In many instances the manufacturers of integrated circuits, and other components, publish "application notes" presenting what they consider optimized demonstrations of how to best use their products. Those notes are especially useful because they are intended to be the best way to do things, so that those products will be utilized and more of them be sold. Back in "the old days"there were even whole books of application literature given away. If you can find those in a used book store or an estate sale they can be rather valuable learning materials. Presently some application information is available on line from the manufacturers, and that is another option. But do not trust any of it from you-tube is my advice.

 

I made the changes to the circuit and now the emitter is connected to the base of the transistor with a 2.2k resistor. I attached the pullup resistor to the collector connected to the power supply. I'm currently using the EPM78V2-05R0-01ROR DC/DC stepdown (5vdc, 1A) to power the rest of the circuit. When I was adjusting the potentiometer with the original power module (120vac to 5vdc, 600mA), I was able to maintain a constant 5v regardless of how I adjusted it. With the DC/DC stepdown, though, the output of the stepdown goes anywhere from 5vdc down to 2.2vdc as I adjust the potentiometer. I'm not entirely sure why this is happening. I was able to test the original circuit (120vac power) with a fan to see if the switching worked with the emitter instead of the collector and it does.

 

Here's the breadboard. The top is the full bridge rectifier outputting 5vdc. The yellow wire feeding into the + terminal of the comparator is the output from the Honeywell sensor and the green wire feeding into the - terminal is from the potentiometer.

I was able to get this circuit to work with a fan by using the power module (120vac power source, 5vdc, 600mA output), but when I attempted to do the same by using the full bridge rectifier + the DC/DC stepdown (24vac power source from transformer), the potentiometer keeps lowering the output voltage from the bridge rectifier. I checked the coil voltage and it drops as low as 2.2vdc (3.75vdc must operate voltage)

TB3/TB4 (black and black+blue stripe wire) is for a 24vac hydraulic actuator I was testing. The other is power (24vac from transformer).