DEW CONTROLLER

Copyright (c) 2005 Javier Martinez. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.

One night I was trying to photograph Andromeda (Messier object #31) when I noticed that dew covered lens in just few minutes after startup, so I was in the need to find a solution to avoid this nasty problem.

There's some interesting projects just Googling a bit, but many of them are a bit old in electronics terms. So I decided to make my own dew controller.

The key elements are:

One Microchip PIC16F628 controller.
Two Dallas DS18S20 temperature sensors for each heater.
A resistor heater.
Automatic control, without knobs or buttons.
Hot-Plug heaters.

Diagram

The PIC16F628 microcontroller will power on heater until the temperature of heater is greater of a given value ( Theater-Tair >= DiffMax ), and will power off heater until lower a given value ( Theater-Tair <= DiffMin ). The key is to keep the lens temperature just a few ºC above the ambient temp.

Temperature graph

The controller have two different differential temps, it will chose the correct one when powering up the system. These differential temps are not related to sensor temps, are related to conexion sockets (RJ45):

Heaters plugged when system is already running: DiffMax = 4 and DiffMin = 3.
Heaters already plugged when powering up system: DiffMax = 6 and DiffMin = 5.

In order to calc the resistive element of the heater I'll take the lens diameter. With a 50mm lens diameter I'll need a 3W heater. So the calcs:

Lengh of circle: 157mm
Heater's resistance: (12v)²/3W = 48 ohm.
Number of resistive elements in parallel: 15 x 720 ohm. Note: my choice is 15 x 680 ohm. (E12 series).
A resistor each cm, all covered with Velcro strip.

Here's some shoots of the already working prototype.

- The box

Box view