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.
I know it's out of season right now, but winter will get here before you know it. I built three of these thermostats last December and they served me quite well. I have received numerous requests from friends and family to build some units for them. They are great for a hobbyist, but they are too labor-intensive to build and sell without printed circuit boards. They're otherwise pretty cheap to make. I used the shells of the three remote controls I hacked up to make two volume controls (GF #192) and a fan control (GF #198). Now, I'll have to buy enclosures and outlets for the power units of any more thermostats I might make. The remote control enclosures were perfect for the job. You can probably buy a remote control at a flea market or a garage sale that has lost its transmitter or doesn't work. Or you could just build one of my remote control gadgets and use the shell from the hacked remote.
Andrew, thank you for coming up with this design. In our case, mountain winters can be especially cold in a sort-of-insulated cabin on stilts, so our thermostat problems are keeping the space heater downstairs on for a longer time at a given heat output.
Given the heat around the country this summer, winter can't come too soon. I never thought I'd say that. This thermostat is a great idea. You identified a real need, Andrew. The thermostats on space heaters are useless.
I agree with you that it is surprising that manufacturers did not think of this. It's such an inexpensive solution to a problem that should have been noticed.
You know what would be cool? Imagine an option where a ZigBee enabled the thermostat to be mounted across the room -- anywhere in the room so that we get to enjoy a more average temperature from the heater. Yeah, that's not as simple and elegant, but it might be an add on.
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.
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!
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.
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.
Well done Andrew with this gadget - once again you've managed to put together a really impressive gadget that is useful in improving power utilization and control of the heater operation. But putting the thermostat controller in the ambient temperature zone, the feedback between the environment and heating element is improved considerably, giving more comfort and control to the user. Impressive!
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.
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.
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.
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.
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?
I am not an attorney but I am almost certain you won't get sued for giving this kind of information away. Otherwise hobby electronics magazines, who actually sell the information, would have long ago ceased to exist as they depend on people like you for their projects. They print stuff like this with a simple warning that high voltages are accessible so use your own "good judgement".
Take a look at the July 2012 projects issue of Elektor USA and you will find four or five line operated projects that are not transformer isolated, usually powered by a resistor/capacitor direct to the power line. And since these have to operate in Europe as well as the US, they expose as much as 230V.
Line voltage safety: We have been using electricity for well over 100 years. If anyone is doing electrical and electronic work and still doesn't know you can get a shock from line voltages, then he pretty much deserves to get zapped.
Good point, Gafisher. Everybody in product development seems to be suing and getting sued these days -- Sony, Apple, MS, Amazon, Samsung. They all have one thing in common -- $$$. I don't believe the small inventor is going to get sued simply because there is no possibility of a big payday.
Never say never when it comes to lawsuits and ridiculous awards. What do hot coffee, circular saws, lawn mowers and highchairs have in common? All were at the base of some outrageous jury awards. Who would have ever guessed that you can get burned by hot coffee?
I would not lose any sleep over it though, because you do not have enough cash to attract one of the real blood sucking attorneys.
What's maddening is you can't anticipate where trouble will come from. In Chuck's Toyota floor mat story, the car company is running a recall because an floor mat not designed for the car model could bump into the accelerator.
Note that when the unit is first plugged in, you must wait at least a half hour for the temperature setting to stabilize. The voltage regulator circuit beneath the hole in the top of the box is located there in order to pull ambient air through the box, but it also heats the box up a bit. This "thermal air pump" was created to speed up the thermostat's response. This wasn't an issue when the circuit was an open breadboard, but enclosing the sensor in the box greatly increased the delay and therefore the temperature overshoot (using a forced-air heater). With the thermal air pump, the response of the thermostat is quite good. With a slow heater, like an oil-filled radiator, the thermal air pump won't be needed and the voltage regulator circuit can be moved to the power unit, if desired.
I use my thermostats with the hysteresis mostly bypassed. That means that the jumper is shorting pins 2 and 3 of J2. This means that the hysteresis is determined by the Thevenin equivalent of R2 through R6 and by R9. This is sufficient for my needs, as I want precise control. I left the option available for increasing the hysteresis in case a user finds that the heater is switching on and off too often.
The SCR voltage regulator is a bit temperature sensitive, which could slightly affect the thermostat's set-point. This will affect the internal heating of the control unit, due to the thermal air pump. The temperature coefficient of the SCR regulator is -80mV/degC. Once the thermostat is stabilizing the room temperature, this won't be an issue at all. If you turn the heater on in a cold room, the thermostat will think the room is a few degrees warmer than it really is and shut off prematurely. Within a half hour, the set-point will stabilize at its proper value. I didn't find this to be a problem because I keep my rooms regulated, but if you do have a problem with it, replace the SCR, R19 and R20 with two 1N4742 (12V, 1 watt) zeners in series. Of course, the standby power consumption will be increased a little bit and the zeners won't be as robust against power surges as the SCR. Definitely use the zeners if you intend to put numbers on the temperature dial.
A microcontroller with a thermistor circuit definitely would resolve the voltage regulator temperature sensitive because temperature monitoring closed loop detection can be embedded within the software code. The closed loop circuit will definitely impact the project BOM cost. Very nice project though!
Thanks! I mounted the sensor inside the box because the circuit is not isolated from the power line. The circuit does not require a regulated 24 volt power supply. The power supply voltage only affects the heat generated by the 6 volt zener regulator, being used as a thermal air pump. Using a microcontroller and a thermistor would not affect this situation. If the sensor is mounted outside the box, or fast sensing response is not required, the 6 volt regulator can be moved to the power unit and the problem goes away entirely.
I frequently work with PIC microcontrollers (see GF#192 and GF#198) and could have gone that route. If I can do the project simply without a microcontroller, I prefer to do it, because not all Gadget Freak readers have the means to program a microcontroller.
This analog circuit is closed-loop and is capable of very high precision.
I have built three of these and they serve my needs quite well, but I will accept no responsibility for any injury or damages arising out of the construction of or the use of this gadget. BUILD AND USE IT AT YOUR OWN RISK! I was not aware of it when I designed this gadget, but there are commercial, UL approved products available that do what this device does. This gadget posting is for educational purposes only. Use any good ideas found in it, but DO NOT BUILD IT! Use of this device may void your fire insurance.
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.
Getting sued? Magizines (hobby and professional) have been publishing plans for projects for well over a century. Can anyone name a single case where an author has suffered any civil or criminal judgment because someone attempted to replicate the published design and got into trouble?
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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