I was thinking of having a ceiling fan come on when my furnace came on. I used to dabble in electronics but it's been a long while and I know there are new and innovative approaches to accomplish the task. But, I'm kind of out of date on ideas.
Does someone have an idea to start with?
There's a handy device which incorporates a temperature sensor and two output drivers - the LM56. I've used these as fan controllers for wall-mounted radiators - a fan on top of the radiator comes on when the radiator heats up, and so spreads the heat away from the wall. You could use the same idea with a suitable interface to your ceiling fan. The only thing to watch is the maximum ratings of the LM56 - 12V supply and 5mA output drive.
I have built one of these systems, because my apartment contains a wall heater / wall furnace which does not circulate air by itself. There are lots of choices in how to go about it.
Controller:
I chose to use an Arduino Uno board. This is a well-supported and easy-to-get-started way to develop and run microcontroller programs, and the physical layout makes it easy to make the necessary input and output connections.
You could also use almost any other microcontroller dev board, or design your own minimal board.
Inputs/sensors:
If you have a modern electronic thermostat, it will almost certainly have an output for fan control — which serves exactly the purpose you intend, to control circulation of air, and allows you to manually turn it on (when you want the fan on but don't want the furnace to shut it off for you). The thermostat may have configuration options for how long the fan runs after heat shuts off.
The output will be a relay, so you can read it from your controller as if it were a switch. This relay closure will be between the terminals usually labeled “R” or “Rh” (heating system signaling power, will have a hopefully-red wire already present) and “G” (fan control; wire would have a green).
If you do not have such a thermostat, you can (as I did) install one in parallel with (if not replacing) your existing thermostat, and gain the additional benefit of being able to schedule temperature changes. The advantage of using this separate circuit is that you do not have to worry about compatibility with the voltage the furnace uses (which may be 24 VAC in an externally powered system, or a small DC voltage in a “millivolt” system powered by a thermopile heated by a gas pilot light.)
A completely different control strategy is to use temperature sensors to detect when the fan should switch on. This has the advantage that the fan is on whenever it is useful, including in the summer when you are trying to push hot air out of the space. I used both this and the thermostat-control strategy in my system — the fan is on whenever either input indicates the fan should be on.
I have two LM34 temperature sensors, mounted on walls near the floor and ceiling. These are analog sensors which can be read by ADC inputs on your choice of microcontroller. (They also have the hilarious feature of reading directly in Fahrenheit at a scale of 10 mV/°F; meaning that colder temperatures require a negative supply to output the corresponding negative voltages. But this is not a concern at the temperatures we care about in this fan controller.)
The controller turns on the fan whenever the temperature difference between the two sensors exceeds 7 °F, and turns it off (if the thermostat is not also requesting fan) when the difference drops below 3 °F. This difference in thresholds, or hysteresis, prevents excessive cycling on and off.
Controlling the fan:
My fan has only traditional mechanical switches — a single switch on the wall and pullchains on the fan itself. Since I do not want to make changes to the apartment's line-voltage wiring, I installed a mechanical switch-flipper driven by a hobby RC servo. It is simple to drive a servo from Arduino; a library for the purpose is provided. The big disadvantage of this system is that it's a visible mess on the wall and noisy (though a quality servo will be quieter, and you can program a movement pattern that sounds less harsh).
If you have a remote control for your fan, then you might be able to replicate its signals; this is easier for IR remotes than for RF remotes, since all that is needed is to drive a pulse train into an IR LED. Unfortunately, my understanding is that the common configuration today is to use RF remotes installed in a wall panel. I do not have any advise on how to go about duplicating one. You could, with some care for voltage and polarity, modify a remote to trigger its button functions externally by wiring to the the switch contacts.
You can add a “smart switch” “IoT” device to switch your fan, and then send commands to that from your controller. I avoid such products out of concern for computer security, so I have no advice here, but it is a good option for elegant integration.
