U2b holds the voltage at the top of R10 to twice the voltage at the temperature set-point, or in other words, the wiper of R4. So the voltage at the top of R10 goes between ground and twice the set-point voltage. This is used as positive feedback, along with the Thevenin equivalent of R2 through R5, in series with R7 and R8 (if not bypassed by the jumper). This positive feedback is the hysteresis.
Actually, space heaters do have thermostats. It's just that they are fixed onto the space heater, so they don't take a measurement of the room. They can only measure the temp right at the heater itself. Andrew has manged to extend it to measure the room apart from the space heater.
Mr. Morris, I still appreciate the idea, even given some safety concerns by readers. The cool thing about Gadget Freak is that it inspires creativity. Your 24-volt idea is a good improvement for safety reasons -- as is a wireless solution. The part I really like is the two op-amp circuit that has a lot of applications. I'm still trying to figure out how the resistors adjust hysteresis. Hints welcome.
A wireless connection between the sensor and the control system would be the best option. That would allow proper temperature sensing and safety. Perhaps, you shouldn't build my gadget after all. I will keep using mine, however.
My only complaint with the unit you mentioned is that the sensor sticks up like a little antenna above the unit. The sensor should have a long wire on it so that it could be mounted at eye level. Of course, then there is the issue, like mine, of circuit isolation, should the cord insulation be damaged. Like mine, this device will not have a power transformer. So, for safety reasons, it's probably best to keep the sensor near the box. It just won't work as well at sensing the room temperature at the height where people are most sensitive to it.
Yes, I thought of that and prepared this statement.
I know I said this in the article, but I want to emphasize that this circuit is NOT ISOLATED from the power line and can be a potential shock hazard if the electrical outlet being used is not properly wired AND the insulation on the wire connecting the control unit to the power unit is damaged. If the thermostat is going to be used around pets or children, use only wire with thick and tough insulation. Of course, a dog can chew through a lamp cord. DO NOT use the headset extension cord that I used, unless you can make it inaccessible to pets and kids. Alternatively, a 24-volt transformer-rectifier circuit could be used in place of the power supply circuit in the schematic as an additional safety measure, in case the cord gets damaged AND the outlet is improperly wired. Please contact me if you need help doing this. I want you to be safe. Nevertheless, build and use this gadget entirely at your OWN RISK. As I have no control over what you do with this gadget, I will accept no responsibility whatsoever for any injury or damages arising out of its construction or use. Do not attempt to build this gadget if you are not VERY familiar with the construction of AC-line connected devices and their safety issues. Properly constructed, this gadget is just as safe as any other AC-operated appliance, as long as the interconnecting cord gets the proper attention.
Not to burst anyone's bubble, but these have been on the market for at least 15 years.
I buy mine from Northern Tool. They are digital. You can get 120V or 240V single phase rated units. They even have a digital readout with the target and actual temp readings. They plug directly into a wall socket and fit flush with the wall socket. Then the heater cord plugs into it.
It's always fun to build things you can call your own. However, ALL electrical appliances must comply with UL. If something were to happen to this home made thermostat and caused a fire you could be held liable! The Insurance Company could try to make you a criminal.
I think I paid $50.00 for mine at Northern. I think Amazon sells them for even less!
Controlling the heater by sensing the actual room temperature is a definite advantage. I plan to do it this winter by using a regular old household thermostat and a relay, either mechanical or solid state.
I had a similar issue with the built-in thermostat on my 5000W Fahrenheat Ceiling-Mount heater when I installed it in my garage. It was pretty easy to add a relay and a standard wall-mount thermostat to get better control over the temperature. A good alternative if you don't want to build a gadget.
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