Very informative thread and funny enough something that I was hoping to work on myself as I'm looking at replacing the simple on/off thermostats that control the heating in my reptiles vivariums.

Can someone (Alain?) post up a schematic for the project. (or settings for the EasyPIC5) - I'm looking at trying to develop this a bit further so the output is pulsed, with the mark / space ratio reducing as the set point is neared. I've used commercial on/off stats, and the temperatures are hard to control. The idea is to keep the hot spot fairly constant and below 35C max, as constant exposure to a surface above this temperature can cause a burn risk to snakes.


With the thermometer probe placed on the shelf near the logger (it wasn't actually touching the same spot), and with the thermostats sensor in the cool part of the shelf, the thermostat was set so the digital thermometer read a max of 34C. Here's are the results - the bit you are interested in is between 18:00 hrs onwards



Obviously the heat was conducting through the wood, which cooled slower than if the sensor was measuring air or the surface of the heater. But you can see that there is a large temperature swing between the min and max readings, and that the surface reached over 40C

To reduce this difference I placed the two probes next to the logger directly above the heater. Here is the results, which still show the same cycling, but with a lower differential (average 5 degree C), but is more stable



The pulse proportional stat reduces the cycling as it's not switching the heater on/off/on, it maintains a steady temperature by pulsing power to it thus the heater never cools.

I could purchase a couple of commercial PP stats for around £100 (possibly less via mail order) but then where's the fun in that Now I know there are a lot of you experience programmers that have dabbled with PWM, so wonder if you would like to add your comments here.