Andrew Morris was frustrated by the inadequate thermostats on most space heaters. The thermostat is typically built inside the heater and has little contact with the outside world. It switches on and off according to the temperature inside the heater, not the ambient air temperature. This makes it little more than a duty-cycle controller, similar to the nonthermostatic control on an electric stove or the temperature control on an electric blanket.
Morris decided to design an electronic thermostat that senses the actual room temperature. He tested and tweaked it to get the proper control range and hysteresis. The new thermostat has an internal adjustment to set the center temperature. It has a range of 20F, centered at 70F. In creating the new thermostat, he did not alter any of the safety functions in the heater.
Wall mounting is more convenient to use and gets a better sense of the room temperature.
Andrew Morris' gadget saves electricity by using the actual room temperature to control the heater.
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.
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.
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.
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.
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.
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.
is available at Amazon for less than it would cost to make mine. It also has a digital readout. You can solve the temperature issue by setting its temperature a couple degrees cooler that you want the eye-level temperature to be. Just keep it well away from the heater. I don't want the liability of someone building a faulty version of my gadget.
I like it, I could use something like this for my gazebo in the winter. Also, if anyone has not already said so, you can probably use a power switch tail from sparkfun. I used one to automatically switch off my soldering iron if it falls off of my workbench.
IANAL either, but I'd say no liability for what someone does by their own choice.
The whole liability issue is fodder for another day, but I have to wonder how many inventions have been suppressed by their inventors over the simple fear that some idiot is going to barbeque himself while misusing the product. Would Edison, Ford, Bell have brought their ideas to market in today's litigious toxic atmosphere?
In our third annual contest, Design News and Allied Electronics are going to crown a winner in early 2016 for the best reader gadget submission this year, and once again, you, the readers, are the judges!
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