Example of Sensing and Control: Thermostat override

This page goes over some considerations which may be useful to amateurs trying to use computers to sense temperatures and turn on heating (or cooling) equipment as needed.

Forgive me repeating something I say elsewhere: Household electricity (i.e. voltages over say 30 volts, e.g. 110v in US, 230v in UK) is very dangerous. Even lower voltages can be dangerous, if not respected. Be sure you don't do things beyond your skill level. If you don't electrocute yourself or others, you might burn something down.

Fooling about with your heating system can be risky in some climates. Have you ever seen the damage caused when pipes burst due to freezing? You really don't want to! And remember Murphy's Law. It will ensure that the uncontrolled gush of water will a) be coming from someplace upstairs, and b) start about an hour after you leave for a long weekend away.

Hence the ideas which follow.

Heating systems can be as simple as.....

simple circuit

"SW1" is the thermostat. It "closes" ("turns on") when the temperature falls below whatever level you have it set for. The circlular symbol labelled "power" is where the circuit connects to the (dangerous) household power.

If you are lucky, your system will be done as follows. (If you are not lucky, it can be changed to the following... by a professional... quite easily)
circuit with relay

SW1 now turns on the coil in the low voltage relay. A relay is just an electrically operated switch. SW1 is still the thermostat.

Notice the two points marked "A" and "B". I'm going to suggest you add some other things to the circuit between them.

detail of final answer

The circuit above shows what I suggest you add to a circuit like the one shown in the second diagram. Note the "A" and "B". They correspond to the "A" and "B" in the second diagram.

SW1 is still the ordinary thermostat.

SW2 is a second thermostat.

SW3 is a computer controlled switch. There are many ways to control a switch by a computer, any of them will do for this. SW3 is just an ordinary toggle switch, and is optional.

(I'll come to SW4 in a moment.)

The core of my suggestion is that you set SW1 to whatever temperature you want to be the "floor" in the environment you are heating. In theory, nothing that happens because of your attempt to introduce computer control of the heating can cause a fault which would leave the heat off if the temperature, as sensed by SW1, falls below the "floor" set in that thermostat.

To make the environment hotter than SW1 requires, just turn on SW3. If the temperature goes above the setting in SW2, the heat will be turned off, regardless of what SW3 thinks of things. You can dispense with SW2, as long as you don't mind your heating bills going through the roof, your plants being cooked, in the event of a failure in the control of SW3.

SW4 is a way to over-ride SW3. If your computer-controlled SW3 isn't working right, you can make SW2 determine the environment temperature by closing SW4. Of course, if SW1's setting of is easily changed, you could also simply it to turn up the heat when SW3 is failing to close.

K.I.S.S.: Keep It Simple. How is the design above "simple"? It isn't... compared to a "dumb" heat control system. But it "works" (at least to the extent of not letting your environment get too cold) regardless of what the computer does. If you "wire in" the computer in a more obvious, less complex, way, you are relying on something very not simple to work all the time.

Enjoy! Happy Controlling.

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