In the "waterbed days" of the 70's when I first got married, I had such a heater and designed a controller with a quiet relay, thermistor and Op Amp with a 12Vdc wallwort supply. It might have cycled every 15 minutes within 0.2'C .
well, more power to you
mine is 0.2ºC in difference only when I go to target like 30ºC when room is like 20ºCish..
when I went to 45 there was a few degrees up/down difference, but in that high temps it doesn't matter much and I can still fine tune the algorithm..
Do you care about the pad temperature or more for the target or interface temperature?
temperature of the environment. as this will be enclosed in the box with some kind of shelf holding something like baking pan which will contain stuff that needs the temp. most of the time some sort of legumes (I make tempeh).
A Raspberry Pi seems massive overkill for such a simple task? You can get an off-the-shelf temperature controller module with dual display & sensing for a few pounds on ebay or amazon etc.
yeah, there were times of water beds and discmans, these are of buffering and booting ;D and even "full blown" raspberry is not of my worries. It's gonna beed to boot like for two minutes until it is operable, but idc.. in average it will run for 3days in most cases.
If you need a custom display and controls, a small PIC or Arduino IC would do it, or Pi Pico, or an ESP32-WROOM-32 etc, if you need WiFi as well. All are a fraction the cost (and far easier to get) than the full blown raspberry pi.
yes sir, I need wifi. and a controll interface on the mobile phone - an app in which you'll be able to set what kind of product are you making and fine tune the heating program. for example, when you are making a tempeh from some robust legume like chickpeas, it is a good idea to go to 42ºC every second hour for an hour and after 8hrs keep it 32ºC then ask an operator after another 8hrs if the fermentation process succesfuly started an in that cast it's gonna lower the temp to 32ºC or so, because the bacteria is generating a heat at this point as it ferments the legume.
also the there is a valid question of where to put the sensor b/c if you decide to grow mushrooms, i'll need some sterile containment and you want to place another temp sond into it, seal it and preferably don't bother it for a month or so. and yes, there will be an optional camera module to be able to take peaks in this lenghty processes.
there are simpler solutions, but trust me, if you don't know the process well enough they'll get you nowhere. you're just gonna waste sources. nobody uses them anyway. those who know what they do will do anyway, others will mostly fail. they all wait until it boots happily
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switching DC at random times in each half cycle with a FET is likely to cause RF interference, which is something you have to be very aware of in any product you are selling.
there seems to be and interesting point however, can't tell exactly. is it about PWM freq or how often I plan to change duty cycle?