Charleston Observatory

Dew Heater Controller - by Len Benschop

Accepts standard Kendrick Heaters or equivalent. I recommend the Kendrick heater elements - they are well made, enclosed in protective fabric, and with suitable care can provide a very long lifespan.


On this page I detail the design of a relatively simple dew heater controller that I built. It is working well, allowing unhindered observing with a dew shield, even during times of high relative humidity. 

NOTES - Cautions
I built this controller with the intent to use it with up to (4) Kendrick heater strips... with only (1) of the (4) being a SCT corrector lens heater, typically never running at 100% duty cycle.

The heaters I use it with are (1) 8" circumference heater on the telescope lens, and several smaller heaters for eyepieces, Telrad etc.  
Since the heaters are connected in parallel, the resistance of all the attached heaters will determine the total current draw. The total wattage of the connected heaters in parallel must be considered by the user!!

The intent of publishing these details is to allow others to build their own simple heater cycle controller, as a DIY project. If you are not inclined to building circuitry yourself, there are much better commercial units available from various sources.

These plans are provided "as is" without any warranty of suitability for your application.

I have been using this controller with 2-3 heaters for approx 12 years.

The primary switching device is an industrial OPTO22 output driver, that switches the 12v to the heaters via a variable duty cycle controlled by the 555 timer. An alternative is to use a Power FET transistor.

The Opto22 ODC5 module is optically isolated and capable of driving 3A continuous with a 5A surge rating for one second. Being that the entire circuit is protected with a 2A fuse, the Opto22 is protected against overcurrent even if shorted out. I chose to use an ODC5 module since I had one in my parts bin. It may be advisable to mount the FET to a heatsink.

The dew heater controller is a simple variable duty cycle on/off control. The LM555 timer provides the oscillator, and the duty cycle is variable with a 50Kohm potentiometer. The duty cycle is adjustable from 10% to 100%. I generally run the corrector heater at only 40%. The LED cycles on and off with the heater, allowing a visual indicator of the heater cycling.

Variable Pulse Width

The duty cycle is variable anywhere between 10% to 100%. The image above displays how the frequency is constant but the duty cycle is variable, dependant upon where the 50K Potentiometer is dialed to.



(2) - 1N4148 Diode
(2) - 10K 1/4 watt resistors
(2) - 1K 1/4 watt resistor
(1) - 100uF Electrolytic Capacitor
(1) - 0.1uF Mylar Capacitor
(1) - LM555 Timer Integrated Circuit
(1) - 50K Potentiometer
(1) - 2A Fuse & Holder
(1) - Red LED
(1) - 4 Jack RCA Connector Strip
(1) - Case suitable for mounting components in
(1) - Lighter Plug and cord
(1) - BUZ71A FET
(1) - Heatsink for the FET
(1) - 8.2K resistor

OR REPLACE FET and 8.2k R6 with

 (1) - Opto22 ODC5 DC Output Module

Several people have enquired about the Pinout for the ODC5 Output module. The schematic has been updated with pin numbers, and a detailed drawing of the module is now posted below...

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