I used a comparator (op-amp) to compare the voltage seen coming back from the temp sensor (johnson replacement probe from morebeer) and compare it to voltages set by the two (blue) pot's. Using the spec sheet for the temp probe I calculated what resistance values should be for 42 and 38F then set the 25 turn precise pot's to be those values.
If the voltage on the temp probe is lower than the voltage on the 42F high setting then the op-amp outputs 5v. Otherwise it outputs 0v. The low temp does the opposite. Both inputs are feed into a SR-Latch made from a quad-nand gate chip. Rising above the high temp sets the latch turning on 5v to the reed relay which then triggers the 12v power supply powering the circuit to trip a 12v relay which switches on the compressor. When the temp gets too cold the SR-Latch resets and the compressor turns off.
Originally Posted by ClaudiusB
When you switch to fermenting temp how many options do I have?
Bare minimum, we need a Lager & Ale setting.
Not all yeast strains have the identical optimum fermentation temp.
With two setting I am forced to select yeast strains with similar fermentation temps.
How can I ferment an Altbier or a Weizen?
Is this design only for aging, conditioning?
Currently temp is set by adjusting a 25 turn variable resistor, checking the resistor value against a chart, and then testing it to see if it works. This method makes the most amount of sense for testing purposes. This is NOT a digital controller that I can program to do amazing things. This is an analog circuit which relies solely on voltages and basic logic to operate.
This means in order to set the temp to x degrees I need to convert x degrees to a resistance value then set the resistor appropriately. I have not thought of another way to do this other than having multiple sets of resistors and switching between them.
This is only accurate to within a few degrees. It works great for serving beer. I set the high temp to 42 and the low to 38 and the beer is always at about 40 degrees. In reality it may be constantly cycling up and down through those temps (its too cold out now to know for sure but thats the logic behind it)
This is meant to be a simple and a bit more diy solution to more costly temp controllers. Above all it is meant to be much less expensive than any of them which start at $60.
Maybe it will not be the best solution for fermenting but for a kegerator it should work great. I plan on testing it out for fermenting soon.
Originally Posted by ClaudiusB
Has to have a dial with temp markings.
Unit must be able to control cooling and heating automatically.
No external load relays.
I could have a selector switch or something like that which switches between common temps like, 38-42, ferment range 1, ferment range 2 etc.... and cover 2-3 ferment temps. If you want exact control you will want to go with a Love controller etc. If you picked up a chest freezer or have an old one and want to turn it into a kegerator or temp controlled fermentor this could be a simple and cost effective solution.
It can control cooling at the moment. I am sure I could redesign it so that it could heat and cool.... in most situations I would assume you would want to do one or the other. In that case a simple toggle switch could flip the inputs so it worked in reverse, heating when it was too cold.
What do you mean by no external load relays? Right now the logic chips are switching on a 5v reed relay which is switching 12v to a 12v auto relay which is turning on the compressor. Is there something wrong with this configuration? is there a better way to make 5v switch 120v outlet?
I appreciate your feedback so far and I have some good ideas I will be testing out. Let me know if you have any more questions or ideas!