Yes, the noise will be interesting to see. I worked little with the firmware and started with a clean "P" controller or "droop controller". It is only slightly better than thermostat on-off with hysteresis. But I soon went for a full PID (maybe I drop the D term later on). One problem is that Tiny85 only got a 8 bit PWM. I found that a really fast updating control loop acts as a overlayed low frequency PWM. But I haven't got the full PID to work yet and I know why. The input range is 1024 steps, the output range is 256 and the integral part must exist in a much bigger interval not to be completely "damped" by windup-protection.
In later years I have become aware of that broking the "float taboo" is often helping a lot in these circumstances. So, I started to use floating point math (please don't flame me on this, I'm perfectly aware of using left shifts and fix precision integers but I also know that it may actually _save_ code space to use floats. I have seen it many times. Floats is not a n00b decision).
Anyhow, when I was leaved the project for work I still hadn't got the PID to work but I have only spent like one hour trying to make it work. I will resume the project in a day or two.
A short word on the power supply. I don't want to load the transformer of the PD 2005 (and its off when the PSU is off) so I will use a 230->24 VAC @ 1.5 A ordinary transformer witch gives 12 v at 115 volts and sufficient current. Hopefully that also reduces noise. But I also have thoughts of using a SMPS with PFC (the power factor can be bad with a common transformer when a rectifier is used). Any ideas? Should I pursue a (PD2005 internally mounted) SMPS or go with the trafo?
Saturation: Do you have an idea about the temperature (or the heater power) of the original heating system? Of course, I do not need to replicate that in detail, as a recalibration theoretically could use any temperature at all, as long as it stable, but it would be nice to know (I primarily want to settle for a little lower temperature to spare the electronics).
I'm amazed that the electronics survived. Damn it was hot! I almost thought the transistors were germanium, but it cannot be, it was so hot! It took ages to get the temperature down and I had time to smell the smoke from meters away, turn it off, unplug it, take it down from the shelf, go find a screwdriver, open the upper cover, the lower one, spray-freeze it from above, from below (the base), notice it to warm up again (the thermostat clicked a few times back and forth), and it was still to hot to touch and smelled "funny". It must have been at least 5 minutes but may have been 15! And when I first noticed something was wrong it must have been several minutes of not-so-regulated heater action. I still don't know what happened. The only data I have is that the oven lamp started to light brighter a couple of minutes before disaster, as it happens that I had mounted a LDR on it to log the oven duty cycle. Well, if someone wants to know the duty cycle they have to do it themselves as my PD2005 gonna be unique.