and Fuse: In the basic set-up above, I am assuming you have added a fuse in series between the heater band and the controller for safety. In the more sophisticated version, you can add the fuse(s) to the controller box for a neater build. Again you will need one fuse per dimmer, with a rating appropriate to the band to be used.
- Chassis Phono (RCA) Socket: Again you will need one socket per dimmer.
- Power Socket or Power Switch: I salvaged a low voltage barrel-type power socket from a broken piece of equipment to use with my 12V laptop power supply. Alternatively you could wire the power supply directly to a switch installed in the project box.
- Wire Cutters/Stripper: This makes life easier, but you could get away with a pen-knife at a push.
- Soldering Iron and Lead-Free Solder: For connecting the phono-sockets and fuse-holders. Heat-shrink tube is useful for insulating connections too.
- Drill and drill-bits: For drilling the project box to add the sockets, leds and fuse-holders.
- Multi-meter: This is optional, but helpful for checking connections and diagnosing faults.
- Hot-glue Gun: Useful for attaching the circuit boards to the project box.
- Remove the dimmer circuit boards carefully from the original enclosures. First, remove the small screws on the underside of the enclosure and the front part should split easily from the back part.
- Note: The potentiometer is attached to the front part of the enclosure and the circuit board is screwed the the back part. The two are connected by a set of fine wires which are easily broken at the point where they attach to the potentiometer so be careful.
- Pull the black knob off the potentiometer shaft using your fingers. Next hold the potentiometer body (behind the enclosure face) to stop it turning and unscrew the large silver nut at the base of the potentiometer shaft on the front of the enclosure. The potentiometer shaft should now slide out of the enclosure face. Retain the knob and nut for later.
- You can detach the potentiometer from the (usually red) circuit board by pulling the white plug out of the socket at the rear of the circuit board. Again this may avoid breaking the fragile connections to the potentiometer whilst you work.
- Finally remove the small screws holding the circuit board to the back of the enclosure. The circuit board and terminal block should come away in one piece.
|Deluxe Version Layout|
- On the right-hand side you can see three dimmer circuit boards (red) with terminal blocks (green) glued in place with hot glue. These are much smaller than their original plastic enclosures (on the right for comparison).
- I drilled a hole to the rear panel of the project box for the black barrel power socket and glued it in place. I attached it to a normal electrical screw terminal block (white, centre) just to make it easy to take things apart if I needed to (you could just run the wires direct to the first set of IN connectors on the dimmer).
- I drilled three holes in the front panel of the project box for the panel phono sockets to the heater bands.
- Finally I drilled the top cover with holes for the potentiometers, holes to contain the panel fuse holders and small holes for the red indicator LEDs. See below for the layout of the top cover.
- On the left-hand side I installed my blue Arduino Uno micro-controller which I am going to use for a future project (so ignore it for this build!)
- Where I had to connect two wires to one screw terminal (e.g. taking power between IN terminals or adding the LEDS), I soldered the wires together and used heat shrink to insulate. This gave me a more reliable connection, but you could simply screw two wires in to one terminal if you wish.
- Note: Make sure you connect the LEDs the right way round. The long (Anode / Positive) leg of the LED should be connected to the positive side of the circuit. Don’t forget to add an appropriate resistor unless you are using a 12V LED and 12V supply (or a 5V LED and 5V supply).
- I didn’t connect the LEDs as shown due to lack of space when my project box was closed. I actually connected them to the OUT terminal side of the fuse holders. This still indicates the power output level correctly, but it doesn’t give an indication that the fuse has blown. I can live with that but it would have been better to connect the LEDs to the terminals on the phono socket if there had been space.
- Optional: I have put my fuses between the heater bands and the dimmers. If you are concerned about one of the dimmers shorting out you could add an extra fuse between the DC power socket and the dimmers. This would need to have a rating sufficient for combined load of all your heater bands.
|Completed Dew Heater Controller|