This is DIY solution I use for preventing dew on my Telescope optics without the need to either setting my dew heaters too hot or constantly having to monitor for any possible dew build up..
The reason was that I did not want to set the Dew Heaters to hot if not needed and only want to keep the optics a few degrees above Dew Point.
So I decided to see if I can put something together using an Arduino Nano (which I have plenty of laying around).

At the end I cam up with this design and it is working rather well. As I used a BME280 to measure temperature, humidity and from that calculate the dew point, I realised that I can use this data to feed back in to your favourite Astro Photography software via an ASCOM driver, which I did.

The unit uses standard dew heater bands like you can get from DewControl.com and is fully Open Source with Schematics, Arduino Code & ASCOM Drivers.
Here the schematic for it (yes, might not be my best).

I used DS18B20 temp sensors that I tuck under the Dew Heater and has direct contact with the telescope housing close to the optics.

This build can be made with or without an OLED LCD. If you decide not to add the LCD, you can also leave out the push button. The push button is there to show the current data for a few seconds and then the LCD will go blank again, this is to minimize light impact on your eyes during visual observation.

I have a PCB design for the electronics to fit a Hammond 1593LALBK or the Hammond 1593KALBK. If you are interested in the PCB for the schematic above, please contact me. The PCB comes without any of the components, so you need to order the components yourself and then solder them on the PCB.

This setup is supported by Arduino Code and ASCOM drivers which is fully open source and can be found on GitHub here.

Yes, this project is entirely opensource and might not be perfect. But if you use this for your own design or use this design for your Observatory, please give me some credit.